acorn.js

Acorn is a tiny, fast JavaScript parser written in JavaScript.

Acorn was written by Marijn Haverbeke and various contributors and released under an MIT license. The Unicode regexps (for identifiers and whitespace) were taken from Esprima by Ariya Hidayat.

Git repositories for Acorn are available at

http://marijnhaverbeke.nl/git/acorn
https://github.com/marijnh/acorn.git

Please use the github bug tracker to report issues.

This file defines the main parser interface. The library also comes with a error-tolerant parser and an abstract syntax tree walker, defined in other files.

(function(root, mod) {
  if (typeof exports == "object" && typeof module == "object") return mod(exports); // CommonJS
  if (typeof define == "function" && define.amd) return define(["exports"], mod); // AMD
  mod(root.acorn || (root.acorn = {})); // Plain browser env
})(this, function(exports) {
  "use strict";

  exports.version = "0.7.1";

The main exported interface (under self.acorn when in the browser) is a parse function that takes a code string and returns an abstract syntax tree as specified by Mozilla parser API, with the caveat that inline XML is not recognized.

  var options, input, inputLen, sourceFile;

  exports.parse = function(inpt, opts) {
    input = String(inpt); inputLen = input.length;
    setOptions(opts);
    initTokenState();
    return parseTopLevel(options.program);
  };

A second optional argument can be given to further configure the parser process. These options are recognized:

  var defaultOptions = exports.defaultOptions = {

ecmaVersion indicates the ECMAScript version to parse. Must be either 3, or 5, or 6. This influences support for strict mode, the set of reserved words, support for getters and setters and other features.

    ecmaVersion: 5,

Turn on strictSemicolons to prevent the parser from doing automatic semicolon insertion.

    strictSemicolons: false,

When allowTrailingCommas is false, the parser will not allow trailing commas in array and object literals.

    allowTrailingCommas: true,

By default, reserved words are not enforced. Enable forbidReserved to enforce them. When this option has the value "everywhere", reserved words and keywords can also not be used as property names.

    forbidReserved: false,

When enabled, a return at the top level is not considered an error.

    allowReturnOutsideFunction: false,

When locations is on, loc properties holding objects with start and end properties in {line, column} form (with line being 1-based and column 0-based) will be attached to the nodes.

    locations: false,

A function can be passed as onToken option, which will cause Acorn to call that function with object in the same format as tokenize() returns. Note that you are not allowed to call the parser from the callback—that will corrupt its internal state.

    onToken: null,

A function can be passed as onComment option, which will cause Acorn to call that function with (block, text, start, end) parameters whenever a comment is skipped. block is a boolean indicating whether this is a block (/* */) comment, text is the content of the comment, and start and end are character offsets that denote the start and end of the comment. When the locations option is on, two more parameters are passed, the full {line, column} locations of the start and end of the comments. Note that you are not allowed to call the parser from the callback—that will corrupt its internal state.

    onComment: null,

Nodes have their start and end characters offsets recorded in start and end properties (directly on the node, rather than the loc object, which holds line/column data. To also add a semi-standardized range property holding a [start, end] array with the same numbers, set the ranges option to true.

    ranges: false,

It is possible to parse multiple files into a single AST by passing the tree produced by parsing the first file as program option in subsequent parses. This will add the toplevel forms of the parsed file to the Program (top) node of an existing parse tree.

    program: null,

When locations is on, you can pass this to record the source file in every node's loc object.

    sourceFile: null,

This value, if given, is stored in every node, whether locations is on or off.

    directSourceFile: null
  };

  function setOptions(opts) {
    options = opts || {};
    for (var opt in defaultOptions) if (!has(options, opt))
      options[opt] = defaultOptions[opt];
    sourceFile = options.sourceFile || null;

    isKeyword = options.ecmaVersion >= 6 ? isEcma6Keyword : isEcma5AndLessKeyword;
  }

The getLineInfo function is mostly useful when the locations option is off (for performance reasons) and you want to find the line/column position for a given character offset. input should be the code string that the offset refers into.

  var getLineInfo = exports.getLineInfo = function(input, offset) {
    for (var line = 1, cur = 0;;) {
      lineBreak.lastIndex = cur;
      var match = lineBreak.exec(input);
      if (match && match.index < offset) {
        ++line;
        cur = match.index + match[0].length;
      } else break;
    }
    return {line: line, column: offset - cur};
  };

  var getCurrentToken = function () {
    var token = {
      type: tokType,
      value: tokVal,
      start: tokStart,
      end: tokEnd
    };
    if (options.locations) {
      token.startLoc = tokStartLoc;
      token.endLoc = tokEndLoc;
    }
    return token;
  };

Acorn is organized as a tokenizer and a recursive-descent parser. The tokenize export provides an interface to the tokenizer. Because the tokenizer is optimized for being efficiently used by the Acorn parser itself, this interface is somewhat crude and not very modular. Performing another parse or call to tokenize will reset the internal state, and invalidate existing tokenizers.

  exports.tokenize = function(inpt, opts) {
    input = String(inpt); inputLen = input.length;
    setOptions(opts);
    initTokenState();

    function getToken(forceRegexp) {
      lastEnd = tokEnd;
      readToken(forceRegexp);
      return getCurrentToken();
    }
    getToken.jumpTo = function(pos, reAllowed) {
      tokPos = pos;
      if (options.locations) {
        tokCurLine = 1;
        tokLineStart = lineBreak.lastIndex = 0;
        var match;
        while ((match = lineBreak.exec(input)) && match.index < pos) {
          ++tokCurLine;
          tokLineStart = match.index + match[0].length;
        }
      }
      tokRegexpAllowed = reAllowed;
      skipSpace();
    };
    return getToken;
  };

State is kept in (closure-)global variables. We already saw the options, input, and inputLen variables above.

The current position of the tokenizer in the input.

  var tokPos;

The start and end offsets of the current token.

  var tokStart, tokEnd;

When options.locations is true, these hold objects containing the tokens start and end line/column pairs.

  var tokStartLoc, tokEndLoc;

The type and value of the current token. Token types are objects, named by variables against which they can be compared, and holding properties that describe them (indicating, for example, the precedence of an infix operator, and the original name of a keyword token). The kind of value that's held in tokVal depends on the type of the token. For literals, it is the literal value, for operators, the operator name, and so on.

  var tokType, tokVal;

Internal state for the tokenizer. To distinguish between division operators and regular expressions, it remembers whether the last token was one that is allowed to be followed by an expression. (If it is, a slash is probably a regexp, if it isn't it's a division operator. See the parseStatement function for a caveat.)

  var tokRegexpAllowed;

When options.locations is true, these are used to keep track of the current line, and know when a new line has been entered.

  var tokCurLine, tokLineStart;

These store the position of the previous token, which is useful when finishing a node and assigning its end position.

  var lastStart, lastEnd, lastEndLoc;

This is the parser's state. inFunction is used to reject return statements outside of functions, inGenerator to reject yields outside of generators, labels to verify that break and continue have somewhere to jump to, and strict indicates whether strict mode is on.

  var inFunction, inGenerator, labels, strict;

This counter is used for checking that arrow expressions did not contain nested parentheses in argument list.

  var metParenL;

This is used by parser for detecting if it's inside ES6 Template String. If it is, it should treat '$' as prefix before '{expression}' and everything else as string literals.

  var inTemplate;

This function is used to raise exceptions on parse errors. It takes an offset integer (into the current input) to indicate the location of the error, attaches the position to the end of the error message, and then raises a SyntaxError with that message.

  function raise(pos, message) {
    var loc = getLineInfo(input, pos);
    message += " (" + loc.line + ":" + loc.column + ")";
    var err = new SyntaxError(message);
    err.pos = pos; err.loc = loc; err.raisedAt = tokPos;
    throw err;
  }

Reused empty array added for node fields that are always empty.

  var empty = [];

Token types

The assignment of fine-grained, information-carrying type objects allows the tokenizer to store the information it has about a token in a way that is very cheap for the parser to look up.

All token type variables start with an underscore, to make them easy to recognize.

These are the general types. The type property is only used to make them recognizeable when debugging.

  var _num = {type: "num"}, _regexp = {type: "regexp"}, _string = {type: "string"};
  var _name = {type: "name"}, _eof = {type: "eof"};

Keyword tokens. The keyword property (also used in keyword-like operators) indicates that the token originated from an identifier-like word, which is used when parsing property names.

The beforeExpr property is used to disambiguate between regular expressions and divisions. It is set on all token types that can be followed by an expression (thus, a slash after them would be a regular expression).

isLoop marks a keyword as starting a loop, which is important to know when parsing a label, in order to allow or disallow continue jumps to that label.

  var _break = {keyword: "break"}, _case = {keyword: "case", beforeExpr: true}, _catch = {keyword: "catch"};
  var _continue = {keyword: "continue"}, _debugger = {keyword: "debugger"}, _default = {keyword: "default"};
  var _do = {keyword: "do", isLoop: true}, _else = {keyword: "else", beforeExpr: true};
  var _finally = {keyword: "finally"}, _for = {keyword: "for", isLoop: true}, _function = {keyword: "function"};
  var _if = {keyword: "if"}, _return = {keyword: "return", beforeExpr: true}, _switch = {keyword: "switch"};
  var _throw = {keyword: "throw", beforeExpr: true}, _try = {keyword: "try"}, _var = {keyword: "var"};
  var _let = {keyword: "let"}, _const = {keyword: "const"};
  var _while = {keyword: "while", isLoop: true}, _with = {keyword: "with"}, _new = {keyword: "new", beforeExpr: true};
  var _this = {keyword: "this"};
  var _class = {keyword: "class"}, _extends = {keyword: "extends", beforeExpr: true};
  var _export = {keyword: "export"}, _import = {keyword: "import"};
  var _yield = {keyword: "yield", beforeExpr: true};

The keywords that denote values.

  var _null = {keyword: "null", atomValue: null}, _true = {keyword: "true", atomValue: true};
  var _false = {keyword: "false", atomValue: false};

Some keywords are treated as regular operators. in sometimes (when parsing for) needs to be tested against specifically, so we assign a variable name to it for quick comparing.

  var _in = {keyword: "in", binop: 7, beforeExpr: true};

Map keyword names to token types.

  var keywordTypes = {"break": _break, "case": _case, "catch": _catch,
                      "continue": _continue, "debugger": _debugger, "default": _default,
                      "do": _do, "else": _else, "finally": _finally, "for": _for,
                      "function": _function, "if": _if, "return": _return, "switch": _switch,
                      "throw": _throw, "try": _try, "var": _var, "let": _let, "const": _const,
                      "while": _while, "with": _with,
                      "null": _null, "true": _true, "false": _false, "new": _new, "in": _in,
                      "instanceof": {keyword: "instanceof", binop: 7, beforeExpr: true}, "this": _this,
                      "typeof": {keyword: "typeof", prefix: true, beforeExpr: true},
                      "void": {keyword: "void", prefix: true, beforeExpr: true},
                      "delete": {keyword: "delete", prefix: true, beforeExpr: true},
                      "class": _class, "extends": _extends,
                      "export": _export, "import": _import, "yield": _yield};

Punctuation token types. Again, the type property is purely for debugging.

  var _bracketL = {type: "[", beforeExpr: true}, _bracketR = {type: "]"}, _braceL = {type: "{", beforeExpr: true};
  var _braceR = {type: "}"}, _parenL = {type: "(", beforeExpr: true}, _parenR = {type: ")"};
  var _comma = {type: ",", beforeExpr: true}, _semi = {type: ";", beforeExpr: true};
  var _colon = {type: ":", beforeExpr: true}, _dot = {type: "."}, _ellipsis = {type: "..."}, _question = {type: "?", beforeExpr: true};
  var _arrow = {type: "=>", beforeExpr: true}, _bquote = {type: "`"}, _dollarBraceL = {type: "${", beforeExpr: true};

Operators. These carry several kinds of properties to help the parser use them properly (the presence of these properties is what categorizes them as operators).

binop, when present, specifies that this operator is a binary operator, and will refer to its precedence.

prefix and postfix mark the operator as a prefix or postfix unary operator. isUpdate specifies that the node produced by the operator should be of type UpdateExpression rather than simply UnaryExpression (++ and --).

isAssign marks all of =, +=, -= etcetera, which act as binary operators with a very low precedence, that should result in AssignmentExpression nodes.

  var _slash = {binop: 10, beforeExpr: true}, _eq = {isAssign: true, beforeExpr: true};
  var _assign = {isAssign: true, beforeExpr: true};
  var _incDec = {postfix: true, prefix: true, isUpdate: true}, _prefix = {prefix: true, beforeExpr: true};
  var _logicalOR = {binop: 1, beforeExpr: true};
  var _logicalAND = {binop: 2, beforeExpr: true};
  var _bitwiseOR = {binop: 3, beforeExpr: true};
  var _bitwiseXOR = {binop: 4, beforeExpr: true};
  var _bitwiseAND = {binop: 5, beforeExpr: true};
  var _equality = {binop: 6, beforeExpr: true};
  var _relational = {binop: 7, beforeExpr: true};
  var _bitShift = {binop: 8, beforeExpr: true};
  var _plusMin = {binop: 9, prefix: true, beforeExpr: true};
  var _modulo = {binop: 10, beforeExpr: true};

'*' may be multiply or have special meaning in ES6

  var _star = {binop: 10, beforeExpr: true};

Provide access to the token types for external users of the tokenizer.

  exports.tokTypes = {bracketL: _bracketL, bracketR: _bracketR, braceL: _braceL, braceR: _braceR,
                      parenL: _parenL, parenR: _parenR, comma: _comma, semi: _semi, colon: _colon,
                      dot: _dot, ellipsis: _ellipsis, question: _question, slash: _slash, eq: _eq,
                      name: _name, eof: _eof, num: _num, regexp: _regexp, string: _string,
                      arrow: _arrow, bquote: _bquote, dollarBraceL: _dollarBraceL};
  for (var kw in keywordTypes) exports.tokTypes["_" + kw] = keywordTypes[kw];

This is a trick taken from Esprima. It turns out that, on non-Chrome browsers, to check whether a string is in a set, a predicate containing a big ugly switch statement is faster than a regular expression, and on Chrome the two are about on par. This function uses eval (non-lexical) to produce such a predicate from a space-separated string of words.

It starts by sorting the words by length.

  function makePredicate(words) {
    words = words.split(" ");
    var f = "", cats = [];
    out: for (var i = 0; i < words.length; ++i) {
      for (var j = 0; j < cats.length; ++j)
        if (cats[j][0].length == words[i].length) {
          cats[j].push(words[i]);
          continue out;
        }
      cats.push([words[i]]);
    }
    function compareTo(arr) {
      if (arr.length == 1) return f += "return str === " + JSON.stringify(arr[0]) + ";";
      f += "switch(str){";
      for (var i = 0; i < arr.length; ++i) f += "case " + JSON.stringify(arr[i]) + ":";
      f += "return true}return false;";
    }

When there are more than three length categories, an outer switch first dispatches on the lengths, to save on comparisons.

    if (cats.length > 3) {
      cats.sort(function(a, b) {return b.length - a.length;});
      f += "switch(str.length){";
      for (var i = 0; i < cats.length; ++i) {
        var cat = cats[i];
        f += "case " + cat[0].length + ":";
        compareTo(cat);
      }
      f += "}";

Otherwise, simply generate a flat switch statement.

    } else {
      compareTo(words);
    }
    return new Function("str", f);
  }

The ECMAScript 3 reserved word list.

  var isReservedWord3 = makePredicate("abstract boolean byte char class double enum export extends final float goto implements import int interface long native package private protected public short static super synchronized throws transient volatile");

ECMAScript 5 reserved words.

  var isReservedWord5 = makePredicate("class enum extends super const export import");

The additional reserved words in strict mode.

  var isStrictReservedWord = makePredicate("implements interface let package private protected public static yield");

The forbidden variable names in strict mode.

  var isStrictBadIdWord = makePredicate("eval arguments");

And the keywords.

  var ecma5AndLessKeywords = "break case catch continue debugger default do else finally for function if return switch throw try var while with null true false instanceof typeof void delete new in this";

  var isEcma5AndLessKeyword = makePredicate(ecma5AndLessKeywords);

  var isEcma6Keyword = makePredicate(ecma5AndLessKeywords + " let const class extends export import yield");

  var isKeyword = isEcma5AndLessKeyword;

Character categories

Big ugly regular expressions that match characters in the whitespace, identifier, and identifier-start categories. These are only applied when a character is found to actually have a code point above 128. Generated by tools/generate-identifier-regex.js.

  var nonASCIIwhitespace = /[\u1680\u180e\u2000-\u200a\u202f\u205f\u3000\ufeff]/;
  var nonASCIIidentifierStartChars = "\xAA\xB5\xBA\xC0-\xD6\xD8-\xF6\xF8-\u02C1\u02C6-\u02D1\u02E0-\u02E4\u02EC\u02EE\u0370-\u0374\u0376\u0377\u037A-\u037D\u037F\u0386\u0388-\u038A\u038C\u038E-\u03A1\u03A3-\u03F5\u03F7-\u0481\u048A-\u052F\u0531-\u0556\u0559\u0561-\u0587\u05D0-\u05EA\u05F0-\u05F2\u0620-\u064A\u066E\u066F\u0671-\u06D3\u06D5\u06E5\u06E6\u06EE\u06EF\u06FA-\u06FC\u06FF\u0710\u0712-\u072F\u074D-\u07A5\u07B1\u07CA-\u07EA\u07F4\u07F5\u07FA\u0800-\u0815\u081A\u0824\u0828\u0840-\u0858\u08A0-\u08B2\u0904-\u0939\u093D\u0950\u0958-\u0961\u0971-\u0980\u0985-\u098C\u098F\u0990\u0993-\u09A8\u09AA-\u09B0\u09B2\u09B6-\u09B9\u09BD\u09CE\u09DC\u09DD\u09DF-\u09E1\u09F0\u09F1\u0A05-\u0A0A\u0A0F\u0A10\u0A13-\u0A28\u0A2A-\u0A30\u0A32\u0A33\u0A35\u0A36\u0A38\u0A39\u0A59-\u0A5C\u0A5E\u0A72-\u0A74\u0A85-\u0A8D\u0A8F-\u0A91\u0A93-\u0AA8\u0AAA-\u0AB0\u0AB2\u0AB3\u0AB5-\u0AB9\u0ABD\u0AD0\u0AE0\u0AE1\u0B05-\u0B0C\u0B0F\u0B10\u0B13-\u0B28\u0B2A-\u0B30\u0B32\u0B33\u0B35-\u0B39\u0B3D\u0B5C\u0B5D\u0B5F-\u0B61\u0B71\u0B83\u0B85-\u0B8A\u0B8E-\u0B90\u0B92-\u0B95\u0B99\u0B9A\u0B9C\u0B9E\u0B9F\u0BA3\u0BA4\u0BA8-\u0BAA\u0BAE-\u0BB9\u0BD0\u0C05-\u0C0C\u0C0E-\u0C10\u0C12-\u0C28\u0C2A-\u0C39\u0C3D\u0C58\u0C59\u0C60\u0C61\u0C85-\u0C8C\u0C8E-\u0C90\u0C92-\u0CA8\u0CAA-\u0CB3\u0CB5-\u0CB9\u0CBD\u0CDE\u0CE0\u0CE1\u0CF1\u0CF2\u0D05-\u0D0C\u0D0E-\u0D10\u0D12-\u0D3A\u0D3D\u0D4E\u0D60\u0D61\u0D7A-\u0D7F\u0D85-\u0D96\u0D9A-\u0DB1\u0DB3-\u0DBB\u0DBD\u0DC0-\u0DC6\u0E01-\u0E30\u0E32\u0E33\u0E40-\u0E46\u0E81\u0E82\u0E84\u0E87\u0E88\u0E8A\u0E8D\u0E94-\u0E97\u0E99-\u0E9F\u0EA1-\u0EA3\u0EA5\u0EA7\u0EAA\u0EAB\u0EAD-\u0EB0\u0EB2\u0EB3\u0EBD\u0EC0-\u0EC4\u0EC6\u0EDC-\u0EDF\u0F00\u0F40-\u0F47\u0F49-\u0F6C\u0F88-\u0F8C\u1000-\u102A\u103F\u1050-\u1055\u105A-\u105D\u1061\u1065\u1066\u106E-\u1070\u1075-\u1081\u108E\u10A0-\u10C5\u10C7\u10CD\u10D0-\u10FA\u10FC-\u1248\u124A-\u124D\u1250-\u1256\u1258\u125A-\u125D\u1260-\u1288\u128A-\u128D\u1290-\u12B0\u12B2-\u12B5\u12B8-\u12BE\u12C0\u12C2-\u12C5\u12C8-\u12D6\u12D8-\u1310\u1312-\u1315\u1318-\u135A\u1380-\u138F\u13A0-\u13F4\u1401-\u166C\u166F-\u167F\u1681-\u169A\u16A0-\u16EA\u16EE-\u16F8\u1700-\u170C\u170E-\u1711\u1720-\u1731\u1740-\u1751\u1760-\u176C\u176E-\u1770\u1780-\u17B3\u17D7\u17DC\u1820-\u1877\u1880-\u18A8\u18AA\u18B0-\u18F5\u1900-\u191E\u1950-\u196D\u1970-\u1974\u1980-\u19AB\u19C1-\u19C7\u1A00-\u1A16\u1A20-\u1A54\u1AA7\u1B05-\u1B33\u1B45-\u1B4B\u1B83-\u1BA0\u1BAE\u1BAF\u1BBA-\u1BE5\u1C00-\u1C23\u1C4D-\u1C4F\u1C5A-\u1C7D\u1CE9-\u1CEC\u1CEE-\u1CF1\u1CF5\u1CF6\u1D00-\u1DBF\u1E00-\u1F15\u1F18-\u1F1D\u1F20-\u1F45\u1F48-\u1F4D\u1F50-\u1F57\u1F59\u1F5B\u1F5D\u1F5F-\u1F7D\u1F80-\u1FB4\u1FB6-\u1FBC\u1FBE\u1FC2-\u1FC4\u1FC6-\u1FCC\u1FD0-\u1FD3\u1FD6-\u1FDB\u1FE0-\u1FEC\u1FF2-\u1FF4\u1FF6-\u1FFC\u2071\u207F\u2090-\u209C\u2102\u2107\u210A-\u2113\u2115\u2119-\u211D\u2124\u2126\u2128\u212A-\u212D\u212F-\u2139\u213C-\u213F\u2145-\u2149\u214E\u2160-\u2188\u2C00-\u2C2E\u2C30-\u2C5E\u2C60-\u2CE4\u2CEB-\u2CEE\u2CF2\u2CF3\u2D00-\u2D25\u2D27\u2D2D\u2D30-\u2D67\u2D6F\u2D80-\u2D96\u2DA0-\u2DA6\u2DA8-\u2DAE\u2DB0-\u2DB6\u2DB8-\u2DBE\u2DC0-\u2DC6\u2DC8-\u2DCE\u2DD0-\u2DD6\u2DD8-\u2DDE\u2E2F\u3005-\u3007\u3021-\u3029\u3031-\u3035\u3038-\u303C\u3041-\u3096\u309D-\u309F\u30A1-\u30FA\u30FC-\u30FF\u3105-\u312D\u3131-\u318E\u31A0-\u31BA\u31F0-\u31FF\u3400-\u4DB5\u4E00-\u9FCC\uA000-\uA48C\uA4D0-\uA4FD\uA500-\uA60C\uA610-\uA61F\uA62A\uA62B\uA640-\uA66E\uA67F-\uA69D\uA6A0-\uA6EF\uA717-\uA71F\uA722-\uA788\uA78B-\uA78E\uA790-\uA7AD\uA7B0\uA7B1\uA7F7-\uA801\uA803-\uA805\uA807-\uA80A\uA80C-\uA822\uA840-\uA873\uA882-\uA8B3\uA8F2-\uA8F7\uA8FB\uA90A-\uA925\uA930-\uA946\uA960-\uA97C\uA984-\uA9B2\uA9CF\uA9E0-\uA9E4\uA9E6-\uA9EF\uA9FA-\uA9FE\uAA00-\uAA28\uAA40-\uAA42\uAA44-\uAA4B\uAA60-\uAA76\uAA7A\uAA7E-\uAAAF\uAAB1\uAAB5\uAAB6\uAAB9-\uAABD\uAAC0\uAAC2\uAADB-\uAADD\uAAE0-\uAAEA\uAAF2-\uAAF4\uAB01-\uAB06\uAB09-\uAB0E\uAB11-\uAB16\uAB20-\uAB26\uAB28-\uAB2E\uAB30-\uAB5A\uAB5C-\uAB5F\uAB64\uAB65\uABC0-\uABE2\uAC00-\uD7A3\uD7B0-\uD7C6\uD7CB-\uD7FB\uF900-\uFA6D\uFA70-\uFAD9\uFB00-\uFB06\uFB13-\uFB17\uFB1D\uFB1F-\uFB28\uFB2A-\uFB36\uFB38-\uFB3C\uFB3E\uFB40\uFB41\uFB43\uFB44\uFB46-\uFBB1\uFBD3-\uFD3D\uFD50-\uFD8F\uFD92-\uFDC7\uFDF0-\uFDFB\uFE70-\uFE74\uFE76-\uFEFC\uFF21-\uFF3A\uFF41-\uFF5A\uFF66-\uFFBE\uFFC2-\uFFC7\uFFCA-\uFFCF\uFFD2-\uFFD7\uFFDA-\uFFDC";
  var nonASCIIidentifierChars = "\u0300-\u036F\u0483-\u0487\u0591-\u05BD\u05BF\u05C1\u05C2\u05C4\u05C5\u05C7\u0610-\u061A\u064B-\u0669\u0670\u06D6-\u06DC\u06DF-\u06E4\u06E7\u06E8\u06EA-\u06ED\u06F0-\u06F9\u0711\u0730-\u074A\u07A6-\u07B0\u07C0-\u07C9\u07EB-\u07F3\u0816-\u0819\u081B-\u0823\u0825-\u0827\u0829-\u082D\u0859-\u085B\u08E4-\u0903\u093A-\u093C\u093E-\u094F\u0951-\u0957\u0962\u0963\u0966-\u096F\u0981-\u0983\u09BC\u09BE-\u09C4\u09C7\u09C8\u09CB-\u09CD\u09D7\u09E2\u09E3\u09E6-\u09EF\u0A01-\u0A03\u0A3C\u0A3E-\u0A42\u0A47\u0A48\u0A4B-\u0A4D\u0A51\u0A66-\u0A71\u0A75\u0A81-\u0A83\u0ABC\u0ABE-\u0AC5\u0AC7-\u0AC9\u0ACB-\u0ACD\u0AE2\u0AE3\u0AE6-\u0AEF\u0B01-\u0B03\u0B3C\u0B3E-\u0B44\u0B47\u0B48\u0B4B-\u0B4D\u0B56\u0B57\u0B62\u0B63\u0B66-\u0B6F\u0B82\u0BBE-\u0BC2\u0BC6-\u0BC8\u0BCA-\u0BCD\u0BD7\u0BE6-\u0BEF\u0C00-\u0C03\u0C3E-\u0C44\u0C46-\u0C48\u0C4A-\u0C4D\u0C55\u0C56\u0C62\u0C63\u0C66-\u0C6F\u0C81-\u0C83\u0CBC\u0CBE-\u0CC4\u0CC6-\u0CC8\u0CCA-\u0CCD\u0CD5\u0CD6\u0CE2\u0CE3\u0CE6-\u0CEF\u0D01-\u0D03\u0D3E-\u0D44\u0D46-\u0D48\u0D4A-\u0D4D\u0D57\u0D62\u0D63\u0D66-\u0D6F\u0D82\u0D83\u0DCA\u0DCF-\u0DD4\u0DD6\u0DD8-\u0DDF\u0DE6-\u0DEF\u0DF2\u0DF3\u0E31\u0E34-\u0E3A\u0E47-\u0E4E\u0E50-\u0E59\u0EB1\u0EB4-\u0EB9\u0EBB\u0EBC\u0EC8-\u0ECD\u0ED0-\u0ED9\u0F18\u0F19\u0F20-\u0F29\u0F35\u0F37\u0F39\u0F3E\u0F3F\u0F71-\u0F84\u0F86\u0F87\u0F8D-\u0F97\u0F99-\u0FBC\u0FC6\u102B-\u103E\u1040-\u1049\u1056-\u1059\u105E-\u1060\u1062-\u1064\u1067-\u106D\u1071-\u1074\u1082-\u108D\u108F-\u109D\u135D-\u135F\u1712-\u1714\u1732-\u1734\u1752\u1753\u1772\u1773\u17B4-\u17D3\u17DD\u17E0-\u17E9\u180B-\u180D\u1810-\u1819\u18A9\u1920-\u192B\u1930-\u193B\u1946-\u194F\u19B0-\u19C0\u19C8\u19C9\u19D0-\u19D9\u1A17-\u1A1B\u1A55-\u1A5E\u1A60-\u1A7C\u1A7F-\u1A89\u1A90-\u1A99\u1AB0-\u1ABD\u1B00-\u1B04\u1B34-\u1B44\u1B50-\u1B59\u1B6B-\u1B73\u1B80-\u1B82\u1BA1-\u1BAD\u1BB0-\u1BB9\u1BE6-\u1BF3\u1C24-\u1C37\u1C40-\u1C49\u1C50-\u1C59\u1CD0-\u1CD2\u1CD4-\u1CE8\u1CED\u1CF2-\u1CF4\u1CF8\u1CF9\u1DC0-\u1DF5\u1DFC-\u1DFF\u200C\u200D\u203F\u2040\u2054\u20D0-\u20DC\u20E1\u20E5-\u20F0\u2CEF-\u2CF1\u2D7F\u2DE0-\u2DFF\u302A-\u302F\u3099\u309A\uA620-\uA629\uA66F\uA674-\uA67D\uA69F\uA6F0\uA6F1\uA802\uA806\uA80B\uA823-\uA827\uA880\uA881\uA8B4-\uA8C4\uA8D0-\uA8D9\uA8E0-\uA8F1\uA900-\uA909\uA926-\uA92D\uA947-\uA953\uA980-\uA983\uA9B3-\uA9C0\uA9D0-\uA9D9\uA9E5\uA9F0-\uA9F9\uAA29-\uAA36\uAA43\uAA4C\uAA4D\uAA50-\uAA59\uAA7B-\uAA7D\uAAB0\uAAB2-\uAAB4\uAAB7\uAAB8\uAABE\uAABF\uAAC1\uAAEB-\uAAEF\uAAF5\uAAF6\uABE3-\uABEA\uABEC\uABED\uABF0-\uABF9\uFB1E\uFE00-\uFE0F\uFE20-\uFE2D\uFE33\uFE34\uFE4D-\uFE4F\uFF10-\uFF19\uFF3F";
  var nonASCIIidentifierStart = new RegExp("[" + nonASCIIidentifierStartChars + "]");
  var nonASCIIidentifier = new RegExp("[" + nonASCIIidentifierStartChars + nonASCIIidentifierChars + "]");

Whether a single character denotes a newline.

  var newline = /[\n\r\u2028\u2029]/;

Matches a whole line break (where CRLF is considered a single line break). Used to count lines.

  var lineBreak = /\r\n|[\n\r\u2028\u2029]/g;

Test whether a given character code starts an identifier.

  var isIdentifierStart = exports.isIdentifierStart = function(code) {
    if (code < 65) return code === 36;
    if (code < 91) return true;
    if (code < 97) return code === 95;
    if (code < 123)return true;
    return code >= 0xaa && nonASCIIidentifierStart.test(String.fromCharCode(code));
  };

Test whether a given character is part of an identifier.

  var isIdentifierChar = exports.isIdentifierChar = function(code) {
    if (code < 48) return code === 36;
    if (code < 58) return true;
    if (code < 65) return false;
    if (code < 91) return true;
    if (code < 97) return code === 95;
    if (code < 123)return true;
    return code >= 0xaa && nonASCIIidentifier.test(String.fromCharCode(code));
  };

Tokenizer

These are used when options.locations is on, for the tokStartLoc and tokEndLoc properties.

  function Position() {
    this.line = tokCurLine;
    this.column = tokPos - tokLineStart;
  }

Reset the token state. Used at the start of a parse.

  function initTokenState() {
    tokCurLine = 1;
    tokPos = tokLineStart = 0;
    tokRegexpAllowed = true;
    metParenL = 0;
    inTemplate = false;
    skipSpace();
  }

Called at the end of every token. Sets tokEnd, tokVal, and tokRegexpAllowed, and skips the space after the token, so that the next one's tokStart will point at the right position.

  function finishToken(type, val, shouldSkipSpace) {
    tokEnd = tokPos;
    if (options.locations) tokEndLoc = new Position;
    tokType = type;
    if (shouldSkipSpace !== false) skipSpace();
    tokVal = val;
    tokRegexpAllowed = type.beforeExpr;
    if (options.onToken) {
      options.onToken(getCurrentToken());
    }
  }

  function skipBlockComment() {
    var startLoc = options.onComment && options.locations && new Position;
    var start = tokPos, end = input.indexOf("*/", tokPos += 2);
    if (end === -1) raise(tokPos - 2, "Unterminated comment");
    tokPos = end + 2;
    if (options.locations) {
      lineBreak.lastIndex = start;
      var match;
      while ((match = lineBreak.exec(input)) && match.index < tokPos) {
        ++tokCurLine;
        tokLineStart = match.index + match[0].length;
      }
    }
    if (options.onComment)
      options.onComment(true, input.slice(start + 2, end), start, tokPos,
                        startLoc, options.locations && new Position);
  }

  function skipLineComment() {
    var start = tokPos;
    var startLoc = options.onComment && options.locations && new Position;
    var ch = input.charCodeAt(tokPos+=2);
    while (tokPos < inputLen && ch !== 10 && ch !== 13 && ch !== 8232 && ch !== 8233) {
      ++tokPos;
      ch = input.charCodeAt(tokPos);
    }
    if (options.onComment)
      options.onComment(false, input.slice(start + 2, tokPos), start, tokPos,
                        startLoc, options.locations && new Position);
  }

Called at the start of the parse and after every token. Skips whitespace and comments, and.

  function skipSpace() {
    while (tokPos < inputLen) {
      var ch = input.charCodeAt(tokPos);
      if (ch === 32) { // ' '
        ++tokPos;
      } else if (ch === 13) {
        ++tokPos;
        var next = input.charCodeAt(tokPos);
        if (next === 10) {
          ++tokPos;
        }
        if (options.locations) {
          ++tokCurLine;
          tokLineStart = tokPos;
        }
      } else if (ch === 10 || ch === 8232 || ch === 8233) {
        ++tokPos;
        if (options.locations) {
          ++tokCurLine;
          tokLineStart = tokPos;
        }
      } else if (ch > 8 && ch < 14) {
        ++tokPos;
      } else if (ch === 47) { // '/'
        var next = input.charCodeAt(tokPos + 1);
        if (next === 42) { // '*'
          skipBlockComment();
        } else if (next === 47) { // '/'
          skipLineComment();
        } else break;
      } else if (ch === 160) { // '\xa0'
        ++tokPos;
      } else if (ch >= 5760 && nonASCIIwhitespace.test(String.fromCharCode(ch))) {
        ++tokPos;
      } else {
        break;
      }
    }
  }

Token reading

This is the function that is called to fetch the next token. It is somewhat obscure, because it works in character codes rather than characters, and because operator parsing has been inlined into it.

All in the name of speed.

The forceRegexp parameter is used in the one case where the tokRegexpAllowed trick does not work. See parseStatement.

  function readToken_dot() {
    var next = input.charCodeAt(tokPos + 1);
    if (next >= 48 && next <= 57) return readNumber(true);
    var next2 = input.charCodeAt(tokPos + 2);
    if (options.ecmaVersion >= 6 && next === 46 && next2 === 46) { // 46 = dot '.'
      tokPos += 3;
      return finishToken(_ellipsis);
    } else {
      ++tokPos;
      return finishToken(_dot);
    }
  }

  function readToken_slash() { // '/'
    var next = input.charCodeAt(tokPos + 1);
    if (tokRegexpAllowed) {++tokPos; return readRegexp();}
    if (next === 61) return finishOp(_assign, 2);
    return finishOp(_slash, 1);
  }

  function readToken_mult_modulo(code) { // '%*'
    var next = input.charCodeAt(tokPos + 1);
    if (next === 61) return finishOp(_assign, 2);
    return finishOp(code === 42 ? _star : _modulo, 1);
  }

  function readToken_pipe_amp(code) { // '|&'
    var next = input.charCodeAt(tokPos + 1);
    if (next === code) return finishOp(code === 124 ? _logicalOR : _logicalAND, 2);
    if (next === 61) return finishOp(_assign, 2);
    return finishOp(code === 124 ? _bitwiseOR : _bitwiseAND, 1);
  }

  function readToken_caret() { // '^'
    var next = input.charCodeAt(tokPos + 1);
    if (next === 61) return finishOp(_assign, 2);
    return finishOp(_bitwiseXOR, 1);
  }

  function readToken_plus_min(code) { // '+-'
    var next = input.charCodeAt(tokPos + 1);
    if (next === code) {
      if (next == 45 && input.charCodeAt(tokPos + 2) == 62 &&
          newline.test(input.slice(lastEnd, tokPos))) {

A --> line comment

        tokPos += 3;
        skipLineComment();
        skipSpace();
        return readToken();
      }
      return finishOp(_incDec, 2);
    }
    if (next === 61) return finishOp(_assign, 2);
    return finishOp(_plusMin, 1);
  }

  function readToken_lt_gt(code) { // '<>'
    var next = input.charCodeAt(tokPos + 1);
    var size = 1;
    if (next === code) {
      size = code === 62 && input.charCodeAt(tokPos + 2) === 62 ? 3 : 2;
      if (input.charCodeAt(tokPos + size) === 61) return finishOp(_assign, size + 1);
      return finishOp(_bitShift, size);
    }
    if (next == 33 && code == 60 && input.charCodeAt(tokPos + 2) == 45 &&
        input.charCodeAt(tokPos + 3) == 45) {

<!--, an XML-style comment that should be interpreted as a line comment

      tokPos += 4;
      skipLineComment();
      skipSpace();
      return readToken();
    }
    if (next === 61)
      size = input.charCodeAt(tokPos + 2) === 61 ? 3 : 2;
    return finishOp(_relational, size);
  }

  function readToken_eq_excl(code) { // '=!', '=>'
    var next = input.charCodeAt(tokPos + 1);
    if (next === 61) return finishOp(_equality, input.charCodeAt(tokPos + 2) === 61 ? 3 : 2);
    if (code === 61 && next === 62 && options.ecmaVersion >= 6) { // '=>'
      tokPos += 2;
      return finishToken(_arrow);
    }
    return finishOp(code === 61 ? _eq : _prefix, 1);
  }

Get token inside ES6 template (special rules work there).

  function getTemplateToken(code) {

'`' and '${' have special meanings, but they should follow string (can be empty)

    if (tokType === _string) {
      if (code === 96) { // '`'
        ++tokPos;
        return finishToken(_bquote);
      } else
      if (code === 36 && input.charCodeAt(tokPos + 1) === 123) { // '${'
        tokPos += 2;
        return finishToken(_dollarBraceL);
      }
    }

    if (code === 125) { // '}'
      ++tokPos;
      return finishToken(_braceR, undefined, false);
    }

anything else is considered string literal

    return readTmplString();
  }

  function getTokenFromCode(code) {
    switch (code) {

The interpretation of a dot depends on whether it is followed by a digit or another two dots.

    case 46: // '.'
      return readToken_dot();

Punctuation tokens.

    case 40: ++tokPos; return finishToken(_parenL);
    case 41: ++tokPos; return finishToken(_parenR);
    case 59: ++tokPos; return finishToken(_semi);
    case 44: ++tokPos; return finishToken(_comma);
    case 91: ++tokPos; return finishToken(_bracketL);
    case 93: ++tokPos; return finishToken(_bracketR);
    case 123: ++tokPos; return finishToken(_braceL);
    case 125: ++tokPos; return finishToken(_braceR);
    case 58: ++tokPos; return finishToken(_colon);
    case 63: ++tokPos; return finishToken(_question);

    case 96: // '`'
      if (options.ecmaVersion >= 6) {
        ++tokPos;
        return finishToken(_bquote, undefined, false);
      }

    case 48: // '0'
      var next = input.charCodeAt(tokPos + 1);
      if (next === 120 || next === 88) return readRadixNumber(16); // '0x', '0X' - hex number
      if (options.ecmaVersion >= 6) {
        if (next === 111 || next === 79) return readRadixNumber(8); // '0o', '0O' - octal number
        if (next === 98 || next === 66) return readRadixNumber(2); // '0b', '0B' - binary number
      }

Anything else beginning with a digit is an integer, octal number, or float.

    case 49: case 50: case 51: case 52: case 53: case 54: case 55: case 56: case 57: // 1-9
      return readNumber(false);

Quotes produce strings.

    case 34: case 39: // '"', "'"
      return readString(code);

Operators are parsed inline in tiny state machines. '=' (61) is often referred to. finishOp simply skips the amount of characters it is given as second argument, and returns a token of the type given by its first argument.

    case 47: // '/'
      return readToken_slash();

    case 37: case 42: // '%*'
      return readToken_mult_modulo(code);

    case 124: case 38: // '|&'
      return readToken_pipe_amp(code);

    case 94: // '^'
      return readToken_caret();

    case 43: case 45: // '+-'
      return readToken_plus_min(code);

    case 60: case 62: // '<>'
      return readToken_lt_gt(code);

    case 61: case 33: // '=!'
      return readToken_eq_excl(code);

    case 126: // '~'
      return finishOp(_prefix, 1);
    }

    return false;
  }

  function readToken(forceRegexp) {
    if (!forceRegexp) tokStart = tokPos;
    else tokPos = tokStart + 1;
    if (options.locations) tokStartLoc = new Position;
    if (forceRegexp) return readRegexp();
    if (tokPos >= inputLen) return finishToken(_eof);

    var code = input.charCodeAt(tokPos);

    if (inTemplate) return getTemplateToken(code);

Identifier or keyword. '\uXXXX' sequences are allowed in identifiers, so '\' also dispatches to that.

    if (isIdentifierStart(code) || code === 92 /* '\' */) return readWord();

    var tok = getTokenFromCode(code);

    if (tok === false) {

If we are here, we either found a non-ASCII identifier character, or something that's entirely disallowed.

      var ch = String.fromCharCode(code);
      if (ch === "\\" || nonASCIIidentifierStart.test(ch)) return readWord();
      raise(tokPos, "Unexpected character '" + ch + "'");
    }
    return tok;
  }

  function finishOp(type, size) {
    var str = input.slice(tokPos, tokPos + size);
    tokPos += size;
    finishToken(type, str);
  }

Parse a regular expression. Some context-awareness is necessary, since a '/' inside a '[]' set does not end the expression.

  function readRegexp() {
    var content = "", escaped, inClass, start = tokPos;
    for (;;) {
      if (tokPos >= inputLen) raise(start, "Unterminated regular expression");
      var ch = input.charAt(tokPos);
      if (newline.test(ch)) raise(start, "Unterminated regular expression");
      if (!escaped) {
        if (ch === "[") inClass = true;
        else if (ch === "]" && inClass) inClass = false;
        else if (ch === "/" && !inClass) break;
        escaped = ch === "\\";
      } else escaped = false;
      ++tokPos;
    }
    var content = input.slice(start, tokPos);
    ++tokPos;

Need to use readWord1 because '\uXXXX' sequences are allowed here (don't ask).

    var mods = readWord1();
    if (mods && !/^[gmsiy]*$/.test(mods)) raise(start, "Invalid regular expression flag");
    try {
      var value = new RegExp(content, mods);
    } catch (e) {
      if (e instanceof SyntaxError) raise(start, "Error parsing regular expression: " + e.message);
      raise(e);
    }
    return finishToken(_regexp, value);
  }

Read an integer in the given radix. Return null if zero digits were read, the integer value otherwise. When len is given, this will return null unless the integer has exactly len digits.

  function readInt(radix, len) {
    var start = tokPos, total = 0;
    for (var i = 0, e = len == null ? Infinity : len; i < e; ++i) {
      var code = input.charCodeAt(tokPos), val;
      if (code >= 97) val = code - 97 + 10; // a
      else if (code >= 65) val = code - 65 + 10; // A
      else if (code >= 48 && code <= 57) val = code - 48; // 0-9
      else val = Infinity;
      if (val >= radix) break;
      ++tokPos;
      total = total * radix + val;
    }
    if (tokPos === start || len != null && tokPos - start !== len) return null;

    return total;
  }

  function readRadixNumber(radix) {
    tokPos += 2; // 0x
    var val = readInt(radix);
    if (val == null) raise(tokStart + 2, "Expected number in radix " + radix);
    if (isIdentifierStart(input.charCodeAt(tokPos))) raise(tokPos, "Identifier directly after number");
    return finishToken(_num, val);
  }

Read an integer, octal integer, or floating-point number.

  function readNumber(startsWithDot) {
    var start = tokPos, isFloat = false, octal = input.charCodeAt(tokPos) === 48;
    if (!startsWithDot && readInt(10) === null) raise(start, "Invalid number");
    if (input.charCodeAt(tokPos) === 46) {
      ++tokPos;
      readInt(10);
      isFloat = true;
    }
    var next = input.charCodeAt(tokPos);
    if (next === 69 || next === 101) { // 'eE'
      next = input.charCodeAt(++tokPos);
      if (next === 43 || next === 45) ++tokPos; // '+-'
      if (readInt(10) === null) raise(start, "Invalid number");
      isFloat = true;
    }
    if (isIdentifierStart(input.charCodeAt(tokPos))) raise(tokPos, "Identifier directly after number");

    var str = input.slice(start, tokPos), val;
    if (isFloat) val = parseFloat(str);
    else if (!octal || str.length === 1) val = parseInt(str, 10);
    else if (/[89]/.test(str) || strict) raise(start, "Invalid number");
    else val = parseInt(str, 8);
    return finishToken(_num, val);
  }

Read a string value, interpreting backslash-escapes.

  function readCodePoint() {
    var ch = input.charCodeAt(tokPos), code;

    if (ch === 123) {
      if (options.ecmaVersion < 6) unexpected();
      ++tokPos;
      code = readHexChar(input.indexOf('}', tokPos) - tokPos);
      ++tokPos;
      if (code > 0x10FFFF) unexpected();
    } else {
      code = readHexChar(4);
    }

UTF-16 Encoding

    if (code <= 0xFFFF) {
      return String.fromCharCode(code);
    }
    var cu1 = ((code - 0x10000) >> 10) + 0xD800;
    var cu2 = ((code - 0x10000) & 1023) + 0xDC00;
    return String.fromCharCode(cu1, cu2);
  }

  function readString(quote) {
    ++tokPos;
    var out = "";
    for (;;) {
      if (tokPos >= inputLen) raise(tokStart, "Unterminated string constant");
      var ch = input.charCodeAt(tokPos);
      if (ch === quote) {
        ++tokPos;
        return finishToken(_string, out);
      }
      if (ch === 92) { // '\'
        out += readEscapedChar();
      } else {
        ++tokPos;
        if (newline.test(String.fromCharCode(ch))) {
          raise(tokStart, "Unterminated string constant");
        }
        out += String.fromCharCode(ch); // '\'
      }
    }
  }

  function readTmplString() {
    var out = "";
    for (;;) {
      if (tokPos >= inputLen) raise(tokStart, "Unterminated string constant");
      var ch = input.charCodeAt(tokPos);
      if (ch === 96 || ch === 36 && input.charCodeAt(tokPos + 1) === 123) // '`', '${'
        return finishToken(_string, out);
      if (ch === 92) { // '\'
        out += readEscapedChar();
      } else {
        ++tokPos;
        if (newline.test(String.fromCharCode(ch))) {
          if (ch === 13 && input.charCodeAt(tokPos) === 10) {
            ++tokPos;
            ch = 10;
          }
          if (options.locations) {
            ++tokCurLine;
            tokLineStart = tokPos;
          }
        }
        out += String.fromCharCode(ch); // '\'
      }
    }
  }

Used to read escaped characters

  function readEscapedChar() {
    var ch = input.charCodeAt(++tokPos);
    var octal = /^[0-7]+/.exec(input.slice(tokPos, tokPos + 3));
    if (octal) octal = octal[0];
    while (octal && parseInt(octal, 8) > 255) octal = octal.slice(0, -1);
    if (octal === "0") octal = null;
    ++tokPos;
    if (octal) {
      if (strict) raise(tokPos - 2, "Octal literal in strict mode");
      tokPos += octal.length - 1;
      return String.fromCharCode(parseInt(octal, 8));
    } else {
      switch (ch) {
        case 110: return "\n"; // 'n' -> '\n'
        case 114: return "\r"; // 'r' -> '\r'
        case 120: return String.fromCharCode(readHexChar(2)); // 'x'
        case 117: return readCodePoint(); // 'u'
        case 85: return String.fromCharCode(readHexChar(8)); // 'U'
        case 116: return "\t"; // 't' -> '\t'
        case 98: return "\b"; // 'b' -> '\b'
        case 118: return "\u000b"; // 'v' -> '\u000b'
        case 102: return "\f"; // 'f' -> '\f'
        case 48: return "\0"; // 0 -> '\0'
        case 13: if (input.charCodeAt(tokPos) === 10) ++tokPos; // '\r\n'
        case 10: // ' \n'
          if (options.locations) { tokLineStart = tokPos; ++tokCurLine; }
          return "";
        default: return String.fromCharCode(ch);
      }
    }
  }

Used to read character escape sequences ('\x', '\u', '\U').

  function readHexChar(len) {
    var n = readInt(16, len);
    if (n === null) raise(tokStart, "Bad character escape sequence");
    return n;
  }

Used to signal to callers of readWord1 whether the word contained any escape sequences. This is needed because words with escape sequences must not be interpreted as keywords.

  var containsEsc;

Read an identifier, and return it as a string. Sets containsEsc to whether the word contained a '\u' escape.

Only builds up the word character-by-character when it actually containeds an escape, as a micro-optimization.

  function readWord1() {
    containsEsc = false;
    var word, first = true, start = tokPos;
    for (;;) {
      var ch = input.charCodeAt(tokPos);
      if (isIdentifierChar(ch)) {
        if (containsEsc) word += input.charAt(tokPos);
        ++tokPos;
      } else if (ch === 92) { // "\"
        if (!containsEsc) word = input.slice(start, tokPos);
        containsEsc = true;
        if (input.charCodeAt(++tokPos) != 117) // "u"
          raise(tokPos, "Expecting Unicode escape sequence \\uXXXX");
        ++tokPos;
        var esc = readHexChar(4);
        var escStr = String.fromCharCode(esc);
        if (!escStr) raise(tokPos - 1, "Invalid Unicode escape");
        if (!(first ? isIdentifierStart(esc) : isIdentifierChar(esc)))
          raise(tokPos - 4, "Invalid Unicode escape");
        word += escStr;
      } else {
        break;
      }
      first = false;
    }
    return containsEsc ? word : input.slice(start, tokPos);
  }

Read an identifier or keyword token. Will check for reserved words when necessary.

  function readWord() {
    var word = readWord1();
    var type = _name;
    if (!containsEsc && isKeyword(word))
      type = keywordTypes[word];
    return finishToken(type, word);
  }

Parser

A recursive descent parser operates by defining functions for all syntactic elements, and recursively calling those, each function advancing the input stream and returning an AST node. Precedence of constructs (for example, the fact that !x[1] means !(x[1]) instead of (!x)[1] is handled by the fact that the parser function that parses unary prefix operators is called first, and in turn calls the function that parses [] subscripts — that way, it'll receive the node for x[1] already parsed, and wraps that in the unary operator node.

Acorn uses an operator precedence parser to handle binary operator precedence, because it is much more compact than using the technique outlined above, which uses different, nesting functions to specify precedence, for all of the ten binary precedence levels that JavaScript defines.

Parser utilities

Continue to the next token.

  function next() {
    lastStart = tokStart;
    lastEnd = tokEnd;
    lastEndLoc = tokEndLoc;
    readToken();
  }

Enter strict mode. Re-reads the next token to please pedantic tests ("use strict"; 010; -- should fail).

  function setStrict(strct) {
    strict = strct;
    tokPos = tokStart;
    if (options.locations) {
      while (tokPos < tokLineStart) {
        tokLineStart = input.lastIndexOf("\n", tokLineStart - 2) + 1;
        --tokCurLine;
      }
    }
    skipSpace();
    readToken();
  }

Start an AST node, attaching a start offset.

  function Node() {
    this.type = null;
    this.start = tokStart;
    this.end = null;
  }

  exports.Node = Node;

  function SourceLocation() {
    this.start = tokStartLoc;
    this.end = null;
    if (sourceFile !== null) this.source = sourceFile;
  }

  function startNode() {
    var node = new Node();
    if (options.locations)
      node.loc = new SourceLocation();
    if (options.directSourceFile)
      node.sourceFile = options.directSourceFile;
    if (options.ranges)
      node.range = [tokStart, 0];
    return node;
  }

Start a node whose start offset information should be based on the start of another node. For example, a binary operator node is only started after its left-hand side has already been parsed.

  function startNodeFrom(other) {
    var node = new Node();
    node.start = other.start;
    if (options.locations) {
      node.loc = new SourceLocation();
      node.loc.start = other.loc.start;
    }
    if (options.ranges)
      node.range = [other.range[0], 0];

    return node;
  }

Finish an AST node, adding type and end properties.

  function finishNode(node, type) {
    node.type = type;
    node.end = lastEnd;
    if (options.locations)
      node.loc.end = lastEndLoc;
    if (options.ranges)
      node.range[1] = lastEnd;
    return node;
  }

Test whether a statement node is the string literal "use strict".

  function isUseStrict(stmt) {
    return options.ecmaVersion >= 5 && stmt.type === "ExpressionStatement" &&
      stmt.expression.type === "Literal" && stmt.expression.value === "use strict";
  }

Predicate that tests whether the next token is of the given type, and if yes, consumes it as a side effect.

  function eat(type) {
    if (tokType === type) {
      next();
      return true;
    } else {
      return false;
    }
  }

Test whether a semicolon can be inserted at the current position.

  function canInsertSemicolon() {
    return !options.strictSemicolons &&
      (tokType === _eof || tokType === _braceR || newline.test(input.slice(lastEnd, tokStart)));
  }

Consume a semicolon, or, failing that, see if we are allowed to pretend that there is a semicolon at this position.

  function semicolon() {
    if (!eat(_semi) && !canInsertSemicolon()) unexpected();
  }

Expect a token of a given type. If found, consume it, otherwise, raise an unexpected token error.

  function expect(type) {
    eat(type) || unexpected();
  }

Raise an unexpected token error.

  function unexpected(pos) {
    raise(pos != null ? pos : tokStart, "Unexpected token");
  }

Checks if hash object has a property.

  function has(obj, propName) {
    return Object.prototype.hasOwnProperty.call(obj, propName);
  }

Convert existing expression atom to assignable pattern if possible.

  function toAssignable(node, allowSpread, checkType) {
    if (options.ecmaVersion >= 6 && node) {
      switch (node.type) {
        case "Identifier":
        case "MemberExpression":
          break;

        case "ObjectExpression":
          node.type = "ObjectPattern";
          for (var i = 0; i < node.properties.length; i++) {
            var prop = node.properties[i];
            if (prop.kind !== "init") unexpected(prop.key.start);
            toAssignable(prop.value, false, checkType);
          }
          break;

        case "ArrayExpression":
          node.type = "ArrayPattern";
          for (var i = 0, lastI = node.elements.length - 1; i <= lastI; i++) {
            toAssignable(node.elements[i], i === lastI, checkType);
          }
          break;

        case "SpreadElement":
          if (allowSpread) {
            toAssignable(node.argument, false, checkType);
            checkSpreadAssign(node.argument);
          } else {
            unexpected(node.start);
          }
          break;

        default:
          if (checkType) unexpected(node.start);
      }
    }
    return node;
  }

Checks if node can be assignable spread argument.

  function checkSpreadAssign(node) {
    if (node.type !== "Identifier" && node.type !== "ArrayPattern")
      unexpected(node.start);
  }

Verify that argument names are not repeated, and it does not try to bind the words eval or arguments.

  function checkFunctionParam(param, nameHash) {
    switch (param.type) {
      case "Identifier":
        if (isStrictReservedWord(param.name) || isStrictBadIdWord(param.name))
          raise(param.start, "Defining '" + param.name + "' in strict mode");
        if (has(nameHash, param.name))
          raise(param.start, "Argument name clash in strict mode");
        nameHash[param.name] = true;
        break;

      case "ObjectPattern":
        for (var i = 0; i < param.properties.length; i++)
          checkFunctionParam(param.properties[i].value, nameHash);
        break;

      case "ArrayPattern":
        for (var i = 0; i < param.elements.length; i++)
          checkFunctionParam(param.elements[i], nameHash);
        break;
    }
  }

Check if property name clashes with already added. Object/class getters and setters are not allowed to clash — either with each other or with an init property — and in strict mode, init properties are also not allowed to be repeated.

  function checkPropClash(prop, propHash) {
    if (prop.computed) return;
    var key = prop.key, name;
    switch (key.type) {
      case "Identifier": name = key.name; break;
      case "Literal": name = String(key.value); break;
      default: return;
    }
    var kind = prop.kind || "init", other;
    if (has(propHash, name)) {
      other = propHash[name];
      var isGetSet = kind !== "init";
      if ((strict || isGetSet) && other[kind] || !(isGetSet ^ other.init))
        raise(key.start, "Redefinition of property");
    } else {
      other = propHash[name] = {
        init: false,
        get: false,
        set: false
      };
    }
    other[kind] = true;
  }

Verify that a node is an lval — something that can be assigned to.

  function checkLVal(expr, isBinding) {
    switch (expr.type) {
      case "Identifier":
        if (strict && (isStrictBadIdWord(expr.name) || isStrictReservedWord(expr.name)))
          raise(expr.start, isBinding
            ? "Binding " + expr.name + " in strict mode"
            : "Assigning to " + expr.name + " in strict mode"
          );
        break;

      case "MemberExpression":
        if (!isBinding) break;

      case "ObjectPattern":
        for (var i = 0; i < expr.properties.length; i++)
          checkLVal(expr.properties[i].value, isBinding);
        break;

      case "ArrayPattern":
        for (var i = 0; i < expr.elements.length; i++) {
          var elem = expr.elements[i];
          if (elem) checkLVal(elem, isBinding);
        }
        break;

      case "SpreadElement":
        break;

      default:
        raise(expr.start, "Assigning to rvalue");
    }
  }

Statement parsing

Parse a program. Initializes the parser, reads any number of statements, and wraps them in a Program node. Optionally takes a program argument. If present, the statements will be appended to its body instead of creating a new node.

  function parseTopLevel(program) {
    lastStart = lastEnd = tokPos;
    if (options.locations) lastEndLoc = new Position;
    inFunction = inGenerator = strict = null;
    labels = [];
    readToken();

    var node = program || startNode(), first = true;
    if (!program) node.body = [];
    while (tokType !== _eof) {
      var stmt = parseStatement();
      node.body.push(stmt);
      if (first && isUseStrict(stmt)) setStrict(true);
      first = false;
    }
    return finishNode(node, "Program");
  }

  var loopLabel = {kind: "loop"}, switchLabel = {kind: "switch"};

Parse a single statement.

If expecting a statement and finding a slash operator, parse a regular expression literal. This is to handle cases like if (foo) /blah/.exec(foo);, where looking at the previous token does not help.

  function parseStatement() {
    if (tokType === _slash || tokType === _assign && tokVal == "/=")
      readToken(true);

    var starttype = tokType, node = startNode();

Most types of statements are recognized by the keyword they start with. Many are trivial to parse, some require a bit of complexity.

    switch (starttype) {
    case _break: case _continue: return parseBreakContinueStatement(node, starttype.keyword);
    case _debugger: return parseDebuggerStatement(node);
    case _do: return parseDoStatement(node);
    case _for: return parseForStatement(node);
    case _function: return parseFunctionStatement(node);
    case _class: return parseClass(node, true);
    case _if: return parseIfStatement(node);
    case _return: return parseReturnStatement(node);
    case _switch: return parseSwitchStatement(node);
    case _throw: return parseThrowStatement(node);
    case _try: return parseTryStatement(node);
    case _var: case _let: case _const: return parseVarStatement(node, starttype.keyword);
    case _while: return parseWhileStatement(node);
    case _with: return parseWithStatement(node);
    case _braceL: return parseBlock(); // no point creating a function for this
    case _semi: return parseEmptyStatement(node);
    case _export: return parseExport(node);
    case _import: return parseImport(node);

If the statement does not start with a statement keyword or a brace, it's an ExpressionStatement or LabeledStatement. We simply start parsing an expression, and afterwards, if the next token is a colon and the expression was a simple Identifier node, we switch to interpreting it as a label.

    default:
      var maybeName = tokVal, expr = parseExpression();
      if (starttype === _name && expr.type === "Identifier" && eat(_colon))
        return parseLabeledStatement(node, maybeName, expr);
      else return parseExpressionStatement(node, expr);
    }
  }

  function parseBreakContinueStatement(node, keyword) {
    var isBreak = keyword == "break";
    next();
    if (eat(_semi) || canInsertSemicolon()) node.label = null;
    else if (tokType !== _name) unexpected();
    else {
      node.label = parseIdent();
      semicolon();
    }

Verify that there is an actual destination to break or continue to.

    for (var i = 0; i < labels.length; ++i) {
      var lab = labels[i];
      if (node.label == null || lab.name === node.label.name) {
        if (lab.kind != null && (isBreak || lab.kind === "loop")) break;
        if (node.label && isBreak) break;
      }
    }
    if (i === labels.length) raise(node.start, "Unsyntactic " + keyword);
    return finishNode(node, isBreak ? "BreakStatement" : "ContinueStatement");
  }

  function parseDebuggerStatement(node) {
    next();
    semicolon();
    return finishNode(node, "DebuggerStatement");
  }

  function parseDoStatement(node) {
    next();
    labels.push(loopLabel);
    node.body = parseStatement();
    labels.pop();
    expect(_while);
    node.test = parseParenExpression();
    semicolon();
    return finishNode(node, "DoWhileStatement");
  }

Disambiguating between a for and a for/in or for/of loop is non-trivial. Basically, we have to parse the init var statement or expression, disallowing the in operator (see the second parameter to parseExpression), and then check whether the next token is in or of. When there is no init part (semicolon immediately after the opening parenthesis), it is a regular for loop.

  function parseForStatement(node) {
    next();
    labels.push(loopLabel);
    expect(_parenL);
    if (tokType === _semi) return parseFor(node, null);
    if (tokType === _var || tokType === _let) {
      var init = startNode(), varKind = tokType.keyword, isLet = tokType === _let;
      next();
      parseVar(init, true, varKind);
      finishNode(init, "VariableDeclaration");
      if ((tokType === _in || (tokType === _name && tokVal === "of")) && init.declarations.length === 1 &&
          !(isLet && init.declarations[0].init))
        return parseForIn(node, init);
      return parseFor(node, init);
    }
    var init = parseExpression(false, true);
    if (tokType === _in || (tokType === _name && tokVal === "of")) {
      checkLVal(init);
      return parseForIn(node, init);
    }
    return parseFor(node, init);
  }

  function parseFunctionStatement(node) {
    next();
    return parseFunction(node, true);
  }

  function parseIfStatement(node) {
    next();
    node.test = parseParenExpression();
    node.consequent = parseStatement();
    node.alternate = eat(_else) ? parseStatement() : null;
    return finishNode(node, "IfStatement");
  }

  function parseReturnStatement(node) {
    if (!inFunction && !options.allowReturnOutsideFunction)
      raise(tokStart, "'return' outside of function");
    next();

In return (and break/continue), the keywords with optional arguments, we eagerly look for a semicolon or the possibility to insert one.

    if (eat(_semi) || canInsertSemicolon()) node.argument = null;
    else { node.argument = parseExpression(); semicolon(); }
    return finishNode(node, "ReturnStatement");
  }

  function parseSwitchStatement(node) {
    next();
    node.discriminant = parseParenExpression();
    node.cases = [];
    expect(_braceL);
    labels.push(switchLabel);

Statements under must be grouped (by label) in SwitchCase nodes. cur is used to keep the node that we are currently adding statements to.

    for (var cur, sawDefault; tokType != _braceR;) {
      if (tokType === _case || tokType === _default) {
        var isCase = tokType === _case;
        if (cur) finishNode(cur, "SwitchCase");
        node.cases.push(cur = startNode());
        cur.consequent = [];
        next();
        if (isCase) cur.test = parseExpression();
        else {
          if (sawDefault) raise(lastStart, "Multiple default clauses"); sawDefault = true;
          cur.test = null;
        }
        expect(_colon);
      } else {
        if (!cur) unexpected();
        cur.consequent.push(parseStatement());
      }
    }
    if (cur) finishNode(cur, "SwitchCase");
    next(); // Closing brace
    labels.pop();
    return finishNode(node, "SwitchStatement");
  }

  function parseThrowStatement(node) {
    next();
    if (newline.test(input.slice(lastEnd, tokStart)))
      raise(lastEnd, "Illegal newline after throw");
    node.argument = parseExpression();
    semicolon();
    return finishNode(node, "ThrowStatement");
  }

  function parseTryStatement(node) {
    next();
    node.block = parseBlock();
    node.handler = null;
    if (tokType === _catch) {
      var clause = startNode();
      next();
      expect(_parenL);
      clause.param = parseIdent();
      if (strict && isStrictBadIdWord(clause.param.name))
        raise(clause.param.start, "Binding " + clause.param.name + " in strict mode");
      expect(_parenR);
      clause.guard = null;
      clause.body = parseBlock();
      node.handler = finishNode(clause, "CatchClause");
    }
    node.guardedHandlers = empty;
    node.finalizer = eat(_finally) ? parseBlock() : null;
    if (!node.handler && !node.finalizer)
      raise(node.start, "Missing catch or finally clause");
    return finishNode(node, "TryStatement");
  }

  function parseVarStatement(node, kind) {
    next();
    parseVar(node, false, kind);
    semicolon();
    return finishNode(node, "VariableDeclaration");
  }

  function parseWhileStatement(node) {
    next();
    node.test = parseParenExpression();
    labels.push(loopLabel);
    node.body = parseStatement();
    labels.pop();
    return finishNode(node, "WhileStatement");
  }

  function parseWithStatement(node) {
    if (strict) raise(tokStart, "'with' in strict mode");
    next();
    node.object = parseParenExpression();
    node.body = parseStatement();
    return finishNode(node, "WithStatement");
  }

  function parseEmptyStatement(node) {
    next();
    return finishNode(node, "EmptyStatement");
  }

  function parseLabeledStatement(node, maybeName, expr) {
    for (var i = 0; i < labels.length; ++i)
      if (labels[i].name === maybeName) raise(expr.start, "Label '" + maybeName + "' is already declared");
    var kind = tokType.isLoop ? "loop" : tokType === _switch ? "switch" : null;
    labels.push({name: maybeName, kind: kind});
    node.body = parseStatement();
    labels.pop();
    node.label = expr;
    return finishNode(node, "LabeledStatement");
  }

  function parseExpressionStatement(node, expr) {
    node.expression = expr;
    semicolon();
    return finishNode(node, "ExpressionStatement");
  }

Used for constructs like switch and if that insist on parentheses around their expression.

  function parseParenExpression() {
    expect(_parenL);
    var val = parseExpression();
    expect(_parenR);
    return val;
  }

Parse a semicolon-enclosed block of statements, handling "use strict" declarations when allowStrict is true (used for function bodies).

  function parseBlock(allowStrict) {
    var node = startNode(), first = true, strict = false, oldStrict;
    node.body = [];
    expect(_braceL);
    while (!eat(_braceR)) {
      var stmt = parseStatement();
      node.body.push(stmt);
      if (first && allowStrict && isUseStrict(stmt)) {
        oldStrict = strict;
        setStrict(strict = true);
      }
      first = false;
    }
    if (strict && !oldStrict) setStrict(false);
    return finishNode(node, "BlockStatement");
  }

Parse a regular for loop. The disambiguation code in parseStatement will already have parsed the init statement or expression.

  function parseFor(node, init) {
    node.init = init;
    expect(_semi);
    node.test = tokType === _semi ? null : parseExpression();
    expect(_semi);
    node.update = tokType === _parenR ? null : parseExpression();
    expect(_parenR);
    node.body = parseStatement();
    labels.pop();
    return finishNode(node, "ForStatement");
  }

Parse a for/in and for/of loop, which are almost same from parser's perspective.

  function parseForIn(node, init) {
    var type = tokType === _in ? "ForInStatement" : "ForOfStatement";
    next();
    node.left = init;
    node.right = parseExpression();
    expect(_parenR);
    node.body = parseStatement();
    labels.pop();
    return finishNode(node, type);
  }

Parse a list of variable declarations.

  function parseVar(node, noIn, kind) {
    node.declarations = [];
    node.kind = kind;
    for (;;) {
      var decl = startNode();
      decl.id = options.ecmaVersion >= 6 ? toAssignable(parseExprAtom()) : parseIdent();
      checkLVal(decl.id, true);
      decl.init = eat(_eq) ? parseExpression(true, noIn) : (kind === _const.keyword ? unexpected() : null);
      node.declarations.push(finishNode(decl, "VariableDeclarator"));
      if (!eat(_comma)) break;
    }
    return node;
  }

Expression parsing

These nest, from the most general expression type at the top to 'atomic', nondivisible expression types at the bottom. Most of the functions will simply let the function(s) below them parse, and, if the syntactic construct they handle is present, wrap the AST node that the inner parser gave them in another node.

Parse a full expression. The arguments are used to forbid comma sequences (in argument lists, array literals, or object literals) or the in operator (in for loops initalization expressions).

  function parseExpression(noComma, noIn) {
    var expr = parseMaybeAssign(noIn);
    if (!noComma && tokType === _comma) {
      var node = startNodeFrom(expr);
      node.expressions = [expr];
      while (eat(_comma)) node.expressions.push(parseMaybeAssign(noIn));
      return finishNode(node, "SequenceExpression");
    }
    return expr;
  }

Parse an assignment expression. This includes applications of operators like +=.

  function parseMaybeAssign(noIn) {
    var left = parseMaybeConditional(noIn);
    if (tokType.isAssign) {
      var node = startNodeFrom(left);
      node.operator = tokVal;
      node.left = tokType === _eq ? toAssignable(left) : left;
      checkLVal(left);
      next();
      node.right = parseMaybeAssign(noIn);
      return finishNode(node, "AssignmentExpression");
    }
    return left;
  }

Parse a ternary conditional (?:) operator.

  function parseMaybeConditional(noIn) {
    var expr = parseExprOps(noIn);
    if (eat(_question)) {
      var node = startNodeFrom(expr);
      node.test = expr;
      node.consequent = parseExpression(true);
      expect(_colon);
      node.alternate = parseExpression(true, noIn);
      return finishNode(node, "ConditionalExpression");
    }
    return expr;
  }

Start the precedence parser.

  function parseExprOps(noIn) {
    return parseExprOp(parseMaybeUnary(), -1, noIn);
  }

Parse binary operators with the operator precedence parsing algorithm. left is the left-hand side of the operator. minPrec provides context that allows the function to stop and defer further parser to one of its callers when it encounters an operator that has a lower precedence than the set it is parsing.

  function parseExprOp(left, minPrec, noIn) {
    var prec = tokType.binop;
    if (prec != null && (!noIn || tokType !== _in)) {
      if (prec > minPrec) {
        var node = startNodeFrom(left);
        node.left = left;
        node.operator = tokVal;
        var op = tokType;
        next();
        node.right = parseExprOp(parseMaybeUnary(), prec, noIn);
        var exprNode = finishNode(node, (op === _logicalOR || op === _logicalAND) ? "LogicalExpression" : "BinaryExpression");
        return parseExprOp(exprNode, minPrec, noIn);
      }
    }
    return left;
  }

Parse unary operators, both prefix and postfix.

  function parseMaybeUnary() {
    if (tokType.prefix) {
      var node = startNode(), update = tokType.isUpdate;
      node.operator = tokVal;
      node.prefix = true;
      tokRegexpAllowed = true;
      next();
      node.argument = parseMaybeUnary();
      if (update) checkLVal(node.argument);
      else if (strict && node.operator === "delete" &&
               node.argument.type === "Identifier")
        raise(node.start, "Deleting local variable in strict mode");
      return finishNode(node, update ? "UpdateExpression" : "UnaryExpression");
    }
    var expr = parseExprSubscripts();
    while (tokType.postfix && !canInsertSemicolon()) {
      var node = startNodeFrom(expr);
      node.operator = tokVal;
      node.prefix = false;
      node.argument = expr;
      checkLVal(expr);
      next();
      expr = finishNode(node, "UpdateExpression");
    }
    return expr;
  }

Parse call, dot, and []-subscript expressions.

  function parseExprSubscripts() {
    return parseSubscripts(parseExprAtom());
  }

  function parseSubscripts(base, noCalls) {
    if (eat(_dot)) {
      var node = startNodeFrom(base);
      node.object = base;
      node.property = parseIdent(true);
      node.computed = false;
      return parseSubscripts(finishNode(node, "MemberExpression"), noCalls);
    } else if (eat(_bracketL)) {
      var node = startNodeFrom(base);
      node.object = base;
      node.property = parseExpression();
      node.computed = true;
      expect(_bracketR);
      return parseSubscripts(finishNode(node, "MemberExpression"), noCalls);
    } else if (!noCalls && eat(_parenL)) {
      var node = startNodeFrom(base);
      node.callee = base;
      node.arguments = parseExprList(_parenR, false);
      return parseSubscripts(finishNode(node, "CallExpression"), noCalls);
    } else if (tokType === _bquote) {
      var node = startNodeFrom(base);
      node.tag = base;
      node.quasi = parseTemplate();
      return parseSubscripts(finishNode(node, "TaggedTemplateExpression"), noCalls);
    } return base;
  }

Parse an atomic expression — either a single token that is an expression, an expression started by a keyword like function or new, or an expression wrapped in punctuation like (), [], or {}.

  function parseExprAtom() {
    switch (tokType) {
    case _this:
      var node = startNode();
      next();
      return finishNode(node, "ThisExpression");

    case _yield:
      if (inGenerator) return parseYield();

    case _name:
      var id = parseIdent(tokType !== _name);
      if (eat(_arrow)) {
        return parseArrowExpression(startNodeFrom(id), [id]);
      }
      return id;

    case _num: case _string: case _regexp:
      var node = startNode();
      node.value = tokVal;
      node.raw = input.slice(tokStart, tokEnd);
      next();
      return finishNode(node, "Literal");

    case _null: case _true: case _false:
      var node = startNode();
      node.value = tokType.atomValue;
      node.raw = tokType.keyword;
      next();
      return finishNode(node, "Literal");

    case _parenL:
      var tokStartLoc1 = tokStartLoc, tokStart1 = tokStart, val, exprList;
      next();

check whether this is generator comprehension or regular expression

      if (options.ecmaVersion >= 6 && tokType === _for) {
        val = parseComprehension(startNode(), true);
      } else {
        var oldParenL = ++metParenL;
        if (tokType !== _parenR) {
          val = parseExpression();
          exprList = val.type === "SequenceExpression" ? val.expressions : [val];
        } else {
          exprList = [];
        }
        expect(_parenR);

if '=>' follows '(...)', convert contents to arguments

        if (metParenL === oldParenL && eat(_arrow)) {
          val = parseArrowExpression(startNode(), exprList);
        } else {

forbid '()' before everything but '=>'

          if (!val) unexpected(lastStart);

forbid '...' in sequence expressions

          if (options.ecmaVersion >= 6) {
            for (var i = 0; i < exprList.length; i++) {
              if (exprList[i].type === "SpreadElement") unexpected();
            }
          }
        }
      }
      val.start = tokStart1;
      val.end = lastEnd;
      if (options.locations) {
        val.loc.start = tokStartLoc1;
        val.loc.end = lastEndLoc;
      }
      if (options.ranges) {
        val.range = [tokStart1, lastEnd];
      }
      return val;

    case _bracketL:
      var node = startNode();
      next();

check whether this is array comprehension or regular array

      if (options.ecmaVersion >= 6 && tokType === _for) {
        return parseComprehension(node, false);
      }
      node.elements = parseExprList(_bracketR, true, true);
      return finishNode(node, "ArrayExpression");

    case _braceL:
      return parseObj();

    case _function:
      var node = startNode();
      next();
      return parseFunction(node, false);

    case _class:
      return parseClass(startNode(), false);

    case _new:
      return parseNew();

    case _ellipsis:
      return parseSpread();

    case _bquote:
      return parseTemplate();

    default:
      unexpected();
    }
  }

New's precedence is slightly tricky. It must allow its argument to be a [] or dot subscript expression, but not a call — at least, not without wrapping it in parentheses. Thus, it uses the

  function parseNew() {
    var node = startNode();
    next();
    node.callee = parseSubscripts(parseExprAtom(), true);
    if (eat(_parenL)) node.arguments = parseExprList(_parenR, false);
    else node.arguments = empty;
    return finishNode(node, "NewExpression");
  }

Parse spread element '...expr'

  function parseSpread() {
    var node = startNode();
    next();
    node.argument = parseExpression(true);
    return finishNode(node, "SpreadElement");
  }

Parse template expression.

  function parseTemplate() {
    var node = startNode();
    node.expressions = [];
    node.quasis = [];
    inTemplate = true;
    next();
    for (;;) {
      var elem = startNode();
      elem.value = {cooked: tokVal, raw: input.slice(tokStart, tokEnd)};
      elem.tail = false;
      next();
      node.quasis.push(finishNode(elem, "TemplateElement"));
      if (eat(_bquote)) { // '`', end of template
        elem.tail = true;
        break;
      }
      inTemplate = false;
      expect(_dollarBraceL);
      node.expressions.push(parseExpression());
      inTemplate = true;
      expect(_braceR);
    }
    inTemplate = false;
    return finishNode(node, "TemplateLiteral");
  }

Parse an object literal.

  function parseObj() {
    var node = startNode(), first = true, propHash = {};
    node.properties = [];
    next();
    while (!eat(_braceR)) {
      if (!first) {
        expect(_comma);
        if (options.allowTrailingCommas && eat(_braceR)) break;
      } else first = false;

      var prop = startNode(), isGenerator;
      if (options.ecmaVersion >= 6) {
        prop.method = false;
        prop.shorthand = false;
        isGenerator = eat(_star);
      }
      parsePropertyName(prop);
      if (eat(_colon)) {
        prop.value = parseExpression(true);
        prop.kind = "init";
      } else if (options.ecmaVersion >= 6 && tokType === _parenL) {
        prop.kind = "init";
        prop.method = true;
        prop.value = parseMethod(isGenerator);
      } else if (options.ecmaVersion >= 5 && !prop.computed && prop.key.type === "Identifier" &&
                 (prop.key.name === "get" || prop.key.name === "set")) {
        if (isGenerator) unexpected();
        prop.kind = prop.key.name;
        parsePropertyName(prop);
        prop.value = parseMethod(false);
      } else if (options.ecmaVersion >= 6 && !prop.computed && prop.key.type === "Identifier") {
        prop.kind = "init";
        prop.value = prop.key;
        prop.shorthand = true;
      } else unexpected();

      checkPropClash(prop, propHash);
      node.properties.push(finishNode(prop, "Property"));
    }
    return finishNode(node, "ObjectExpression");
  }

  function parsePropertyName(prop) {
    if (options.ecmaVersion >= 6) {
      if (eat(_bracketL)) {
        prop.computed = true;
        prop.key = parseExpression();
        expect(_bracketR);
        return;
      } else {
        prop.computed = false;
      }
    }
    prop.key = (tokType === _num || tokType === _string) ? parseExprAtom() : parseIdent(true);
  }

Initialize empty function node.

  function initFunction(node) {
    node.id = null;
    node.params = [];
    if (options.ecmaVersion >= 6) {
      node.defaults = [];
      node.rest = null;
      node.generator = false;
    }
  }

Parse a function declaration or literal (depending on the isStatement parameter).

  function parseFunction(node, isStatement, allowExpressionBody) {
    initFunction(node);
    if (options.ecmaVersion >= 6) {
      node.generator = eat(_star);
    }
    if (isStatement || tokType === _name) {
      node.id = parseIdent();
    }
    parseFunctionParams(node);
    parseFunctionBody(node, allowExpressionBody);
    return finishNode(node, isStatement ? "FunctionDeclaration" : "FunctionExpression");
  }

Parse object or class method.

  function parseMethod(isGenerator) {
    var node = startNode();
    initFunction(node);
    parseFunctionParams(node);
    var allowExpressionBody;
    if (options.ecmaVersion >= 6) {
      node.generator = isGenerator;
      allowExpressionBody = true;
    } else {
      allowExpressionBody = false;
    }
    parseFunctionBody(node, allowExpressionBody);
    return finishNode(node, "FunctionExpression");
  }

Parse arrow function expression with given parameters.

  function parseArrowExpression(node, params) {
    initFunction(node);

    var defaults = node.defaults, hasDefaults = false;

    for (var i = 0, lastI = params.length - 1; i <= lastI; i++) {
      var param = params[i];

      if (param.type === "AssignmentExpression" && param.operator === "=") {
        hasDefaults = true;
        params[i] = param.left;
        defaults.push(param.right);
      } else {
        toAssignable(param, i === lastI, true);
        defaults.push(null);
        if (param.type === "SpreadElement") {
          params.length--;
          node.rest = param.argument;
          break;
        }
      }
    }

    node.params = params;
    if (!hasDefaults) node.defaults = [];

    parseFunctionBody(node, true);
    return finishNode(node, "ArrowFunctionExpression");
  }

Parse function parameters.

  function parseFunctionParams(node) {
    var defaults = [], hasDefaults = false;

    expect(_parenL);
    for (;;) {
      if (eat(_parenR)) {
        break;
      } else if (options.ecmaVersion >= 6 && eat(_ellipsis)) {
        node.rest = toAssignable(parseExprAtom(), false, true);
        checkSpreadAssign(node.rest);
        expect(_parenR);
        break;
      } else {
        node.params.push(options.ecmaVersion >= 6 ? toAssignable(parseExprAtom(), false, true) : parseIdent());
        if (options.ecmaVersion >= 6 && tokType === _eq) {
          next();
          hasDefaults = true;
          defaults.push(parseExpression(true));
        }
        if (!eat(_comma)) {
          expect(_parenR);
          break;
        }
      }
    }

    if (hasDefaults) node.defaults = defaults;
  }

Parse function body and check parameters.

  function parseFunctionBody(node, allowExpression) {
    var isExpression = allowExpression && tokType !== _braceL;

    if (isExpression) {
      node.body = parseExpression(true);
      node.expression = true;
    } else {

Start a new scope with regard to labels and the inFunction flag (restore them to their old value afterwards).

      var oldInFunc = inFunction, oldInGen = inGenerator, oldLabels = labels;
      inFunction = true; inGenerator = node.generator; labels = [];
      node.body = parseBlock(true);
      node.expression = false;
      inFunction = oldInFunc; inGenerator = oldInGen; labels = oldLabels;
    }

If this is a strict mode function, verify that argument names are not repeated, and it does not try to bind the words eval or arguments.

    if (strict || !isExpression && node.body.body.length && isUseStrict(node.body.body[0])) {
      var nameHash = {};
      if (node.id)
        checkFunctionParam(node.id, nameHash);
      for (var i = 0; i < node.params.length; i++)
        checkFunctionParam(node.params[i], nameHash);
      if (node.rest)
        checkFunctionParam(node.rest, nameHash);
    }
  }

Parse a class declaration or literal (depending on the isStatement parameter).

  function parseClass(node, isStatement) {
    next();
    node.id = tokType === _name ? parseIdent() : isStatement ? unexpected() : null;
    node.superClass = eat(_extends) ? parseExpression() : null;
    var classBody = startNode(), methodHash = {}, staticMethodHash = {};
    classBody.body = [];
    expect(_braceL);
    while (!eat(_braceR)) {
      var method = startNode();
      if (tokType === _name && tokVal === "static") {
        next();
        method['static'] = true;
      } else {
        method['static'] = false;
      }
      var isGenerator = eat(_star);
      parsePropertyName(method);
      if (tokType === _name && !method.computed && method.key.type === "Identifier" &&
          (method.key.name === "get" || method.key.name === "set")) {
        if (isGenerator) unexpected();
        method.kind = method.key.name;
        parsePropertyName(method);
      } else {
        method.kind = "";
      }
      method.value = parseMethod(isGenerator);
      checkPropClash(method, method['static'] ? staticMethodHash : methodHash);
      classBody.body.push(finishNode(method, "MethodDefinition"));
      eat(_semi);
    }
    node.body = finishNode(classBody, "ClassBody");
    return finishNode(node, isStatement ? "ClassDeclaration" : "ClassExpression");
  }

Parses a comma-separated list of expressions, and returns them as an array. close is the token type that ends the list, and allowEmpty can be turned on to allow subsequent commas with nothing in between them to be parsed as null (which is needed for array literals).

  function parseExprList(close, allowTrailingComma, allowEmpty) {
    var elts = [], first = true;
    while (!eat(close)) {
      if (!first) {
        expect(_comma);
        if (allowTrailingComma && options.allowTrailingCommas && eat(close)) break;
      } else first = false;

      if (allowEmpty && tokType === _comma) elts.push(null);
      else elts.push(parseExpression(true));
    }
    return elts;
  }

Parse the next token as an identifier. If liberal is true (used when parsing properties), it will also convert keywords into identifiers.

  function parseIdent(liberal) {
    var node = startNode();
    if (liberal && options.forbidReserved == "everywhere") liberal = false;
    if (tokType === _name) {
      if (!liberal &&
          (options.forbidReserved &&
           (options.ecmaVersion === 3 ? isReservedWord3 : isReservedWord5)(tokVal) ||
           strict && isStrictReservedWord(tokVal)) &&
          input.slice(tokStart, tokEnd).indexOf("\\") == -1)
        raise(tokStart, "The keyword '" + tokVal + "' is reserved");
      node.name = tokVal;
    } else if (liberal && tokType.keyword) {
      node.name = tokType.keyword;
    } else {
      unexpected();
    }
    tokRegexpAllowed = false;
    next();
    return finishNode(node, "Identifier");
  }

Parses module export declaration.

  function parseExport(node) {
    next();

export var|const|let|function|class ...;

    if (tokType === _var || tokType === _const || tokType === _let || tokType === _function || tokType === _class) {
      node.declaration = parseStatement();
      node['default'] = false;
      node.specifiers = null;
      node.source = null;
    } else

export default ...;

    if (eat(_default)) {
      node.declaration = parseExpression(true);
      node['default'] = true;
      node.specifiers = null;
      node.source = null;
      semicolon();
    } else {

export * from '...' export { x, y as z } [from '...']

      var isBatch = tokType === _star;
      node.declaration = null;
      node['default'] = false;
      node.specifiers = parseExportSpecifiers();
      if (tokType === _name && tokVal === "from") {
        next();
        node.source = tokType === _string ? parseExprAtom() : unexpected();
      } else {
        if (isBatch) unexpected();
        node.source = null;
      }
    }
    return finishNode(node, "ExportDeclaration");
  }

Parses a comma-separated list of module exports.

  function parseExportSpecifiers() {
    var nodes = [], first = true;
    if (tokType === _star) {

export * from '...'

      var node = startNode();
      next();
      nodes.push(finishNode(node, "ExportBatchSpecifier"));
    } else {

export { x, y as z } [from '...']

      expect(_braceL);
      while (!eat(_braceR)) {
        if (!first) {
          expect(_comma);
          if (options.allowTrailingCommas && eat(_braceR)) break;
        } else first = false;

        var node = startNode();
        node.id = parseIdent();
        if (tokType === _name && tokVal === "as") {
          next();
          node.name = parseIdent(true);
        } else {
          node.name = null;
        }
        nodes.push(finishNode(node, "ExportSpecifier"));
      }
    }
    return nodes;
  }

Parses import declaration.

  function parseImport(node) {
    next();

import '...';

    if (tokType === _string) {
      node.specifiers = [];
      node.source = parseExprAtom();
      node.kind = "";
    } else {
      node.specifiers = parseImportSpecifiers();
      if (tokType !== _name || tokVal !== "from") unexpected();
      next();
      node.source = tokType === _string ? parseExprAtom() : unexpected();

only for backward compatibility with Esprima's AST (it doesn't support mixed default + named yet)

      node.kind = node.specifiers[0]['default'] ? "default" : "named";
    }
    return finishNode(node, "ImportDeclaration");
  }

Parses a comma-separated list of module imports.

  function parseImportSpecifiers() {
    var nodes = [], first = true;
    if (tokType === _star) {
      var node = startNode();
      next();
      if (tokType !== _name || tokVal !== "as") unexpected();
      next();
      node.name = parseIdent();
      checkLVal(node.name, true);
      nodes.push(finishNode(node, "ImportBatchSpecifier"));
      return nodes;
    }
    if (tokType === _name) {

import defaultObj, { x, y as z } from '...'

      var node = startNode();
      node.id = parseIdent();
      checkLVal(node.id, true);
      node.name = null;
      node['default'] = true;
      nodes.push(finishNode(node, "ImportSpecifier"));
      if (!eat(_comma)) return nodes;
    }
    expect(_braceL);
    while (!eat(_braceR)) {
      if (!first) {
        expect(_comma);
        if (options.allowTrailingCommas && eat(_braceR)) break;
      } else first = false;

      var node = startNode();
      node.id = parseIdent(true);
      if (tokType === _name && tokVal === "as") {
        next();
        node.name = parseIdent();
      } else {
        node.name = null;
      }
      checkLVal(node.name || node.id, true);
      node['default'] = false;
      nodes.push(finishNode(node, "ImportSpecifier"));
    }
    return nodes;
  }

Parses yield expression inside generator.

  function parseYield() {
    var node = startNode();
    next();
    if (eat(_semi) || canInsertSemicolon()) {
      node.delegate = false;
      node.argument = null;
    } else {
      node.delegate = eat(_star);
      node.argument = parseExpression(true);
    }
    return finishNode(node, "YieldExpression");
  }

Parses array and generator comprehensions.

  function parseComprehension(node, isGenerator) {
    node.blocks = [];
    while (tokType === _for) {
      var block = startNode();
      next();
      expect(_parenL);
      block.left = toAssignable(parseExprAtom());
      checkLVal(block.left, true);
      if (tokType !== _name || tokVal !== "of") unexpected();
      next();

of property is here for compatibility with Esprima's AST which also supports deprecated [for (... in ...) expr]

      block.of = true;
      block.right = parseExpression();
      expect(_parenR);
      node.blocks.push(finishNode(block, "ComprehensionBlock"));
    }
    node.filter = eat(_if) ? parseParenExpression() : null;
    node.body = parseExpression();
    expect(isGenerator ? _parenR : _bracketR);
    node.generator = isGenerator;
    return finishNode(node, "ComprehensionExpression");
  }

});