JavaScript cheat sheet

Trivial expressions

A plain word refers to a variable in the current environment.
A quoted word is a string, a value containing piece of text.
A number value.
-12 1.5 3.4e10
Other ways to write numbers (negative, fractional, with exponent).
Boolean (yes/no) value. true for yes, false for no.

Operator expressions

a + b
Binary operator applied to two values. + to add, - to subtract, * to multiply, / to divide.
(a + b) * c
Parenthesis for explicit grouping.
a < b
Comparison operators ==, != (not equal), <, >, <= (less or equal), >=.
a = b
Assignment, set variable a to value b. Not to be confused with == comparison. a += b is a shorthand for a = a + b, also for -= etc.
a && b
Logical operators — && for AND, || for OR.
Unary (one-operand) operator. - to negate, ! for boolean negation.

Composite expressions

Subscript, fetch the field named by b from value a.
Shorthand for a["x"].
Function call. Call the function value a with b as argument. Zero or more argument expressions can be given, separated by spaces. a(1, 2, 3, 4)
Method call. Call the function found in field x of value a, and pass a as the this argument.
[1, 2, 3, 4]
Array value with zero or more elements.
{a: 1, b: 2}
Object value with zero or more name: value field definitions.
function(arg1, arg2) { /* ... body ... */ }
Function value. Zero or more argument names. Any statements may appear in body.


Any expression, followed by a semicolon, is a statement.
{a; b; c;}
A series of statements, wrapped in braces, form a composite statement.
var a = b;
Variable definition. The variable with name a is defined and given value b. Value is optional. var a; sets a to undefined.
function foo(arg1, arg2) { /* ... body ... */ }
Function definition. Defines variable foo to have a function value. Zero or more arguments, any statements may appear in body.
if (a) { /* ... */ } else { /* ... */ }
Conditional statement. If value a is true, the first statement, otherwise the else statement executes. Else part may be left off. Can be chained as in if (a) {} else if (b) {} else {}.
while (a) { /* ... */ }
A loop. The loop body statement will be executed as long as a produces a true value.
for (var a = 0; a < 10; a = a + 1) { /* ... */ }
Example for-loop statement. var a = 1 initializes the loop, a < 10 checks whether it has ended yet, and a = a + 1 moves to the next step.
return a;
Only valid inside a function. Returns value a as the result of the function call.

Useful functions

Converts v to a number. Number("5") gives 5.
Converts v to a string.
Show a dialog window saying 'hello'.
confirm("are you sure?")
Show a yes/no dialog. Returns a true/false value indicating whether the user clicked yes.
prompt("what is your name?", "")
Show a dialog asking for input. First argument is the message, second argument is the initial value of the input.

Useful string properties

The length (number of characters) of the string.
Get the character at position n. (Zero is the first character.)
"foo".slice(from, to)
Get a piece of the string. "012345".slice(1, 4) gives "123".
"a b c".split(" ")
Split the string on a character, producing an array of strings (["a", "b", "c"]).

Useful array properties

If i is an integer, this will access the element at that position.
The number of elements in the array.
Add value b to the end of the array.
Remove the last element of the array, and return it.
a.slice(from, to)
Get a piece of the array, similar to the slice method on strings.

Useful math properties

Produce a random number between 0 and 1.
Round x to an integer.
Returns the absolute (positive) value of x.
Math.max(a, b, c, ...) Math.min(a, b, c, ...)
Given any number of values, returns the greatest (max) or smallest (min) one.
The pi (π) constant.
Math.cos(x) Math.sin(x) Math.tan(x)
Trigonometric functions.
Math.acos(x) Math.asin(x) Math.atan(x)
Inverse trigonometric functions.