Marijn Haverbeke's blog (license)

JavaScript closure vs. object look-up performance

Thursday, September 13, 2012 javascript performance codemirror

Note: the question asked in this post, "why aren't closures super-fast?", was thorougly answered by Vyacheslav Egorov in his followup. Reading that is probably more informative than reading the text below.

I originally structured CodeMirror instances as one huge closure that contained all the internal variables. The constructor would create local variables for all internal state, and local functions for everything that needed access to that state, and then return an object that contained the API methods, which themselves also closed over all those internals. Something like this:

function CodeMirror(args) {
  // Internal state
  var doc = something, selection = somethingElse;
  // Internal functions
  function modifyDoc(from, to, newText) {
    /* directly access doc, selection, etc */
  function drawDoc() { /* ... */ }
  return {
    getLine: function(n) { return getLineFrom(doc, n); },
    refresh: drawDoc
    /* etc */

I had several reasons for doing it like this. Firstly, it minifies well—local variables are very easy and non-invasive to rename, so if most of your fields and functions are regular variables rather than object fields, simple minification works very well. There are tools like Google's Closure compiler in 'advanced' mode, which do try to rename properties, but those are harder to use (they need to be told which properties they may rename, and which are exported or come from external component, such as .style.display on a DOM node).

Secondly, it makes for uncluttered code—I can write foo instead of or That really does make a big difference in the overall succinctness of the code.

Thirdly, and lastly, I believed that this would be faster. I reasoned thusly: a lexical variable in a closure is in static, known-at-compile-time place. The closure data is allocated as a structure with a known layout, and the closing function will have a pointer to that structure in a known place. Thus, a reference to a closed-over variable involves:

That sounds like two, three instructions at most, plus maybe two more to get at the current function's data. In case it's a nested closure, where the current function closes over an environment that in turn closed over the variable we're getting at, that'd add another hop, but still, nothing more than a simple following of pointers is involved.

Compare that to accessing an object field. This has been the target of much optimization work, since it used to be one of JavaScript's main bottlenecks, but it still requires a baseline amount of work. Assuming the JavaScript engine implements polymorphic inline caching, which all relevant ones do at this point, you'll still have to:

In my mind, this might come close to the speed of accessing a closed-over variable, but would definitely not surpass it.

However, benchmarks, both micro and a more elaborate one that I'll discuss in a moment, do show that on modern engines, object access is in fact faster than closure access.

I don't have an explanation for this. I'd be happy if someone can enlighten me on the subject. My current assumption is that the people working on these engines were just so busy optimizing object access and Sunspider performance that, unlike the compiler implementers in most other functional-language communities, closures just haven't been that well-optimized.

I spent the past few days on an experiment. I rewrote CodeMirror's core to use a set of objects rather than a single big closure for its central state. I have a version that passes the test suite now (though, since this change involved touching pretty much every single one of the almost 2000 lines that lived in that monster closure, there's probably some breakage that I haven't caught).

The reasons for this mostly had to do with modularity and code clarity. Closures are, in a way, too convenient. I can just define a new variable to hold some state, access it from everywhere, and it'll work. This is good, in most respects, when the system modelled by the closure is small (to medium) in size. But CodeMirror had long ago crossed the threshold where the proliferation of stateful local variables became hard to see through. Grouping them into a set of objects with well defined roles and lifetimes definitely made the data model easier to understand and reason about. It also, by lifting all internal functions out of the closure, forces me to specify the inputs that the functions act on in their argument lists.

The overhauled implementation did become noisier, obviously. Field access had to be prefixed by object names all over the place, and it is often necessary to create locals like var doc = cm.view.doc; to prevent typing out the chain of property accesses twenty times in a function.

These are the file size numbers, in bytes, using UglifyJS for minification:

            Full   Minified  Min+gzip
Closure   150453      65381     22733
Objects   154752      74655     24448

So the raw file became 2.7% bigger, the minified file 12.4% (!), and the gzipped file 7.0%. Zipping absorbs some of the extra size of the minified version, because the repeated property names are exactly the kind of thing that compression is supposed to handle well, but there still remains a significant bloat of the file size.

My conclusion: closures really do help with minification, but not to a huge enough extent to justify sticking to it for the CodeMirror project.

Finally, performance. I only did some ad-hoc benchmarks on Chrome 21 and Firefox 15, comparing the old closure-based CodeMirror codebase to the equivalent, object-using one. But those showed, on both browsers, a consistent speedup in the 2% to 3% range. Thus, my intuition about closure access being fast was once again debunked.