Function.prototype.toString to discover function dependencies

Mark S. Miller erights at
Sun Sep 19 12:41:02 PDT 2010

On Wed, Sep 15, 2010 at 5:24 PM, James Burke <jrburke at> wrote:

> First time posting to the es-discuss list:
> Over on the CommonJS list, there is a thread about how to provide a
> module format that works in today's browsers. The traditional CommonJS
> module format cannot run in the browser via script tags without a
> transform, and some of us want a format that will work without
> transforms but can be easily hand-coded.
> Tom Robinson made the following suggestion, is this an acceptable use
> of Function.prototype.toString:
> require.def(function (require, exports, module) {
>    var foo = require('foo'),
>        bar = require('bar');
> });
> Where require.def would call .toString() on the function passed to it,
> regexp for the require() calls, then make sure to load those
> dependencies before executing the function that defines the module.
> So the toString() does not have to be an exact copy of the function,
> just enough that allows the require() calls to be found.

Hi James, at <> I've
been trying to state a usefully stronger but not impractically stronger
spec. I like your usage pattern of "require" above, especially that it can
be reliably recognized without a full lexing or parsing by regexp tricks,
without getting misled by comments or quotes. (I try a similar but different
pattern at <>.
I like your's better.)

Given source code, of course, either recognition trick works. Given only
meaning preserving source code, which is the most I was willing to propose
on that strawman page, these recognition tricks fail <>.
Would we be willing to specify enough fidelity with the original source code
that your trick would work? I don't know. Perhaps AST preserving? Perhaps
preserving of some abstraction over equivalent ASTs? I would like to clean
up Function.prototype.toString for ES-Harmony. Opinions?

> I am leaving out a bunch of other stuff about the browser-friendly
> module format (for instance there is a transform for this format to
> allow grouping more than one module in a file for optimized browser
> delivery and that form does not use .toString() and this approach
> *would not* be used on minified code), but the main question for the
> es-discuss list is:
> Is this an acceptable use of Function.prototype.toString? What
> problems would there be now or in the future?
> What I have found so far:
> The ECMAScript 3rd and 5th edition specs say the following:
> ------
> Function.prototype.toString ( )
> An implementation-dependent representation of the function is
> returned. This representation has the syntax of a FunctionDeclaration.
> Note in particular that the use and placement of white space, line
> terminators, and semicolons within the representation string is
> implementation-dependent.
> The toString function is not generic; it throws a TypeError exception
> if its this value is not a Function object. Therefore, it cannot be
> transferred to other kinds of objects for use as a method.
> ------
> In a 2008 es-discuss thread[1], Erik Arvidsson thought the
> "implementation-dependent" part allowed for low-memory devices to not
> keep a reversible implementation in memory.
> A test was run in a few browsers to test the approach[2]: modern
> desktop browsers (including IE 6+) give a usable toString() value, and
> there are many mobile browsers that also work well: Android, iOS,
> Windows Mobile, webOS and latest BlackBerry. So far BlackBerry 4.6 and
> some version of Opera Mobile may not, and there are some like Symbian
> and Bada that have not been tested yet.
> So one issue is not universal browser support, but enough of today's
> browsers both on desktop and mobile that it may make sense using it
> going forward. While the spec technically allows an implementation to
> produce something that might cause the above mentioned Function
> toString() approach to fail, it seems in practice it may be workable.
> I am a bit wary of the approach, but initial testing seems to indicate
> it may work, so I am looking to have an expert review done before
> proceeding with the approach. If any of you have other other
> information or concerns it would be good to know.
> James
> [1 ]Thread starts with this post:
> [2] Browser test page:
> _______________________________________________
> es-discuss mailing list
> es-discuss at

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