Is there a way to look inside a (anonymous) function?

Question

In JavaScript, let's say you have:

function doSomething(callback) {
    if (callback instanceof Function) callback();
}

doSomething(function() {
    alert('hello world');
});

Is there a way to check what is inside 'callback' (like, the fact that alert() is called) from doSomething()? Something like:

function doSomething(callback) {
    alert(callback.innards().indexOf('alert('));
}

I'm just curious

Good discussion of typeof vs. instanceof here: http://stackoverflow.com/questions/899574/which-is-best-to-use-typeof-or-instanceof — ScottE, May 24 '12 at 01:53

score 2 · Accepted Answer · answered May 24 '12 at 01:53

Function.prototype.toString() gives an implementation-dependent representation of the function. However built-in functions will return something like:

function Array() {
   /* [native code] */
}

and host methods can return anything, even throw an error. So the strict answer is yes, maybe. But in a practical sense, it is not reliable.

score 1 · Answer 2 · answered May 24 '12 at 01:50

1

Some browsers support toString() on functions.

function doSomething(callback) {
    console.log( callback.toString().indexOf('alert(') )
    if (callback instanceof Function) callback();
}

answered May 24 '12 at 01:50

epascarello

204,599
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`Some browsers`? It's in [ECMA-262](http://es5.github.com/#x15.3.4.2) so I would expect all browsers to support it. Do you know of any browser that doesn't? – RobG May 24 '12 at 01:55
@Saxoier duh, it is a text match. It is not going to be accurate. And you missed `var foo = "alert()";` – epascarello May 24 '12 at 02:11
And i 'missed' `alert (` and 1000 other possibilities. – Saxoier May 24 '12 at 02:15
@RobG should be good for all modern day browsers, brain does not remember if IE6 supports it and I am not going to whip open a VM to test it. – epascarello May 24 '12 at 02:15

score 0 · Answer 3 · answered May 24 '12 at 01:51

You should be able to say:

alert( callback.toString().indexOf('alert(') );

However that won't distinguish between the beginning of a function alert(, the beginning of a function myspecialalert(, and the text "alert(" if it happens to be in a string literal - so you may need to do a bit of parsing.

Not sure how cross-browser this is, but in Firefox at least .toString() returns the whole text of the function including the word "function" and the parameter list.

xdazz · Answer 4 · 2012-05-24T01:57:32.847

0

callback.toString().indexOf('alert(');

OR do a regex match like:

/alert *\(/.test(callback.toString());

edited May 24 '12 at 01:57

answered May 24 '12 at 01:52

xdazz

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Is there a way to look inside a (anonymous) function?

4 Answers4