How big are JavaScript function objects?

First of all, it's common to place methods in the object constructor prototype, so they'll be shared among all instances of a given object:

function MyObject() {
    ....
}

MyObject.prototype.do_this = function() {
   ...
}

MyObject.prototype.do_that = function() {
   ...
}

Also note that a "function object" is a constant code-only block or a closure; in both cases the size is not related to the code:

x = [];
for (var i=0; i<1000; i++) {
    x.push(function(){ ... });
}

The size of each element of the array is not going to depend on the code size, because the code itself will be shared between all of the function object instances. Some memory will be required for each of the 1000 instances, but it would be roughly the same amount required by other objects like strings or arrays and not related to how much code is present inside the function.

Things would be different if you create functions using JavaScript's eval: In that case I'd expect each function to take quite a bit and proportional to code size unless some super-smart caching and sharing is done also at this level.

This is how Chrome handles functions, and other engines may do different things.

Let's look at this code:

var funcs = [];
for (var i = 0; i < 1000; i++) {
    funcs.push(function f() {
        return 1;
    });
}
for (var i = 0; i < 1000; i++) {
    funcs[0]();
}

http://jsfiddle.net/7LS6B/4/

Now, the engine creates 1000 functions.

The individual function itself takes up almost no memory at all (36 bytes in this case), since it merely holds a pointer to a so-called SharedFunctionInfo object, which is basically a reference to the function definition in your source code*. This is called lazy parsing.

Only when you run it frequently does the JIT kick in, and creates a compiled version of the function, which requires more memory. So, funcs[0] takes up 256 bytes in the end:

_{*) This is not exactly true. It also holds scope information and the function's name and other metadata, which is why it has a size of 592 bytes in this case.}