Accessing the two 'this' variables in Function.prototype

Question

I'd like to extend Function.prototype with a custom method:

Function.prototype.myMethod = function() {
     var fn = this;
     var owner = ???;
     /** ... */
};

this in that scope refers to the original function. But how do I access the this that refers to the object that "owns" the function (or whatever the outer this is at the time that fn is called)?

I think you can't know it because you are actually not calling the "original" function. There is no way to know whether the function is assigned to a property or not. What are you trying to do? Maybe there is a better way than extending Function.prototype. — Felix Kling
– Felix Kling, Commented Jun 3, 2011 at 15:07
I'm trying to do the following: Function.prototype.future = function() { var fn = this; var owner = ???; return fn.bind(owner, arguments); }; — kpozin
– kpozin, Commented Jun 3, 2011 at 15:13
There aren't "two this variables." Your only choice is to pass it in as a parameter. — Pointy
– Pointy, Commented Jun 3, 2011 at 15:15
I don't think fn.bind.apply(owner) would work as you expect (if owner is not a function). And it still does not explain what your actual goal is. I can only assume so far that extending Function.prototype is not the right way. — Felix Kling
– Felix Kling, Commented Jun 3, 2011 at 15:16
In this case, my goal is to make it easy to create futures/promises out of a function and a set of arguments. The result can be used for lazy evaluation. But there are probably other applications as well. — kpozin
– kpozin, Commented Jun 3, 2011 at 15:18

Ben Roux · Accepted Answer · 2011-06-03 15:09:55Z

1

You would need to pass it in as a parameter to the function call... not sure if this is possible in your specific usage.

myMethod(this);

function myMethod( parent ) {
    // ...

answered Jun 3, 2011 at 15:09

Ben Roux

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jishi · Accepted Answer · 2011-06-03 15:30:11Z

0

SpiderMonkey and JScript implements a non-standard "caller" property on Function-objects. I don't know the extent of the support in other browsers.

I belive it should be accessible like this:

Function.prototype.myMethod = function() {
   var fn = this;
   var owner = this.caller;
   /** ... */
};

but haven't tested it. As I said, it's not part of ECMA standard.

https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Function/Caller

answered Jun 3, 2011 at 15:30

jishi

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