JavaScript get function value

Question

I'm trying to override Node.js require function. However in JavaScript functions can have values and properties

Assuming code like this

a = function() {};
a.someParam1 = 1;
a.someParam2 = 2;

If I console log it. It'd look like:

{ [Function] someParam1: 1, someParam2: 2 }

I know I can call this function by simply doing

a()

But what If I wanted to override it's behavior without getting rid of properties?

Doing

a = function(){ console.log("Some other function")}
console.log(a.someParam1)

would yield undefined.

How can I override function value without touching it's parameters?

Overriding require() is not a good idea. What are you trying to accomplish? — SLaks
– SLaks, Commented Sep 28, 2014 at 17:56
a bracket is missing from console.log("Some other function"... but i guess that was just a typo... ;) — benomatis
– benomatis, Commented Sep 28, 2014 at 17:56

David P. Caldwell · Accepted Answer · 2014-09-28 17:58:12Z

1

a = (function(previously) {
  var newImplementation = function() {
    // new implementation
  }
  for (var x in previously) {
     newImplementation[x] = previously[x];
  }
  return newImplementation;
})(a);

answered Sep 28, 2014 at 17:58

David P. Caldwell

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2 Comments

Krzysztof Wende Over a year ago

This is a little bit way around. Aren't there any properties that can give me just the value?

David P. Caldwell Over a year ago

No. JavaScript functions are objects. When you replace the value of an object, you are not "overriding" anything; you are replacing an object reference with another object reference. So if you want the second reference to have the first reference's properties, you have to copy them.

Winchestro · Accepted Answer · 2014-09-28 18:15:01Z

1

You can inherit, but that's not recommended*. (But since what you want to do isn't recommended in the first place you might as well give it a shot.)

a = function() {return "something"};
a.someParam1 = 1;
a.someParam2 = 2;
a = Object.setPrototypeOf(function(){return "something else";},a);

*Warning: Mutating the [[Prototype]] of an object, using either this method or the deprecated Object.prototype.__proto__, is strongly discouraged, because it is very slow and unavoidably slows down subsequent execution in modern JavaScript implementations. source: MDN

edited Sep 28, 2014 at 18:15

answered Sep 28, 2014 at 18:08

Winchestro

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Comments

user663031 · Accepted Answer · 2014-09-28 18:26:36Z

1

The function object is the function. The function is not some kind of property of an abstract object whose "value" (the function) can be overridden. You cannot override the value of a function any more than you can override the number 1 with the number 2.

You can attach properties to the function, being an object, but the function is the function and cannot be changed out from underneath itself.

I don't know in what environment a console.log would print out { [Function] someParam1: 1, someParam2: 2 }. It certainly does not in any environment I'm familiar with. console.log on a function, with or without associated properties, would print out the function.

> a = function() {};
> a.someParam1 = 1;
> a.someParam2 = 2;

> console.log(a)
function () {}

If you want to move the properties onto a different function, feel free with Object.assign(function newFunc() { }, oldFunc) or _.extend or whatever you prefer.

answered Sep 28, 2014 at 18:26

user663031

2 Comments

Winchestro Over a year ago

nice, didn't know about Object.assign. It's currently FF only, but good to know it's coming :)

user663031 Over a year ago

Also available in traceur.

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