javascript - what does fn.apply(fn, []) do? - Stack Overflow

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I have a piece of code which accepts fn as the argument and stores it in an object property.

var obj = {};

function anotherFn(fn){
   obj["name"] = fn.apply(fn, []);
}

function google(){
    console.log("hello");
}
anotherFn(google);
console.log(obj.name);

What i am not understanding is the fn.apply(fn,[]) code and its purpose. Normally we use call and apply method when we want to execute a fn in a different this/context.

But what does the fn.apply(fn, []) do here?. The confusion is why can't i just do

obj["name"] = fn();

I have a piece of code which accepts fn as the argument and stores it in an object property.

var obj = {};

function anotherFn(fn){
   obj["name"] = fn.apply(fn, []);
}

function google(){
    console.log("hello");
}
anotherFn(google);
console.log(obj.name);

What i am not understanding is the fn.apply(fn,[]) code and its purpose. Normally we use call and apply method when we want to execute a fn in a different this/context.

But what does the fn.apply(fn, []) do here?. The confusion is why can't i just do

obj["name"] = fn();

• javascript

Share Improve this question Follow edited Dec 5, 2014 at 19:25 Pointy 414k6262 gold badges595595 silver badges629629 bronze badges asked Dec 5, 2014 at 19:21 ShaneShane 5,6971515 gold badges5757 silver badges8282 bronze badges 2

• That call does execute fn in a different this context, that being the function object itself. – Pointy Commented Dec 5, 2014 at 19:26
• I would remend never doing something like this in production. – tengbretson Commented Dec 5, 2014 at 19:56

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2 Answers 2

Sorted by: Reset to default 8

fn.apply(fn, []) calls the function stored in fn with a context (the value of this while executing the function) of fn, and the arguments contained within [] (no arguments).

It seems odd to call apply in that way, when it would have been equivalent to call fn.call(fn).

fn() would not be an appropriate replacement, as fn() will execute in the global context, which means that the value of this within the function will be window (assuming a browser environment).

Here's a contrieved sample showing how it can be different:

var obj = {};

function anotherFn(fn){
   obj["name"] = fn.apply(fn, []);
}

function anotherFn2(fn){
   obj["name"] = fn(fn, [])
}

function google() {
    console.log(this.world);
}
google.world = "yay!"

anotherFn(google);
anotherFn2(google);

The output is:

yay!
undefined

jsFiddle: http://jsfiddle/c56ja3jL/

Depending on the context, this might be useful. The basic idea is that you always have this equal to the function itself, instead of the global context.

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