Javascript "apply" function seems not working for global "this"?

Question

I tested the scope of 'this' keyword, in nodejs:

color = 'red';
var o = {
    color: 'blue'
};

function say() {
    console.log(this.color);
};
say.apply(this);  //'undefined'
say.apply(global);//'red'

In browser:

color = 'red';
var o = {
    color: 'blue'
};

function say() {
    alert(this.color);
};
say.apply(this);  //'undefined'
say.apply(window);//'red'

var color = 'red';
function say() {
  alert(this.color);
}
say.apply(this);   //'red'
say.apply(window); //'red'

The result is a bit weird to me: as long as "this" is a pointer to "global" or "window", why say.apply(this) will output 'undefined'?

Thanks.

Then you're not in the global scope or in any scope where `this` is not `window`. The `o` object isn't used in either example. If you run that code in the global scope, you'll get `'red'` both times. In NodeJS, if you're in a module, then you're not in the global scope. — , Aug 08 '16 at 02:39
If you run the browser code as global code, you'll get "red" for both calls to *say*. — RobG, Aug 08 '16 at 03:43

score 1 · Accepted Answer · edited May 23 '17 at 12:22

In this case your this is really underfined, because color = 'red'; has this equivalent: global.color = 'red';. To make it related with this you need to do it by yourself: this.color = 'red';

To get value of color from o object you need to write something like this:

say.apply(o);

This is the good example of using apply():

function printArgs() {
  var join = [].join; // copy link of array.join into variable

  // call join with "this=arguments",
  // this call is equivalent with "arguments.join(':')"
  // here we have guarantee that method 'join()' was called from 'arguments'
  // (not from other object)
  var argStr = join.apply(arguments, [':']);

  console.log( argStr ); // output will looks like '1:2:3'
}

printArgs(1, 2, 3);

Also you can find useful information here:

Javascript "apply" function seems not working for global "this"?

1 Answers1