What is the value of "this" within an anonymous function in setTimeout below?

Question

I've rewritten the underscore.js delay function to look like the below. It works after tinkering with using apply() but I don't fully understand what the "this" in the apply is pointing to in the anonymous function within setTimeout.

_.delay = function(func, wait) {
   var args = Array.prototype.slice.call(arguments, 2);
     setTimeout(function() {
       return func.apply(this, args);
     }, wait);
 };

For solutions: http://stackoverflow.com/questions/2130241/pass-correct-this-context-to-settimeout-callback — user2864740, Sep 12 '15 at 19:28
@ВидулПетров It's a good solution question, but it doesn't really explain the mechanics of `this`.. — user2864740, Sep 12 '15 at 19:30
Ahh, that is what I was looking for: http://stackoverflow.com/questions/3127429/how-does-the-this-keyword-work — user2864740, Sep 12 '15 at 19:31
@user2864740 See also [this](http://stackoverflow.com/a/14049482/1367611) answer. — Vidul, Sep 12 '15 at 19:37

score 1 · Answer 1 · answered Sep 19 '15 at 12:56

this refers to window inside the setTimeout() and setInterval() functions. If you want to avoid this side effect, you can "bind" the value of this:

_.delay = function(func, wait, thisArg) {
   var args = Array.prototype.slice.call(arguments, 2);
   thisArg = thisArg || this;//assume a default value
   setTimeout(function() {
     return boundFunction.apply(thisArg, args);
   }, wait);
};

Then you can pass what will be the value of this to _.delay() or let it be _ by default.

Another way is to bind your function before passing it to _.delay():

function a() {console.log(this);}
_.delay(a.bind({foo:'bar'}), 1000); //prints {foo: 'bar'}

What is the value of "this" within an anonymous function in setTimeout below?

1 Answers1

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