Understanding JavaScript Function Invocation and “this”

August 11th, 2011

Over the years, I’ve seen a lot of confusion about JavaScript function invocation. In particular, a lot of people have complained that the semantics of `this` in function invocations is confusing.

In my opinion, a lot of this confusion is cleared up by understanding the core function invocation primitive, and then looking at all other ways of invoking a function as sugar on top of that primitive. In fact, this is exactly how the ECMAScript spec thinks about it. In some areas, this post is a simplification of the spec, but the basic idea is the same.

The Core Primitive

First, let’s look at the core function invocation primitive, a Function’s call method[1]. The call method is relatively straight forward.

1. Make an argument list (argList) out of parameters 1 through the end
2. The first parameter is thisValue
3. Invoke the function with this set to thisValue and the argList as its argument list

For example:

function hello(thing) {
  console.log(this + " says hello " + thing);
}
 
hello.call("Yehuda", "world") //=> Yehuda says hello world

As you can see, we invoked the hello method with this set to "Yehuda" and a single argument "world". This is the core primitive of JavaScript function invocation. You can think of all other function calls as desugaring to this primitive. (to “desugar” is to take a convenient syntax and describe it in terms of a more basic core primitive).

[1] In the ES5 spec, the call method is described in terms of another, more low level primitive, but it’s a very thin wrapper on top of that primitive, so I’m simplifying a bit here. See the end of this post for more information.

Simple Function Invocation

Obviously, invoking functions with call all the time would be pretty annoying. JavaScript allows us to invoke functions directly using the parens syntax (hello("world"). When we do that, the invocation desugars:

function hello(thing) {
  console.log("Hello " + thing);
}
 
// this:
hello("world")
 
// desugars to:
hello.call(window, "world");

This behavior has changed in ECMAScript 5 only when using strict mode[2]:

// this:
hello("world")
 
// desugars to:
hello.call(undefined, "world");

The short version is: a function invocation like fn(...args) is the same as fn.call(window [ES5-strict: undefined], ...args).

Note that this is also true about functions declared inline: (function() {})() is the same as (function() {}).call(window [ES5-strict: undefined).

[2] Actually, I lied a bit. The ECMAScript 5 spec says that undefined is (almost) always passed, but that the function being called should change its thisValue to the global object when not in strict mode. This allows strict mode callers to avoid breaking existing non-strict-mode libraries.

Member Functions

The next very common way to invoke a method is as a member of an object (person.hello()). In this case, the invocation desugars:

var person = {
  name: "Brendan Eich",
  hello: function(thing) {
    console.log(this + " says hello " + thing);
  }
}
 
// this:
person.hello("world")
 
// desugars to this:
person.hello.call(person, "world");

Note that it doesn't matter how the hello method becomes attached to the object in this form. Remember that we previously defined hello as a standalone function. Let's see what happens if we attach is to the object dynamically:

function hello(thing) {
  console.log(this + " says hello " + thing);
}
 
person = { name: "Brendan Eich" }
person.hello = hello;
 
person.hello("world") // still desugars to person.hello.call(person, "world")
 
hello("world") // "[object DOMWindow]world"

Notice that the function doesn't have a persistent notion of its 'this'. It is always set at call time based upon the way it was invoked by its caller.

Using Function.prototype.bind

Because it can sometimes be convenient to have a reference to a function with a persistent this value, people have historically used a simple closure trick to convert a function into one with an unchanging this:

var person = {
  name: "Brendan Eich",
  hello: function(thing) {
    console.log(this.name + " says hello " + thing);
  }
}
 
var boundHello = function(thing) { return person.hello.call(person, thing); }
 
boundHello("world");

Even though our boundHello call still desugars to boundHello.call(window, "world"), we turn right around and use our primitive call method to change the this value back to what we want it to be.

We can make this trick general-purpose with a few tweaks:

var bind = function(func, thisValue) {
  return function() {
    return func.apply(thisValue, arguments);
  }
}
 
var boundHello = bind(person.hello, person);
boundHello("world") // "Brendan Eich says hello world"

In order to understand this, you just need two more pieces of information. First, arguments is an Array-like object that represents all of the arguments passed into a function. Second, the apply method works exactly like the call primitive, except that it takes an Array-like object instead of listing the arguments out one at a time.

Our bind method simply returns a new function. When it is invoked, our new function simply invokes the original function that was passed in, setting the original value as this. It also passes through the arguments.

Because this was a somewhat common idiom, ES5 introduced a new method bind on all Function objects that implements this behavior:

var boundHello = person.hello.bind(person);
boundHello("world") // "Brendan Eich says hello world"

This is most useful when you need a raw function to pass as a callback:

var person = {
  name: "Alex Russell",
  hello: function() { console.log(this.name + " says hello world"); }
}
 
$("#some-div").click(person.hello.bind(person));
 
// when the div is clicked, "Alex Russell says hello world" is printed

This is, of course, somewhat clunky, and TC39 (the committee that works on the next version(s) of ECMAScript) continues to work on a more elegant, still-backwards-compatible solution.

On jQuery

Because jQuery makes such heavy use of anonymous callback functions, it uses the call method internally to set the this value of those callbacks to a more useful value. For instance, instead of receiving window as this in all event handlers (as you would without special intervention), jQuery invokes call on the callback with the element that set up the event handler as its first parameter.

This is extremely useful, because the default value of this in anonymous callbacks is not particularly useful, but it can give beginners to JavaScript the impression that this is, in general a strange, often mutated concept that is hard to reason about.

If you understand the basic rules for converting a sugary function call into a desugared func.call(thisValue, ...args), you should be able to navigate the not so treacherous waters of the JavaScript this value.

PS: I Cheated

In several places, I simplified the reality a bit from the exact wording of the specification. Probably the most important cheat is the way I called func.call a "primitive". In reality, the spec has a primitive (internally referred to as [[Call]]) that both func.call and [obj.]func() use.

However, take a look at the definition of func.call:

1. If IsCallable(func) is false, then throw a TypeError exception.
2. Let argList be an empty List.
3. If this method was called with more than one argument then in left to right order starting with arg1 append each argument as the last element of argList
4. Return the result of calling the [[Call]] internal method of func, providing thisArg as the this value and argList as the list of arguments.

As you can see, this definition is essentially a very simple JavaScript language binding to the primitive [[Call]] operation.

If you look at the definition of invoking a function, the first seven steps set up thisValue and argList, and the last step is: "Return the result of calling the [[Call]] internal method on func, providing thisValue as the this value and providing the list argList as the argument values."

It's essentially identical wording, once the argList and thisValue have been determined.

I cheated a bit in calling call a primitive, but the meaning is essentially the same as had I pulled out the spec at the beginning of this article and quoted chapter and verse.

There are also some additional cases (most notably involving with) that I didn't cover here.

This entry was posted on Thursday, August 11th, 2011 at 2:54 am and is filed under JavaScript. You can follow any responses to this entry through the RSS 2.0 feed. You can skip to the end and leave a response. Pinging is currently not allowed.

20 Responses to “Understanding JavaScript Function Invocation and “this””

Finbarr Taylor, Posted August 11, 2011, 11:59 am

This is extremely helpful to me at this moment in time and I’m glad I found it. Well written and explained. Thanks very much!

Karl Swedberg, Posted August 11, 2011, 12:35 pm

Great post, Yehuda! Check out the last code block, though. It’ll log “[object Object] says hello world”. I suppose you either wanted to do console.log(this.name… or person.hello.bind(person.name), in which case, it would log “Alex Russell…” not “Brendan Eich…”

Brock, Posted August 11, 2011, 12:37 pm

This is a very clear explanation, thanks!

Marcel M., Posted August 11, 2011, 12:46 pm

var boundHello = bind(person.hello, person);
boundHello(“world”) // “Brendan Eich says hello world”

should be:

var boundHello = bind(person.hello, person.name);
boundHello(“world”) // “Brendan Eich says hello world”

Don Burks, Posted August 11, 2011, 1:10 pm

Yehuda,
Brilliantly written. Thank you for writing on this topic. Invaluable to those who don’t already understand it, and insightful for those of us who do.

Ben, Posted August 11, 2011, 1:16 pm

on the same note: http://www.aminutewithbrendan.com/pages/20110303

vjeux, Posted August 11, 2011, 1:21 pm

Nice post.

You are missing one way to set “this”: using the “new” operator. I’ve written a blog post about it there http://blog.vjeux.com/2011/javascript/how-prototypal-inheritance-really-works.html

Guillermo, Posted August 11, 2011, 1:38 pm

After Member Functions title you use this example, that i think it has a typo:

var person = {
name: “Brendan Eich”,
hello: function(thing) {
console.log(this + ” says hello ” + thing);
}
}

You use this inside the function, that i think should be “this.name”

wycats, Posted August 11, 2011, 2:12 pm

To all those who reported the typo, it’s fixed!

toolbear, Posted August 11, 2011, 5:02 pm

Great explanation. Complete and concise. Now I can point people to this article instead of subjecting them to my attempts at an explanation.

fnl, Posted August 11, 2011, 5:33 pm

Great post! One suggestion – add the pattern for making Function bind available in pre-ES5, eg. (my version, but essentially just stolen from MDN):

[pre]
if (typeof Function.prototype.bind === ‘undefined’) {
Function.prototype.bind = function (target) {
if (typeof this !== “function”)
throw new TypeError(“bind – this is not a function”);
var tail = Array.prototype.slice.call(arguments, 1),
func = this,
noop = function () {},
bound = function () {
return func.apply(this instanceof noop ? this : target || window,
tail.concat(Array.prototype.slice.call(arguments))); // copy arguments array!
};
noop.prototype = this.prototype;
bound.prototype = new noop();
return bound;
};
}
[/pre]

Dmitry A. Soshnikov, Posted August 12, 2011, 2:27 am

Don’t forget some subtle cases, when the table you’ve described doesn’t reflect some things. For example: func(...args) not always sets this to global object (or undefined in the strict mode) — in case of with, it will be with-object.

Moreover, some very subtle cases can also arise, when it seems that if a function would be called as a method (your third case in the table with path.to.obj), but the this value nevertheless is set to global again.

E.g.:

[js]

var o = {m: function () {this}};

o.m(); // o
(o.m)(); // o
(o.m = o.m)(); // global
(o.m || o.n)(); // global
//etc.

[/js]

The which should be understood in order to determine this value always correctly is values of Reference type, because exactly this type affects this value in every single function invocation.

P.S.:

If you’re interested in more deeper details of this value in JS and function invocations, here is additional literature:

http://dmitrysoshnikov.com/ecmascript/chapter-3-this/
http://dmitrysoshnikov.com/ecmascript/javascript-the-core/#this-value

Dmitry.

Michael Edmondson, Posted August 12, 2011, 4:34 pm

hello(“world”) // “[object DOMWindow]world”

…should be…

hello(“world”) // “[object DOMWindow] says hello world”

Pablo Fernandez, Posted August 12, 2011, 8:26 pm

Yehuda,

I think a case is missing: When the function is used as a constructor (as new FunctionName())

That case would be something like:

FunctionName.call({}, args);

But would also imprint the __proto__ and constructor properties, right?

Shihua Ma, Posted August 14, 2011, 5:19 am

Excellent work, Yehuda! I am in confusion with the code:
var person = {
hello: function(thing) {
console.log(this + ” says hello ” + thing);
}
};

(person.hello)(“world”);
(person.hello,person.hello)(‘word’);

Could you explain why (person.hello,person.hello) is set this to window.
Thanks.

Laurens Holst, Posted August 15, 2011, 1:44 pm

Maybe also worth mentioning, it’s a nice elaboration on how this is really just the first call parameter:

var boundHello = person.hello.bind(person, “world”);
boundHello() // “Brendan Eich says hello world”

John Fearnside, Posted August 18, 2011, 11:13 am

Another typo? In “Simple Function Invocation” section:

“a function invocation like fn(…args) is the same as fn(window [ES5-strict: undefined], …args)”

should be:

“a function invocation like fn(…args) is the same as fn.call(window [ES5-strict: undefined], …args)”

Justin Meyer, Posted August 22, 2011, 12:58 am

Everyone struggles with this. People are used to thinking about ‘this’ having value set by where ‘this’ is in the code. The first thing I say ‘this’ gets set only by how a function is called.

TiTi, Posted October 28, 2011, 11:43 am

Thank you for lighting me up with the “desugars” term.
Because my explanation of the `this` resolution was kinda wrong in my code : http://pastebin.com/tWprtttK
It is a very specific piece of code regarding “this” & “window” using 2 windows :-p
I encourage you to take a look because it’s not that obvious.

Michael A. Smith, Posted November 16, 2011, 11:28 pm

One thing that threw me until the very end was that Fn.call is a function. The whole time I was reading this I was thinking Fn.call.call.call.call.call.call…

I’m glad you wrote the “I Cheated” section, as I think it explains why this isn’t actually the case.