Unschooled

Nick

Hi, I'm Nick. This is my blog. I'm a life-long unschooler living in New York. You can find more about me here.

I did Y Combinator (S10) with my friend Dave. We currently run Hacker School and write LinkedListNYC.

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Understanding the "this" keyword in JavaScript

March 20, 2012

Many people get tripped up by the this keyword in JavaScript. I think the confusion comes from people reasonably expecting this to work like “this” does in Java or the way people use “self” in Python. Although this is sometimes used to similar effect, it’s nothing like “this” in Java or other languages. And while it’s a little harder to understand, its behavior isn’t magic. In fact, this follows a relatively small set of simple rules. This post is an explanation of those rules.

But first, I want to give some terminology and information about JavaScript runtime environments which will hopefully help you develop a mental model to better understand this.[1]

Execution contexts

Every line of JavaScript code is run in an “execution context.” The JavaScript runtime environment maintains a stack of these contexts, and the top execution context on this stack is the one that’s actively running.

There are three types of executable code: Global code, function code, and eval code. Roughly speaking, global code is code at the top level of your program that’s not inside any functions, function code is code that’s inside the body of a function, and eval code is global code evaluated by a call to eval.

The object that this refers to is redetermined every time control enters a new execution context and remains fixed until control shifts to a different context. The value of this is dependent upon two things: The type of code being executed (i.e., global, function, or eval) and the caller of that code.

The global object

All JavaScript runtimes have a unique object called the global object. Its properties include built-in objects like Math and String, as well as extra properties provided by the host environment.

In browsers, the global object is the window object. In Node.js, it’s just called the “global object.” (I’m going to assume you’re running code in a browser, however, everything I say should apply to Node.js as well.)

Determining the value of this

The first rule is simple: _this refers to the global object in all global code._ Since all programs start by executing global code, and this is fixed inside of a given execution context, we know that, by default, this is the global object.

What happens when control shifts to a new execution context? There are only three cases where the value of this changes: method invocations, functions called with the new operator, and functions called using call and apply. I’ll explain each of these in turn.

Method invocations

If we call a function as a property of an object using either dot (i.e., obj.foo()) or bracket (i.e., obj[“foo”]()) notation, this will refer to the parent object in the body of the function:

var counter = {
  val: 0,
  increment: function () {
    this.val += 1;
  }
};
counter.increment();
console.log(counter.val); // 1
counter['increment']();
console.log(counter.val); // 2

This is our second rule: this refers to the parent object inside function code if the function is called as a property of the parent.

Note that if we call the same function directly, that is, not as a property of the parent object, this does not refer to the counter object:

var inc = counter.increment;
inc();
console.log(counter.val); // 2
console.log(val); // NaN 

Here, this was not changed when inc was called, so it still referred to the global object. When inc ran

this.val += 1;

it was effectively running:

window.val += 1;

window.val is undefined, and adding 1 to undefined yields NaN.

The new operator

Any JavaScript function can be used as a constructor function with new. The new operator creates a new object and sets this to the new object inside the function it was called with. For example:

function F (v) {
  this.val = v;
}
var f = new F(“Woohoo!”);
console.log(f.val); // Woohoo!
console.log(val); // ReferenceError

This leads to our third rule: this in function code invoked using the new operator refers to the newly created object.

Note that there’s nothing special about F. If we call it without using new, this will refer to the global object:

var f = F(“Oops!”);
console.log(f.val); // undefined
console.log(val); // Oops!

In this case, F(“Oops!”) is a regular function call, and this doesn’t get set to a new object, because no new object is created since the new operator isn’t used. this remains set to the global object.

Call and apply

All JavaScript functions have two methods, call and apply, which let you call functions and explicitly set the value of this. The apply method takes two arguments: an object to set this to, and an (optional) array of arguments to pass to the function:

var add = function (x, y) {
      this.val = x + y;
    },
    obj = {
      val: 0
    };
add.apply(obj, [2, 8]);
console.log(obj.val); // 10

The call method works exactly the same as apply, but you pass the arguments individually rather than in an array:

add.call(obj, 2, 8);
console.log(obj.val); // 10

This is our fourth rule: this is set to the first argument passed to call or apply inside function code when that function is called with either call or apply.

Summary

That’s it! You can figure out what object this refers to by following a few simple rules:

If you understand and follow those four rules, you will always know what this is.

…Addendum: Eval breaks all the rules

Remember when I said that code evaluated inside eval is its own type of executable code? Well, that’s true, and it means that the rules for determining what this refers to inside of eval code are a little more complex.

As a first pass, you might think that this directly inside eval refers to the same object as it does in eval’s caller’s context. For example:

var obj = {
  val: 0,
  func: function() { 
    eval(“console.log(this.val)”);
  }
};
obj.func(); // 0

That works likely as you expect it to. However, there are many cases with eval where this will probably not work as you expect:

eval.call({val: 0}, “console.log(this.val)”); // depends on browser

The output of the above code depends on your browser. If your JavaScript runtime implements ECMAScript 5, this will refer to the global object and the above should print undefined, because it’s an “indirect” call of eval. That’s what the latest versions of Chrome and Firefox do. Safari 5.1 actually throws an error (“The ‘this’ value passed to eval must be the global object from which eval originated”), which is kosher according to ECMAScript 3.

If you want know how this works with eval, you should read Juriy Zaytsev’s excellent, “Global eval. What are the options?” You’ll learn more about eval, execution contexts, and direct vs. indirect calls than you probably ever wanted to know.

In general, you should just avoid using eval, in which case the simple rules about this given previously will always hold true.

  1. This is a somewhat simplified view of things to make developing a basic mental model easier. For a more detailed explanation, read the section starting at page 37 of the ECMAScript Language Specification. I’ve based this post in large part off of that document, however, my understanding of it is far from perfect or complete, so please let me know (me @ nicholasbs.net) if I’ve misunderstood or misrepresented anything.