The Complete Rules to `this`

Master 'this' by learning exactly how its rules work in JavaScript. Eliminate the cloud of uncertainty surrounding this keyword. Learn exactly how 'this' is determined, how to predict what it will be before you run your code, and how to leverage its uses.

We'll cover the following

this is set in several different ways inside functions. This lesson will make you a master of this.

Rules

1 - If the new keyword is used when calling the function, this inside the function is a brand new object created by the JavaScript engine.

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function ConstructorExample() {
    console.log(this);
    this.value = 10;
    console.log(this);
}
new ConstructorExample();
// -> ConstructorExample {}
// -> ConstructorExample { value: 10 }

2 - If apply, call, or bind are used to call a function, this inside the function is the object that is passed in as the argument.

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function fn() {
    console.log(this);
}
var obj = {
    value: 5
};
var boundFn = fn.bind(obj);
boundFn(); // -> { value: 5 }
fn.call(obj); // -> { value: 5 }
fn.apply(obj); // -> { value: 5 }

3 - If a function is called as a method — that is, if dot notation is used to invoke the function — this is the object that the function is a property of. In other words, when a dot is to the left of a function invocation, this is the object to the left of the dot. (ƒ symbolizes function in the code blocks)

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const obj = {
    value: 5,
    printThis: function() {
      console.log(this);
    }
};
obj.printThis(); // -> { value: 5, printThis: ƒ }

4 - If a function is invoked as a free function invocation, meaning it was invoked without any of the conditions present above, this is the global object. In a browser, it’s window.

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function fn() {
    console.log(this);
}
// If called in browser:
fn(); // -> Window {stop: ƒ, open: ƒ, alert: ƒ, ...}

Note that this rule is the same as rule 3 — the difference is that a function that is not declared as a method automatically becomes a property of the global object, window. This is therefore an implicit method invocation. When we call fn(), it’s interpreted as window.fn(), so this is window.

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function fn() {
    console.log(this);
}
// In browser:
console.log(fn === window.fn); // -> true

5 - If multiple of the above rules apply, the rule that is higher wins and will set the this value.

Applying the Rules

Let’s go over a code example and apply our rules. Try figuring out what this will be with the two different function calls.

Determining Which Rule Applies

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const obj = {
    value: 'hi',
    printThis: function() {
        console.log(this);
    }
};
const print = obj.printThis;
obj.printThis(); // -> {value: "hi", printThis: ƒ}
print(); // -> Window {stop: ƒ, open: ƒ, alert: ƒ, ...}

obj.printThis() falls under rule 3 — invocation using dot notation. On the other hand, print() falls under rule 4 as a free function invocation. For print() we don’t use new, bind/call/apply, or dot notation when we invoke it, so we go to rule 4 and this is the global object, window.

This goes back to value vs. reference. The value of printThis on the object is a reference to the function. When we assign obj.printThis to print, print receives the reference of the function. It’s not bound to obj in any way - obj just happens to have a reference to it.

When we invoke it without obj, it’s an FFI. It really is the use of the dot (.) that makes rule 3 apply.

When Multiple Rules Apply

When multiple rules apply, the rule higher on the list wins. If rules 2 and 3 both apply, rule 2 takes precedence.

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const obj1 = {
    value: 'hi',
    print: function() {
        console.log(this);
    },
};
const obj2 = { value: 17 };
obj1.print.call(obj2); // -> { value: 17 }

If rules 1 and 3 both apply, rule 1 takes precendence.

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const obj1 = {
    value: 'hi',
    print: function() {
        console.log(this);
    },
};
new obj1.print(); // -> print {}

Libraries

Libraries will sometimes intentionally bind the value of this inside their functions. this is bound to the most useful value for use in the function. jQuery, for example, binds this to the DOM element triggering an event in the callback to that event. If a library has an unexpected this value that doesn’t seem to follow the rules, check its documentation. It’s likely being bound using bind.

Arrow Functions

ES2015 arrow functions get their this value lexically.