This binding in JavaScript – 4. New binding

This post (This binding in JavaScript – 4. New binding) was originally published on Sargalias.

In this series we talk about this binding in JavaScript.

This is a very important topic. It's also something that even experienced developers frequently get wrong and / or have to think about.

Basically in JavaScript there are 4 modes for this binding. Make that 5 if we include arrow functions.

In order of lowest priority to highest priority, here they are:

1. Default binding
2. Implicit binding
3. Explicit binding
4. New binding
5. Arrow functions
6. Gotchas and final notes

In this post we'll talk about new binding.

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How new binding works

new is a special keyword in JavaScript.

It does a lot of things, but we'll only talk in detail about how it relates to binding.

To start off, note that new has even higher precedence than even hard binding. Another way of thinking about it is that it ignores the normal binding rules and does its own thing.

You can use new when calling functions like so: new foo().

new does 4 things:

1. It creates a new empty object.
2. It makes this be the new object.
3. It makes foo.prototype be the prototype of the object.
4. It implicitly returns this if nothing else is returned from the function.

For now ignore points 3 and 4 until a different blog post. Let's focus on points 1 and 2, the binding.

To recap, when you call a function with new before it, you create a brand new empty object which is assigned to this inside the function.

For example:

function foo() {
  console.log(this);
}

new foo(); // outputs an empty object

As mentioned, new has higher precedence than even hard binding.

const objForBind = { name: 'objForBind' };

function foo() {
  console.log(this);
}

const boundFoo = foo.bind(objForBind); // hard bind foo to objForBind

new boundFoo(); // logs a new empty object to the console, not objForBind

Explanation:
new has higher precedence than explicit and implicit binding. It ignores them, creates a new object, and binds it to this.