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JavaScript Function bind()


Function Borrowing

With the bind() method, an object can borrow a method from another object.

The example below creates 2 objects (person and member).

The member object borrows the fullname method from the person object:

Example

const person = {
  firstName:"John",
  lastName: "Doe",
  fullName: function () {
    return this.firstName + " " + this.lastName;
  }
}

const member = {
  firstName:"Hege",
  lastName: "Nilsen",
}

let fullName = person.fullName.bind(member);
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Preserving this

Sometimes the bind() method has to be used to prevent loosing this.

In the following example, the person object has a display method. In the display method, this refers to the person object:

Example

const person = {
  firstName:"John",
  lastName: "Doe",
  display: function () {
    let x = document.getElementById("demo");
    x.innerHTML = this.firstName + " " + this.lastName;
  }
}

person.display();
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When a function is used as a callback, this is lost.

This example will try to display the person name after 3 seconds, but it will display undefined instead:

Example

const person = {
  firstName:"John",
  lastName: "Doe",
  display: function () {
    let x = document.getElementById("demo");
    x.innerHTML = this.firstName + " " + this.lastName;
  }
}

setTimeout(person.display, 3000);
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The bind() method solves this problem.

In the following example, the bind() method is used to bind person.display to person.

This example will display the person name after 3 seconds:

Example

const person = {
  firstName:"John",
  lastName: "Doe",
  display: function () {
    let x = document.getElementById("demo");
    x.innerHTML = this.firstName + " " + this.lastName;
  }
}

let display = person.display.bind(person);
setTimeout(display, 3000);
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What is this?

In JavaScript, the this keyword refers to an object.

Which object depends on how this is being invoked (used or called).

The this keyword refers to different objects depending on how it is used:

In an object method, this refers to the object.
Alone, this refers to the global object.
In a function, this refers to the global object.
In a function, in strict mode, this is undefined.
In an event, this refers to the element that received the event.
Methods like call(), apply(), and bind() can refer this to any object.

Note

this is not a variable. It is a keyword. You cannot change the value of this.

See Also:

The JavaScript this Tutorial