JavaScript call, apply and bind methods

In this article, I briefly discuss the bind(), call() and apply() methods in JavaScript.

These methods are available to every function in JavaScript and are used to control what this in a function points to. Let’s see the following examples.

`bind()`

The bind() method creates a new function that, when called, has its this keyword set to the provided value.

const person = {
  name: "Erons",
  greet: function () {
    return(`Hello ${this.name}`)
  }
}

function greetPerson() {
  console.log(this.greet())
}

greetPerson();
// -> Uncaught TypeError: this.greet is not a function

An error would be thrown when the code above is run because there is no greet function in the global object(the browser in this case) that this in greetPerson references.

With bind(), we can tell the JavaScript engine where to look for this.

const person = {
  name: "Erons",
  greet: function () {
    return(`Hello ${this.name}`)
  }
}

function greetPerson() {
  console.log(this.greet())
}

const bindPerson = greetPerson.bind(person)

bindPerson();
// -> Hello Erons

The bind() method did some interesting things

It created bindPerson, a copy of the greetPerson function
bindPerson when called will have it’s this variable pointing to the person object

bind() is also used in what is known as function currying. The act of creating a copy of a function with some preset parameters.

function multiply(a, b) {
  return a * b;
}

const multiplyByTen = multiply.bind(this, 10)

multiplyByTen(5)
// -> 50

multiplyByTen is now a copy of multiply but with 10 as a default parameter. So multiplyByTen can now be called with only one parameter.

`call()`

The call() method calls a function with a given this value and arguments provided individually.

call() essentially does the same thing as bind() except that call() actually executes the function(it doesn’t create a copy of it) and can accept additional parameters while bind() only creates a copy of the function.

const erons = {
  name: "Erons",
  speech: function () {
    return(`Hi, I'm ${this.name}`)
  }
}

function moreSpeech(age, hobby) {
  console.log(`${this.speech()}, I'm ${age} years old and I love ${hobby}`)
}

moreSpeech.call(erons, 17, 'cooking')

// -> Hi, I'm Erons, I'm 17 years old and I love cooking

`apply()`

call() and apply() do the exact same thing except that call() expects all parameters to be passed in individually, but apply() expects all additional parameters to be passed as an array.

const erons = {
  name: "Erons",
  speech: function () {
    return(`Hi, I'm ${this.name}`)
  }
}

function moreSpeech(age, hobby) {
  console.log(`${this.speech()}, I'm ${age} years old and I love ${hobby}`)
}

moreSpeech.apply(erons, 17, 'cooking')

// -> Uncaught TypeError: CreateListFromArrayLike called on non-object

We got an error because we used apply() but didn’t pass our extra parameters as an array.

const erons = {
  name: "Erons",
  speech: function () {
    return(`Hi, I'm ${this.name}`)
  }
}

function moreSpeech(age, hobby) {
  console.log(`${this.speech()}, I'm ${age} years old and I love ${hobby}`)
}

moreSpeech.apply(erons, [17, 'cooking'])

// -> Hi, I'm Erons, I'm 17 years old and I love cooking

bind(), call() and apply() can also be used in function borrowing. Function borrowing allows us to use the methods of one object on a different object without having to make copies of the methods.

const person = {
  name: 'Erons',
  age: 17,
  speech: function() {
    return(`Hi, I'm ${this.name} and I'm ${this.age} years old`)
  }
}

const person2 = {
  name: 'Ehis',
  age: 11
}

const bindSpeech = person.speech.bind(person2)
bindSpeech()
// -> My name is Ehis and I am 11 years old

console.log(person.speech.call(person2))
// -> My name is Ehis and I am 11 years old

console.log(person.speech.apply(person2))
// -> My name is Ehis and I am 11 years old

We’ve seen how to control the this variable in functions and methods with three powerful JavaScript built-in methods.

Published 3 Aug 2019

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