JavaScript apply vs. call vs. bind

JavaScript apply vs. call vs. bind.md

JavaScript has three different methods that allow you to change the value of this for a given function.

Those methods are Function.prototype.call(), Function.prototype.apply() and Function.prototype.bind().

Methods

1. call invokes the function and allows you to pass in arguments one by one.

2. apply invokes the function and allows you to pass in arguments as an array.

3. bind returns a new function, allowing you to pass in a this array and any number of arguments.

Some Examples

###Call

var person1 = {firstName: 'Jon', lastName: 'Kuperman'};
var person2 = {firstName: 'Kelly', lastName: 'King'};

function say(greeting) {
console.log(greeting + ' ' + this.firstName + ' ' + this.lastName);
}

say.call(person1, 'Hello'); // Hello Jon Kuperman
say.call(person2, 'Hello'); // Hello Kelly King

###Apply

var person1 = {firstName: 'Jon', lastName: 'Kuperman'};
var person2 = {firstName: 'Kelly', lastName: 'King'};

function say(greeting) {
console.log(greeting + ' ' + this.firstName + ' ' + this.lastName);
}

say.apply(person1, ['Hello']); // Hello Jon Kuperman
say.apply(person2, ['Hello']); // Hello Kelly King

###Bind

var person1 = {firstName: 'Jon', lastName: 'Kuperman'};
var person2 = {firstName: 'Kelly', lastName: 'King'};

function say() {
console.log('Hello ' + this.firstName + ' ' + this.lastName);
}

var sayHelloJon = say.bind(person1);
var sayHelloKelly = say.bind(person2);

sayHelloJon(); // Hello Jon Kuperman
sayHelloKelly(); // Hello Kelly King

When To Use Each

Call and apply are pretty interchangeable. Just decide whether it’s easier to send in an array or a comma separated list of arguments.

I always remember which one is which by remembering that Call is for comma (separated list) and Apply is for Array.

Bind is a bit different. It returns a new function. Call and Apply execute the current function immediately.

Bind is great for a lot of things. We can use it to curry functions like in the above example. We can take a simple hello function and turn it into a helloJon or helloKelly. We can also use it for events like onClick where we don’t know when they’ll be fired but we know what context we want them to have.