I have read lot of theory regarding call(), apply() and bind().

I am not able to understand if we can call method directly why we need call() , apply() or bind()?

Can somebody explain in laymen terms and little in terms of JavaScript?

CodePudding user response：

That's the flexibility offered by Javascript, an object and it's method don't have to be coupled with each other.

Given following object 'a' with a member method 'play'

var a = { play: function (content) { console.log("what is this:", this, content)} }

Use

a.play('Hello')

Is equivalent to

a.play.call(a, 'Hello')

As for your question why need the second way 'call'. Because 'call' gives you a way to call something else instead of 'this', which is 'a' in the example above, so you can do:

a.play.call(document, 'Hello')

About 'apply', it's just another version of 'call', which needs you to pass arguments as an array instead of comma separated parameters. To use apply, you do:

a.play.apply(a, ['Hello'])

As for 'bind', it is to link a function with a 'this' object at first, then get a returned callable object, which you can either call with the rest arguments directly, or pass it into anything else to be called later.

A typical 'bind' use case is React component's event handler. To define an event handler to be passed into a component. You need to use 'bind' in a React class component like this:

handleClick = this.handleClick.bind(this)

Then, in render function:

<Button onClick={this.handleClick} />

Check https://reactjs.org/docs/handling-events.html for the full information.