Know this JavaScript Question and excel your interview — Apply/Call/Bind.

Every JavaScript programmer goes through the same confusing thought process i.e. when to use Apply, Call, Bind; what is the difference between call, apply, and bind ; and why do we need this function?
After reading this article you will understand Apply/Call/Bind. In order for you to fully understand bind, call, and apply; you have to understand the JavaScript concept of this.

What is ‘this’?

In JavaScript, the this keyword refers to an object.

One bemusing thing about JavaScript is that this value changes depending on the context in which this is called. By default, it refers to the global scope, but when it is called inside a function, it relates to that function.

JavaScript allows an object to contain functions as properties. These properties are commonly known as methods.

It’s common that an object method needs to access the information stored in the object to do its job.

objects by deepajarout

For instance, the code inside user.fullName() may require the name of the user.

To access the object, a method can use the this keyword.

Here, during the execution of user.fullName(), the value of this will be user.

There’s no syntax error in the below example:

function by deepajarout

The value of this is evaluated during the run-time, depending on the context.

For instance, here the same function is assigned to two different objects and has a different “this” in the calls:

object by deepajarout

The rule: if obj.fullName() is called, then this is obj during the call of fullName. So it can either beuser or user2 in the above example.

When and why to use Apply/Call/Bind?

There are two important concepts:

# First is to adopt another object’s methods and

# Second is to construct a custom value for this

Adopting another object’s methods

Call() Method- The call()method allows a method or a function belonging to one object to be assigned and called for a different object. Call invokes the function and allows you to pass in arguments one by one.

call() method by deepaJarout

Apply() Method- It is analogous to call. It immediately calls a function and lets you specify this value. The only distinction between the “apply” and the “call” method is that, rather than specifying each argument one by one. You can pass the arguments as an array.

apply() method by deepajarout

Bind() Method- The Bind method is a little bit different from call and apply method, rather than instantly invoking the function it actually makes and returns a new function. The new function has a defined this value attached to it.

bind() method by deepajarout

Constructing a custom value for “this”

Create custom value by deepajarout

The differences between call, apply and bind

Both call method and apply method invokes a function. The only difference between them is that call accepts arguments in a comma-separated fashion while in the case of apply method, the requiring arguments need to be passed as an array or an array-like object.Bind returns a new function.