call, bind and apply in Javascript

March 28th, 2020 | 3 mins read

#javascript

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These methods in Javascript are used to scope the keyword this in a function. In my article (Almighty this, demystified), I wrote:

this is an inherently (automatically) created variable at the creation of every function.

But it's scope varies depending on how it is been called. Check out the article to learn more about this.

For example:

const me = {
  name: "Dillion",
  printName: function () {
    console.log(this.name)
  },
}
me.printName()

In the above, printName() would work as expected (log 'Dillion') because this at that point in time refers to the object (me) calling the method.

But look at this example:

const me = {
  name: "Dillion",
  age: 56,
  print: function () {
    function printName() {
      console.log(this.name)
    }
    printName()
    function printAge() {
      console.log(this.age)
    }
    printAge()
  },
}
me.printName()

The expected outputs are undefined and undefined. This is because, the functions printName and printAge do not use me as their this. me did not directly call them, so their default this is window. And on window, name and age property do not exist.

bind, apply and call help us scope this for situations like this, so that the inner functions of print can use the this which it uses - me. Let's see them in action.

call

Syntax

function.call(this, arg1, arg2, ...)

This method is used set this in a function which is afterwards, executed. Example:

const me = {
  name: "Dillion",
  print: function () {
    function printName() {
      console.log(this.name)
    }
    printName.call(this)
  },
}
me.printName()

The expected output now is 'Dillion'.

call is used on the printName() function with an argument of this. printName and the this are on the same level, so what the method does is, "Use this this to replace your own automatically generated this and immediately, execute yourself". Hence, printName has access to me and it prints the name property.

Another example is;

const me = {
  name: "Dillion",
}
function printName(arg1) {
  console.log(arg1 + " " + this.name)
}
printName()
printName.call(me, "Hello")

The expected outputs are undefined and 'Hello Dillion'. This is because, printName() called without an object uses the global this which points to window (and doesn't have the name property) whie printName.call(me) uses me to define it's this.

The extra argument of call as seen above replaces arg1 parameter in the function.

bind

Syntax

newFunction = function.bind(this, arg1, arg2, ...)

This method is very similar to call. The only difference is that while call sets this and immediately executes the function, bind creates a new function and sets the this.

Example:

const me = {
  name: "Dillion",
}
function printName(arg1) {
  console.log(arg1 + " " + this.name)
}
const newPrint = printName.bind(me, "Hello")
newPrint()

The expected output is 'Hello Dillion'.

apply

Syntax

function.apply(this, [arg1, arg2, ...])

This is also similar to call. The slight difference being that call accepts this argument and other other arguments seperated with commas while apply accepts this argument and other arguments seperated with commas but in an array.

Example:

const me = {
  name: "Dillion",
}
function printName(arg1, arg2) {
  console.log(arg1 + " " + this.name + " >> " + arg2)
}
printName.apply(me, ["Hello", "Wow"])

The expected output is 'Hello Dillion >> Wow'.

Wrap up

bind, apply and call simply set the this in a function. Whichever method to use depends on your choice, but I hope with this article, you have learnt how each of them works.

Thanks for reading : )

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Dillion Megida

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