Easier Way to Understand apply and call in JS

The first time I know apply was when I met this code:

Math.max.apply(null, [1, 2, 3, 4])

As the mdn shows, the syntax is:

Actually, in case above, thisArg has no influence which means code below also works:

Math.max.apply(undefined, [1, 2, 3, 4])
Math.max.apply(Math, [1, 2, 3, 4])

The only effect of apply in the code above is that it can pass the values in array to the function max. So, code above equal

Math.max(1, 2, 3, 4)

Why would I mention this? Because we don't need this anymore because we already have ... which works like:

Math.max(...[1, 2, 3, 4])

The reason that we still need apply and call is the thisArg. They can help us call some powerful methods.

thisArg in apply and call

I guess you might have seen this code:

Array.prototype.slice.call({ length: 2 })
function fn() {
  console.log(Array.prototype.slice.call(arguments))
}
fn(1, 2, 3, 4) //[1,2,3,4]

Today, we don't need this either because of Array.from. But I still want to talk about it for explanation. In the case above, call was used because we want to do something like:

let obj = { length: 2 }
obj.slice() //Uncaught TypeError: obj.slice is not a function

It would cause error because slice was defined in Array.prototype. Only Array instance can call that method. But actually in the implementation of slice, it doesn't need to be called by Array instance and there is a lot of methods like this. So, in this case, call or apply would let non Array instance call these methods which means

Array.prototype.slice.call({ length: 2 })
//help you do
let obj = { length: 2 }
obj.slice = Array.prototype.slice
obj.slice()

And to help it easier to understand , you can remember it like:

method.call(thisArg, ...args)
//works like in most cases
thisArg.method = method
thisArg.method(...args)
//for apply
method.apply(thisArg, args)
//works like in most cases
thisArg.method = method
thisArg.method(...args)

Wasn't that easy ?

So, let get back to Math.max.apply({}, [1, 2, 3, 4]). You can remember it like:

let thisArg = {}
thisArg.max = Math.max
thisArg.max(...[1, 2, 3, 4])

And more cases:

Object.prototype.toString.call([]) //"[object Array]"
//help you do this
let thisArg = []
thisArg.toString = Object.prototype.toString
thisArg.toString() //"[object Array]"
//while
[].toString()//""

Or

;[' sd ', 1, 3].map(Function.prototype.call, String.prototype.trim) //['sd','1','3']
//help you do
;[' sd ', 1, 3].map(function(...args) {
  return String.prototype.trim.call(...args)
})
//help you do
;[' sd ', 1, 3].map(function(...args) {
  let thisArg = args[0]
  thisArg.trim = String.prototype.trim
  return thisArg.trim(...args.slice(1)) //Uncaught TypeError: thisArg.trim is not a function
})

In the case above, it will got error because args[0] is Primitive values. You can't call methods in Primitive values. But it can still help you understand.

More in apply

As apply can accept an array-like object. So, what would happen if coding like:

Array.apply(null, { length: 2 })

Actually, it equals

Array.apply(null, [undefined, undefined])

So, you can understand it like:

let thisArg = {} //set null would get error in code below, also thisArg in above case is not important
thisArg.Array = Array
thisArg.Array(undefined, undefined)

Hope it's easier to understand apply and call.

Original Post