JS: Function.prototype.call ❌

By Xah Lee. Date: . Last updated: .

🟢 TIP: better is Reflect.apply.

f.call(thisBinding)

Call f with its this (binding) having value of thisBinding.

/*
demo of what Function.prototype.call does.
*/

// define a function that returns the implicit argument this-binding.
function ff() { return this; }

// create a object
const jj = {};

// use ff.call(jj)
console.log(ff.call(jj) === jj);
// true
f.call(thisBinding, arg1, arg2, etc)

Feed f with arguments arg1, arg2, etc.

// example of using Function.prototype.call

function ff(a, b) {
  this.x = a;
  this.y = b;
}

const jj = {};

ff.call(jj, 7, 8);

console.log(jj);
// { x: 7, y: 8 }

Purpose of Function.prototype.call

This method was added in 1996. It lets you call a method on a object that does not inherit that method. e.g. you have data x, you want to apply a array method on it, but x cannot access array method because it is not a array type.

Typically, Function.prototype.call is used in a form where the function name is a sequence of property chain, like this (Array.prototype.slice).call(args).

it's most famously used before 2015 as a way to convert Array-Like Object to Array, like this: 

/*
Most famous use of Function.prototype.call is to convert Array-Like Object to Array.
used before 2015.
Like this:
*/
const xalike = { 0: "a", 1: "b", length: 2 };
const xresult = Array.prototype.slice.call(xalike);
console.log(Array.isArray(xresult));
// true

What's the difference between .call vs .apply

JavaScript. Apply Function