Apply.md

apply() method

The apply() method in JavaScript is a method that is used to call another function with a given this value and arguments provided as an array (or an array-like object). It is used for function borrowing and for invoking a function with a specific this context.

Here's how it works:

1. It is a method available on all functions.
2. It calls the function with a given this value and arguments provided as an array (or an array-like object).
3. It returns the result of the function call.

Here's a simple example:

let car1 = {
    color: "Red",
    company: "Ferrari"
}

let car2 = {
    color: "Blue",
    company: "BMW"
}

function purchaseCar(price){
    console.log(`I have purchased ${this.color} ${this.company} for Rs. ${price}`)
}

purchaseCar.apply(car1, ["70 lakh"]) // Output: I have purchased Red Ferrari for Rs. 70 lakh
purchaseCar.apply(car2, ["90 lakh"]) // Output: I have purchased Blue BMW for Rs. 90 lakh

In this example, apply() is called on the purchaseCar function with car1 as the this value. It calls purchaseCar with this set to car1, so this.color and this.company inside purchaseCar refer to car1.color and car1.company, respectively. The purchaseCar function is not changed. Similar happens when apply() is called on the purchaseCar function with car2 as the this value. This time the this refers to car2, therefore the same purchaseCar function can be borrowed by both the objects car1 and car2. Also the arguments passed to the function purchaseCar are provided as array.

Syntax

The syntax for the apply() method in JavaScript is as follows:

func.apply(thisObj, argsArray);

Here's what each parameter means:

• thisObj: The value of this provided for the call to function. Note that this may not be the actual value seen by the method: if the method is a function in non-strict mode, null and undefined will be replaced with the global object, and primitive values will be boxed.

• argsArray (optional): An array-like object, specifying the arguments with which function should be called, or null or undefined if no arguments should be provided to the function. Starting with ECMAScript 5 these arguments can be a generic array-like object instead of an array.

Polyfill

Function.prototype.apply = function(context, args) {
    context = context || window;
    args = args || [];

    context.fn = this;

    context.fn(...args);

};