what is 'this'?

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In this post we will explore what this keyword refers under various conditions and there by understand the keyword and how it works in javascript.

The this keyword can point to

  • The object when referenced within the obejcts method.
  • The window object - when referenced within a function.
  • The instance of the object when the new keyword is used.
  • The object when arrow function is used`

this within method of an object - no arrow function

const obj = {
    name: "obj",
    printName() {
        console.log(this.name);
    },
    printObj() {
        console.log(this); // this refers to the object `obj`
    }
}
obj.printName() // obj
obj.printObj() //{name: "obj", printName: ƒ, printObj: ƒ}

this within method of an object - arrow function

The value of this in an arrow function is always the value of this in the parent non arrow function.

const obj = {
    name: "obj",
    printObj: () => {
        console.log(this); // this refers to the window
    }
}
obj.printObj() // Window {window: Window, self: Window, document: document, name: "", location: Location, …}

this within function body

function greet() {
    console.log(this);  
}
greet(); // Window {window: Window, self: Window, document: document, name: "", location: Location, …}

this when new keyword is used

function Game(inTitle) {
    this.title = inTitle;
    console.log(this); // Game {title: "pong"}
}

const game = new Game("pong");

this within callback in a method of an object - no arrow function

const fruits = {
    name: "fruits",
    fruits: ["apple", "mango", "banana"],
    printFruits() {
        console.log(this); // {name: "fruits", fruits: Array(3), printFruits: ƒ}
        this.fruits.forEach(function(fruit) {
            console.log(this); // Window {window: Window, self: Window, document: document, name: "", location: Location, …} * 3 times
        })
    }
}

fruits.printFruits();

this within callback in a method of an object - arrow function

const fruits = {
    name: "fruits",
    fruits: ["apple", "mango", "banana"],
    printFruits() {
        console.log(this); // {name: "fruits", fruits: Array(3), printFruits: ƒ}
        this.fruits.forEach((fruit) => {
            console.log(this); // {name: "fruits", fruits: Array(3), printFruits: ƒ} * 3 times
        })
    }
}

fruits.printFruits();