Mordern JavaScript

JavaScript.js

hola

Symbol

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prototypal chaining

let arr = [1,2,3,4];
let obj = {}
let fn = ()=>2;
console.log(arr.__proto__.__proto__)
console.log(obj.__proto__)
console.log(fn.__proto__.__proto__)
console.log(Function.prototype.__proto__)
console.log(Array.prototype.__proto__)
console.log(Object.prototype)

All is same

console

Proxy in JavaScript

It basically wraps our object and then pass the handler to it to override the existing method

Promise Catching at the place

let p1 = new Promise((resolve, reject) => {
    resolve("done1")
});

let p2 =  new Promise((resolve, reject) => {
    resolve("done2");
})

let p3 =  new Promise((resolve, reject) => {
    reject("reject3")
}).catch((error) => { // catching error directly here so that in Promise.all it is considered as successs
    return undefined;    
});

Promise.all([p1,p2,p3]).then(res=>{
    console.log({res});  // { res: [ 'done1', 'done2', undefined ] } 
    
}).catch((error) => {
    console.log({error});
})

what is output

var foo = 'bim'
var getFoo = function () { return foo }
foo = 'boum'
console.log(getFoo());  // boum

The answer is ‘boum’ because foo is changed to ‘boum’ on line 3, and function getFoo will return a variable foo within its scope.

Output

In the following code, I expected both a and b to be 3. However, a is undefined and b is 3. Why?

(function(){
var a = b = 3;
})();

console.log(typeof a);//"undefined"
console.log(b);//3

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Actually it is

var a = 3;
b = 3

b = 3 is an assignment without a declaration. Since there is no b in the current execution context (the IIFE), JavaScript engine looks for b in the outer lexical environment. In this case, the outer environment is the global execution context.
Since b is not found in the global execution context either, and assuming non-strict mode, JavaScript implicitly creates b as a property of the global object (e.g., window.b in browsers). This makes b a global variable.

Destructuring

Prototype

class Hola{
    constructor(){
        this.name="yogendra";
        console.log(this);
    }

    static div(){
        console.log("div");
    }

    sum(){
        console.log("sum");
    }
}

const h = new Hola();

console.log(Object.getPrototypeOf(h))   // {constructor: ƒ, sum: ƒ}

console.log(h.__proto__)   // {constructor: ƒ, sum: ƒ}

console.log(Hola.prototype); // {constructor: ƒ, sum: ƒ}

Array.at()

Get the value at that position -> index passed
const arr = ["jack","yogi","abc"]
console.log(arr.at(0)) // "jack"
console.log(arr.at(-1)) // "abc"

This keyword

Akshay -> https://www.youtube.com/watch?v=9T4z98JcHR0&list=PLlasXeu85E9eWOpw9jxHOQyGMRiBZ60aX&index=9