Printing object value after deleting it

const obj = {
  lang: "js"
};
let x = Object.create(obj);
delete x.lang;
console.log(x.lang) // "js"

Explanation

This is beacause of the act of "Object.create" it stores the values of the object in the prototype (proto) and the delete keyword does not delete anything from the prototype (proto)

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Printing undefined instead of error

console.log(x) // undefined

var x = 10;

Explanation

This is called hoisting this code will be compiled and be like this

var x;

console.log(x) // undefined

x = 10;

This only works with var keyword and does not work for let and const, so make that in mind. this works also with functions for example

sayHello();

function sayHello() {
console.log("Hello, World!")
}

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Making self calling function

(function (lang) {
  console.log(`Hello, ${lang}!`)
})("javascript") // Hello, javascript!

Explanation

Yes you can do this. this is called IIFE it stands for immediately invoked function expression

Use === instead of ==

[10] === 10    // is false
[10]  == 10    // is true
'10' == 10     // is true
'10' === 10    // is false
 []   == 0     // is true
 [] ===  0     // is false
 '' == false   // is true but true == "a" is false
 '' ===   false // is false 

Explanation

The == (or !=) operator performs an automatic type conversion if needed. The === (or !==) operator will not perform any conversion. It compares the value and the type, which could be considered faster than ==.