Behind the Scenes of JavaScript

How it works?

The weird part of ES5 vs ES6+?

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var allows variables to be used in functional and global scopes

let and const uses a concept called block scope

Using the let keyword, javascript simply shadows the global variables if the global variable and the local variable have the same variable name

let name = "dummy"

function hi(){ 
	let age = 90;
	let name = "Harry"
	console.log(name, age) //prints Harry, 90
}

console.log(name) //prints dummy

let name = "nail"
let name = "zyan"

console.log(name) //this thowrs error saying same variable name declared again

But by using var javascript wont throw this error

let name = "harry"
let name = "liam"
console.log(name) //prints liam

Consider this example for strange behavior of var

var name = "louis"
if(name === 'louis'){
	var hobbies = ['football']
	console.log(hobbies) //prints the array ['football']
}

function greet(){
	var age = 24;
	var name = "zyan"
	console.log(name, age) //prints zyan 24
}

console.log(name, hobbies) //prints louis, ['football']

greet()

This means that other than function declared variables, if the variable is declared with var keyword, then it will be considered as a global variable

But in the above code snippet, if you try to create the hobbies variable with let keyword, and execute the script, you will get an error on at console.log(name, hobbies) this line, saying hobbies variable is not defined

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[[JavaScript/Hoisting|Hoisting]]