var

var should be avoided for initalising variables, since those variables are hoisted (Hoisting) to the nearest function scope, or else globally. This brings with it a few problems:

• The variable can technically be used before its initialisation in the code. e.g. the following is completely valid. Trying to set somethingElse to the value of an undefined variable won’t cause an issue because JavaScript implicitly pulls any var declarations to the top of the containing function and sets them to undefined

function myFunc() {
	let somethingElse = myVar;

	if (1 == 1) {
		var myVar = 'a string'
	}
}

• The variable can be used and reassigned from anywhere within a function, since var variables are not block-scoped. For example in the following snippet, myVar is accessible to the console log function despite being in a separate block

function myFunc() {
	{
		var myVar = 'hello world'
	}
	
	console.log(myVar) // 'hello world'
}

let

let allows the value of the variable to be changed. The variable is block-scoped, so won’t be accessible outside of its containing { } block

const

const will not allow the value of the variable to be changed. A variable defined with const is also block-scoped

Hoisting

JavaScript Hoisting ****refers to the process whereby the interpreter appears to move the declaration of functions, variables or classes to the top of their scope, prior to execution of the code