	var	let	const
scope	global	block-level	block-level
initialised	yes	no	no
can be reassigned	yes	yes	no

var

If a var variable is declared in a conditional, it ends up being hoisted to the top of the scope. (Unique to JavaScript!!)

-> if a var is referenced outside the block-level in which it’s defined, it is ‘undefined’ - a let returns an uncaught reference error!

-> good practice to declare them at the top, not inside conditional, to make hoisting clear.

let

When using let, no hoisting takes place, as the declaration is block-level, and the variable is only valid within the block it’s defined in.

-> if a let is referenced outside the block-level in which it’s defined, it throws an exception.

-> for this reason, many devs prefer let to var, as hoisting is unexpected behaviour when coming from other languages => consensus is to use let, unless we have a really good use case for var

This code will not work with let (or const):

function checkPassword(password) {
  const valid = password.length > 6;
  if (valid) {
    var message = "Your password is valid";
  } else {
    var message = "Your password is not valid";
  }
  console.log(message);
}

checkPassword("1234");
checkPassword("1234567");

const

The const keyword makes a variable partly immutable, i.e. it can’t be reassigned completely, but its contents can be changed (e.g. through pushing a new value into an array).

-> the variable is not like constant values in php, or even 100% immutable.

=> immutability can be enforced with Object.freeze([…]), e.g.:

const months = Object.freeze([])

-> const keyword is block-scoped too!

When do you use const, when do you use let?

Many companies: default to using const. Change to let if you decide to change variable later on.

Others advise defaulting to var, as const indicates that you are intending immutability, plus “const” is two characters more :-) Plus, it’s premature optimisation…

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