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Variables

Any type of value can be stored in any variable.

If your JavaScript environment supports the const or let keywords (modern browsers, Node.js, TypeScript), you should use them. const makes it so a variable can only be assigned once, at the time it is declared. let allows you to assign to the variable later, but it uses "block scoping" instead of "function scoping", which is a really good thing.

const

Prefer const. It has block scoping and encourages functional code.

let

Use let instead of var. It has block scoping and works like expected.

var

It has function scoping and tends to crawl out of its hole.

Example

hoistedToTop = 'can you believe this works?'

let input = `something a user typed in`

input = 0.66322 // not type checked at all, ever

const hostname = `example.com`     // look ma,
const url = `https://${hostname}/` // functional code!

var hoistedToTop

input = null // best way to clear a variable, if needed