Discussion on: The Difference Between Null and Undefined in JavaScript

Note that var and let keywords let you make undefined but const does not

let x

// syntax error
const y

For bonus points, add a fallback for undefined or null or falsy (zero, empty string)

let x

const y = x || "default"
console.log(y.toUpperCase())

Or in newer versions of JS, you can use this to handle null and undefined, but it won't replace zero or empty string.

let y

const x = y ?? "default"

michaelcurrin.github.io/dev-cheats...

TypeScript can enforce missing values for you so you don't have to throw errors or use a fallback.