coming-from-javascript.md

🟨 Coming from JavaScript / TypeScript?

You've been shipping undefined is not a function to production for years, and you've made peace with it. F# offers you something radical: a language where the type system is actually on your side, where undefined is not a concept, and where hot reload is so fast it feels like cheating.

Pain you're leaving behind: node_modules eating your disk, any creep in TypeScript, undefined vs null vs "" vs 0 all being falsy, and webpack rebuilds that take longer than your lunch break.

What you'll love immediately:

• Option<'T> means "might not exist" — compiler-enforced, no runtime surprise
• |> pipelines are .filter().map().reduce() but for any function, not just array methods
• No this binding bugs — functions are just functions
• Fable compiles F# to clean JavaScript — the SageFs VS Code extension is F# all the way down

→ Start here: samples/from-javascript/hello.fsx

// JS/TS: type Shape = { kind: "circle"; r: number } | { kind: "rect"; w: number; h: number }
// F# (compiler checks exhaustiveness — no forgotten cases at runtime):
type Shape =
  | Circle    of radius: float
  | Rectangle of width: float * height: float

let area = function
  | Circle r          -> System.Math.PI * r * r
  | Rectangle (w, h) -> w * h
// Forget the Rectangle case? Warning. Add Triangle without updating area? Warning.