coming-from-javascript.md

🟨 Coming from JavaScript / TypeScript?

F# catches undefined is not a function at compile time, because there is no undefined in the language to begin with. And eval is fast enough that it feels like the browser console, not a build step.

You leave behind node_modules eating your disk, any creep in TypeScript, the undefined/null/""/0 falsy mess, and webpack rebuilds that outlast your lunch break.

What you'll notice right away:

• Option<'T> means "might not exist," and the compiler makes you handle it. No runtime surprise three functions later.
• |> pipelines work like .filter().map().reduce(), but for any function, not just array methods.
• No this binding bugs. Functions are just functions.
• Fable compiles F# to JavaScript. The SageFs VS Code extension itself is written in F# and shipped through Fable, so this is a real, working example.

→ 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.