Narrowing with if + typeof

What is narrowing

A parameter can arrive as many types. Narrowing is splitting your code into branches per type so each branch only handles ONE shape.

function format(x) {
  if (typeof x === "number") return x.toFixed(2);
  if (typeof x === "string") return x.toUpperCase();
  if (typeof x === "boolean") return x ? "yes" : "no";
  return "unknown";
}

Inside each if branch the value is guaranteed to be that type. Editors with TypeScript-aware tooling (VS Code's JS server, JetBrains) will auto-complete the right methods inside each branch — no .ts file required.

The "fall through" guard

Always end with a default branch that handles "everything else". It's documentation: "I thought about other types, here's what happens."