Type Narrowing & Casting

When a value has a union type, TypeScript narrows it inside checks that rule types out:

function len(x: string | string[]) { if (typeof x === "string") return x.length; // x: string here return x.join("").length; // x: string[] here }

Narrowing follows your control flow: if/else, switch, early return, &&, ||, ternaries.