Milan Pantelić
Software Engineering ·TypeScript· 1 min read

Type narrowing in TypeScript

Let the compiler prove what a value is, instead of casting your way past it.

Narrowing is how TypeScript turns a wide type into a specific one inside a branch.

Discriminated unions#

ts
type Result = { ok: true; value: number } | { ok: false; error: string };

function unwrap(r: Result) {
  if (r.ok) return r.value; // narrowed to the first member
  throw new Error(r.error); // narrowed to the second
}