Narrowing and Type Guards

Core idea

TypeScript becomes much more useful when you narrow uncertain values before using them.

Revision points

• typeof, in, and custom predicates all help narrowing.
• Unions stay ergonomic when the branches are modeled clearly.
• Good narrowing reduces non-null assertions.

type Result = { ok: true; data: string } | { ok: false; error: string };

function handle(result: Result) {
  if (result.ok) {
    return result.data.toUpperCase();
  }

  return result.error;
}

What to remember

Push uncertainty to the edges, then narrow aggressively.