Type Narrowing

Narrow with typeof

After a typeof check, TypeScript treats the value as that type inside the branch.

function padLeft(value: string | number) {
  if (typeof value === "number") {
    return " ".repeat(value);
  }
  return value;
}

console.log(padLeft(3) + "hi");

Narrow null with equality

Check for null or undefined before reading properties.

function lengthOf(text: string | null): number {
  if (text === null) {
    return 0;
  }
  return text.length;
}

Narrow objects with in

The in operator checks for a property and narrows custom unions.

type Success = { ok: true; data: string };
type Failure = { ok: false; error: string };

function message(result: Success | Failure) {
  if (result.ok) {
    return result.data;
  }
  return result.error;
}

Keep narrowing honest

• Write real runtime checks; types erase at compile time
• Use else branches so every case is handled
• Prefer early returns for simple guards
• Discriminated unions with a shared tag field scale well