typeof type guards

typeof type guards

Let’s go back and write the code for the version of padLeft that uses union types. We could write it with type predicates as follows:

function isNumber(x: any): x is number {
  return typeof x === "number";
}

function isString(x: any): x is string {
  return typeof x === "string";
}

function padLeft(value: string, padding: string | number) {
  if (isNumber(padding)) {
    return Array(padding + 1).join(" ") + value;
  }
  if (isString(padding)) {
    return padding + value;
  }
  throw new Error(`Expected string or number, got '${padding}'.`);
}

However, having to define a function to figure out if a type is a primitive is kind of a pain. Luckily, you don’t need to abstract typeof x === "number" into its own function because TypeScript will recognize it as a type guard on its own. That means we could just write these checks inline.

function padLeft(value: string, padding: string | number) {
  if (typeof padding === "number") {
    return Array(padding + 1).join(" ") + value;
  }
  if (typeof padding === "string") {
    return padding + value;
  }
  throw new Error(`Expected string or number, got '${padding}'.`);
}

These typeof type guards are recognized in two different forms: typeof v === "typename" and typeof v !== "typename", where "typename" must be "number", "string", "boolean", or "symbol". While TypeScript won’t stop you from comparing to other strings, the language won’t recognize those expressions as type guards.