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Type guards: isDefined, isString, isOneOf

Write three checks after which TypeScript narrows the value's type.

isDefined(value): not null and not undefined; [1, null, 2].filter(isDefined) should give number[], and 0 and "" are values too. isString(value): a string. isOneOf(values) returns a check for one of the listed values: after isOneOf(["red", "green"] as const)(c) the type of c is "red" | "green".

function isDefined(value: unknown): boolean {
  // ...
  return false;
}

function isString(value: unknown): boolean {
  // ...
  return false;
}

function isOneOf(values: readonly unknown[]): (value: unknown) => boolean {
  // ...
  return () => false;
}

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Tests (6)

Type tests are checked by the TypeScript compiler in strict mode. Expect<Equal<A, B>> passes when the types match exactly, and // @ts-expect-error passes when the next line does not compile. The first run downloads the compiler, so it takes a few seconds.

types

const xs = [1, null, 2, undefined].filter(isDefined);
type _ = Expect<Equal<typeof xs, number[]>>;

types

const v: unknown = 'x';
if (isString(v)) {
  type _ = Expect<Equal<typeof v, string>>;
}

types

const isColor = isOneOf(['red', 'green'] as const);
const c: unknown = 'red';
if (isColor(c)) {
  type _ = Expect<Equal<typeof c, 'red' | 'green'>>;
}

expect([1, null, 2, undefined, 0, ''].filter(isDefined)).toEqual([1, 2, 0, '']);

expect(isString('a')).toBe(true);
expect(isString(1)).toBe(false);
expect(isString(null)).toBe(false);

const isColor = isOneOf(['red', 'green']);
expect(isColor('red')).toBe(true);
expect(isColor('blue')).toBe(false);