user_defined_type_guard.ts

// https://github.com/denoland/deno_doc/pull/105
export function f1(val1: A | B): val1 is A {}
export function f2(val2: any): asserts val2 is string {}
export function f3(val3: any): asserts val3 {}
export function assertIsDefined<T>(val4: T): asserts val4 is NonNullable<T> {
  if (val === undefined || val === null) {
    throw new AssertionError(
      `Expected 'val' to be defined, but received ${val}`
    );
  }
}
export class C {
  isSomething(): this is Something {
    return this instanceof Something;
  }
}

export type A = string | number;