TypeScript 3.7 Beta(devblogs.microsoft.com)

    const types = ['x', 'y', 'z'] as const;
    const a: typeof types[number] = 'a'; // not ok
    const b: typeof types[number] = 'z'; // ok

    const dir = "up"; // TS infers type "string"
    test(dir); // error

    const dir = "up" as const;
    test(dir); // works

  if (typeof num === 'number') {
    assertIsInRange(num);
  }

  function something(num : number) {
    assertIsInRange(num);

    return num + 1;
  }

  function assertIsInRange(num: number, msg?: string): asserts num {
    if (num < 50 || num > 100) {
      throw new Error('not in range');
    }
  }

  something(50);

  function something(num) {
    assertIsInRange(num);
    return num + 1;
  }
  function assertIsInRange(num, msg) {
    if (num < 50 || num > 100) {
      throw new Error('not in range');
    }
  }
  something(50);

    if (typeof x === "number")
         throw new Error();