type checking works with `if` test inside a function body but not via a function call with the same test

Issue

Consider the following MWE:

function print (x: number) {
  console.log(x)
}

function noString (a: number | string) {
  if (typeof a === 'string') {
    throw new Error('error')
  }
}

export function funOne () {
  const a: number | string = 'abc'

  if (typeof a === 'string') {
    throw new Error('error')
  }

  print(a)
}

export function funTwo () {
  const a: number | string = 'abc'

  noString(a)

  print(a)
}

I think funOne and funTwo are essentially the same. However, I got [tsserver 2345] [E] Argument of type 'string' is not assignable to parameter of type 'number'. on the last line in funTwo.

Why does this happen and how should I avoid this?

Solution

This happens because typescript doesn't know that noString is used as a type check. But you can tell typescript about it, for example like this:

function noString(a: number | string): number {
  if (typeof a === 'string') {
    throw new Error('error');
  }
  return a;
}

export function funTwo() {
  const a: number | string = 'abc';
  print(noString(a));
}

Or you could use a type predicate:

function noString(a: string | number): a is number {
    if (typeof a === 'string') {
        throw new Error('error');
    }
    return true;
}

export function funTwo() {
    const a: number | string = 'abc';
    if (noString(a)) {
        print(a);
    }
}

As mentioned in the comments by @Artyer and @futur, you could also use asserts:

function noString(a: string | number): asserts a is number {
    if (typeof a === 'string') {
        throw new Error('error');
    }
}

export function funTwo() {
    const a: number | string = 'abc';
    noString(a)
    print(a);
}

Answered By - Kirill Simonov