Suppressing the "not all paths return a value" Typescript warning when a code path is unreachable

Question

I have a function foobar(x:string) which will not return if x is the empty string. Otherwise, it return another string.

So I can't declare it to return "never", and if I declare it to return "string", TS doesn't realise the code afterwards may not be reached

(Yes, this is a horrid design. No, I can't change it)

Sometimes, I need to use it with an empty x in another function which in other branches returns a value. So something like

function f(n:number) 
{

    if (n==0) {
        foobar("")
    }
    else
    {
        return n - 1
    }

}

TS complains because not all paths return a value. So I would like to tell the compiler that foobar does not return (or, equivalently the code after it is unreachable). Making something up, something like:

function f(n:number) 
{

    if (n==0) {
        foobar("")
        /** @notreachable */
    }
    else
    {
        return n - 1
    }

}

Can I do this? What else can I do? I could return something of type never, maybe, but how do I get a never "value" to return?

Answer 1

Have you tried function overloads:

function foobar(val: string): string;
function foobar(val: ''): never;
function foobar(val: string): string {
  if (val === '') {
    // while (true) {
    //   console.log('Infinite loop also works fine');
    // }
    throw new Error('never returns');
  } else {
    return 'new value';
  }
}


function f(n: number) {
  if (n === 0) {
    foobar('');
  } else {
    return n - 1;
  }
}

Update I think similar approach will work for empty objects as well:

interface NoProps {
  [a: string]: never;
}

interface X {
  a?: string;
}

function foobar(val: NoProps): never;
function foobar(val: X): string;
function foobar(val: NoProps | X): string {
  if (val.a === undefined) {
    throw new Error('never returns');
  } else {
    return 'new value';
  }
}


function f(n: number) {
  if (n === 0) {
    foobar({});
  } else {
    return n - 1;
  }
}