Ternary return type inferred from parameter failure (Typescript)

Question

After a long time reading questions and looking at the docs I still need help. My problem is quite simple. I want to define a function that returns an array when an array is given as parameter but Typescript never let me return the wanted value.

I simplified the function i want to code here as the error is still the same.

function notWorkingTernary(val: number | number[]) : typeof val extends number[] ? string[] : string{
  if(Array.isArray(val)){
    return val.map(v=>String(v));
  }
  else{
    return String(val);
  }
}

This function is getting the following error at the first return (line 3) : Type 'string[]' is not assignable to type 'string'.ts(2322)

I don't understand why would Typescript want me to return a string type even when the type of val is clearly an array of number here.

If I avoid giving a return type then the return type is always string | string[] and I have to cast the return type myself each time i use the function.

What am I doing wrong here?

Answer 1

You want to use function signature overloading. In your case it should look like this:

function notWorkingTernary(val: number): string;
function notWorkingTernary(val: number[]): string[];

function notWorkingTernary(val: number | number[]): (string | string[]) {
  if(Array.isArray(val)){
    return val.map(v=>String(v));
  }
  else{
    return String(val);
  }
}

Basically, you've got one function with several "type overloads" which are handled in runtime and by declaring types you promise that runtime is compatible with typings.

Typescript playground.