[SOLVED] Getting Typescript strictNullChecks to work with undefined returning vanilla js functions

Getting Typescript strictNullChecks to work with undefined returning vanilla js functions

With idiomatic js returning undefined on error, converted to TS

function multiply(foo: number | undefined){
   if (typeof foo !== "number"){
      return;
   };
   return 5 * foo;
}

When using multiply in new TS code i get the problem of the compiler believing doStuff can return undefined, when it cannot.

So i tried to write an "unsafe" version of this function called by safe TS code, leaving the safe version for regular js code.

function unsafeMultiply(num: number){
   return multiply(num);
}

Since unsafeMultiply can only accept a number, the type guard in multiply should consider that multiply will only return a number since unsafeMultiply can only process number. If this is too complicated for the compiler, how do i force him to accept i know what i'm doing ?

Solution

When using multiply in new TS code i get the problem of the compiler believing doStuff can return undefined, when it cannot.

Yes, it can: multiply(undefined) returns undefined.

If this is too complicated for the compiler, how do i force him to accept i know what i'm doing ?

You can do a type assertion, since you know that multiply will only return undefined if it is called with a non-number:

function unsafeMultiply(num: number) {
   return multiply(num) as number;
}

Or you can add code for a type guard at runtime:

function unsafeMultiply(num: number) {
  let result = multiply(num);
  if (typeof result === "undefined") {
    throw new Error("Invalid num argument");
  }
  return result;
}

But if it were me, I'd make the multiply function fail or return NaN rather than returning undefined, if given undefined. Then unsafeMultiply wouldn't be needed.