Theo's Typesafe Cult•10mo ago

Why the infered type of this function is `string | undefined`?

Why the infered type of this function is string | undefined? How TypeScript is unable to see that the returned value can never be undefined?

export const myFunction = (
  { value1 }: { value1?: string },
  value2?: string
) => {
  if (!value1 && !value2) {
    throw new Error("No value provided.");
  }

  return value1 ?? value2;
};

export const myFunction = (
  { value1 }: { value1?: string },
  value2?: string
) => {
  if (!value1 && !value2) {
    throw new Error("No value provided.");
  }

  return value1 ?? value2;
};

18 Replies

Benjamin•10mo ago

Even a simple version returns string | undefined:

export const myFunction = (value1?: string, value2?: string) => {
  if (value1 == undefined && value2 == undefined) {
    throw new Error("No value provided.");
  }

  return value1 != undefined ? value1 : value2;
};

export const myFunction = (value1?: string, value2?: string) => {
  if (value1 == undefined && value2 == undefined) {
    throw new Error("No value provided.");
  }

  return value1 != undefined ? value1 : value2;
};

Anna | DevMiner•10mo ago

this should be

  if (value1 == undefined || value2 == undefined) {
    throw new Error("No value provided.");
  }

  if (value1 == undefined || value2 == undefined) {
    throw new Error("No value provided.");
  }

then it'd be fine

Benjamin•10mo ago

Nope, you're changing the logic, I only want to throw if neither are defined (= both undefined). That's why TS should know at least one of them is defined and therefore the result is never undefined.

Anna | DevMiner•10mo ago

hmmm

Benjamin•10mo ago

I have to add as string and I hate that 😢

dan•10mo ago

Its one of those cases where you cant get it to be inferred. The way it infers it technically is correct but I think as string is the only way.

Anna | DevMiner•10mo ago

how about

export const myFunction = (value1?: string, value2?: string) => {
  if (value1 !== undefined) return value1;
  if (value2 !== undefined) return value2;

  throw new Error("No value provided.");
};

export const myFunction = (value1?: string, value2?: string) => {
  if (value1 !== undefined) return value1;
  if (value2 !== undefined) return value2;

  throw new Error("No value provided.");
};

Benjamin•10mo ago

It was not exactly my use case, I am calling another function that takes a string as parameter. It works but it means I have to repeat the function call twice. Not ideal but maybe better than as string, I don't know. I guess that's the 2 solutions. ¯\_(ツ)_/¯

Anna | DevMiner•10mo ago

or you just use this as a separate function and then do something like this

export const actualFunction = (value1?: string, value2: string) => doSomeWork(myFunction(value1, value2));

export const actualFunction = (value1?: string, value2: string) => doSomeWork(myFunction(value1, value2));

Benjamin•10mo ago

yeah...

Kasper•10mo ago

export const myFunction = ({ value1 }: { value1?: string }, value2?: string) => {
  if (value1 && !value2) {
    return value1
  }

  if (!value1 && value2) {
    return value2
  }

  if (value1 && value2) {
    return value2
  }

  throw new Error("No value provided."); 
};

export const myFunction = ({ value1 }: { value1?: string }, value2?: string) => {
  if (value1 && !value2) {
    return value1
  }

  if (!value1 && value2) {
    return value2
  }

  if (value1 && value2) {
    return value2
  }

  throw new Error("No value provided."); 
};

Not the best, but yeah

Benjamin•10mo ago

Yeah I think the as string is the most readable 😅

Kasper•10mo ago

Yup

Benjamin•10mo ago

My post purpose was mainly to understand why TypeScript doesn't understand

dan•10mo ago

Because to the type system after the first if check, both are typed as string | undefined. and it cant pickup on the fact that if value1 is undefined, value2 has to be defined.

Kasper•10mo ago

Yeah, because it cant know which of them is not undefined.

Benjamin•10mo ago

but why if I only have one condition (value !== undefined), it manages, but not 2?

Anna | DevMiner•10mo ago

because then it's logical what changed

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Why the infered type of this function is `string | undefined`?