Typescript conditional typings strange error case

user_2116361 0
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Conditional typing in Typescript is not very intuitive. The code bellow returns an error in the Typescript playground. Can someone explain why since this seems like a perfect easy example for the conditional typings.

function process<T extends string | null>(text: T): T extends string ? string : null {
  return text;
}

//Type 'T' is not assignable to type 'T extends string ? string : null'.
//Type 'string | null' is not assignable to type 'T extends string ? string : null'.
//Type 'null' is not assignable to type 'T extends string ? string : null'.(2322)
typescript
1 Answers

Typescript is not able to narrow the return type of a function with conditional types, which has parameters defined as a union.

function process<T extends string | null>(text: T): T extends string ? string : null {
  // the problem here is that type guard can't narrow over the type parameter
  if (typeof text === 'string') {
      return text; // error
  }

  return null; // error
}

The best you can do is to return text as any; and go on with your life. The alternative here is to use overloads to accomplish what you want.

user_7112060
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