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Use conditional type to narrow return type of function with a union-typed argument #24929

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@aneilbaboo

Description

@aneilbaboo

Search Terms

conditional return type narrowing generics

Suggestion

It seems like it should be possible to constrain the return type of a function using a conditional expression:

For example,

// I want my function to have a different return type depending on input
export function foo<T extends string|number>(
  val: T                                  // input type is a union
): T extends string ? string : number {   // output type is either string or number
  return val;
} 
// expected this to work, but received:
// Type 'T' is not assignable to type 'T extends string ? string : number'.
//  Type 'string | number' is not assignable to type 'T extends string ? string : number'.
//    Type 'string' is not assignable to type 'T extends string ? string : number'.

Use Cases

A common use case is a function which can process single objects or arrays of objects, and returns single objects or an array, respectively.

Examples

// capitalize a string or each element of an array of strings
function capitalize<T extends string | string[]>(
  input: T
): T extends string[] ? string[] : string {
  if (isString(input)) {
    return input[0].toUpperCase() + input.slice(1);
  } else {
	return input.map(elt => capitalize(elt));
  }
}

Checklist

My suggestion meets these guidelines:

  • This wouldn't be a breaking change in existing TypeScript / JavaScript code
  • This wouldn't change the runtime behavior of existing JavaScript code
  • This could be implemented without emitting different JS based on the types of the expressions
  • This isn't a runtime feature (e.g. new expression-level syntax)

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