Typescript: conditional return type not working-CodePudding

I have a function whose return type should depend on the provided arguments.

const test = (flag: boolean): typeof flag extends true ? "yes" : "no" => {
  if (flag === true) {
    return "yes"
  } else {
    return "no"
  }
}

Why does TSC raise the error Type '"yes"' is not assignable to type '"no"'.ts(2322) for the line return "yes"?

Using Typescript 4.8.4

CodePudding user response：

Why does TSC raise the error

I believe you need to specify the returned value explicitly.

const test = (flag: boolean): typeof flag extends true ? 'yes' : 'no' =>
   (flag === true ? 'yes' : 'no') as typeof flag extends true ? 'yes' : 'no';

Or either using generics:

const test = <T extends boolean>(flag: T): T extends true ? "yes" : "no" =>
   (flag === true ? "yes" : "no") as T extends true ? "yes" : "no";

CodePudding user response：

For this, you can use a feature called overloading, where you define the possible inputs and outputs for the function.

function test(flag: true): "yes";
function test(flag: false): "no";

function test(flag: true | false): "yes" | "no" {
  return flag === true ? "yes" : "no"
}