I have the following code:

type allPossibileTypes = 'my-first-type' | 'my-second-type' | 'my-third-type';
type genericFunction = (innerVariable: any) => void;

class A {
   doSomethingWith(inputType: 'my-first-type', callback: (innerVariable: string) => void);
   doSomethingWith(inputType: allPossibileTypes, callback: genericFunction);
   doSomethingWith(inputType: string, callback: any) {
      // function logic here
   }
}

If I try to call the function like this:

AInstance.doSomething('my-first-type', ...);

I expect that typescript will hint me to the (innerVariable: string) => void for the second parameter, yet instead, it hints me to the genericFunction type. Until now, I thought that when picking type hints, typescript will choose the most specific implementation that it can use, am I mistaken in this?

If yes, how could I achieve to define a function, which allows me to use as the first parameter, only specific strings, and on some concrete occasions, based on the first parameter, give me additional type hints?

CodePudding user response：

You need to exclude 'my-first-type' from allPossibileTypes in your overloads:

type allPossibileTypes = 'my-first-type' | 'my-second-type' | 'my-third-type';
type genericFunction = (innerVariable: any) => void;

class A {
    doSomethingWith(inputType: 'my-first-type', callback: (innerVariable: string) => void): void
    doSomethingWith(inputType: Exclude<allPossibileTypes, 'my-first-type'>, callback: genericFunction): void
    doSomethingWith(inputType: string, callback: any) {
        // function logic here
    }
}

new A().doSomethingWith('my-first-type', (a: string) => void 0); // ok
new A().doSomethingWith('my-first-type', (a: number) => void 0) // error

Playground

Without Exclude both versions with number and string are valid

P.S.

interface test { prop1: string, prop2: string };

type ToBool<T> = {
  [Prop in keyof T]: boolean
}

type Result = ToBool<test>