typescriptvoid

Why does () => void return something?


I know that below does NOT mean the return 'type' is void. What I understood is that voidFunc returns nothing since it is returning 'void'. Why does it return any type?

type voidFunc = () => void

const myFunc: voidFunc = () => {
  return 'hello'
}

How is it different from writing it as below? type voidFunc = () => any


Solution

  • See Assignability of Functions

    Return type void

    The void return type for functions can produce some unusual, but expected behavior.

    Contextual typing with a return type of void does not force functions to not return something. Another way to say this is a contextual function type with a void return type (type vf = () => void), when implemented, can return any other value, but it will be ignored.

    Thus, the following implementations of the type () => void are valid:

    type voidFunc = () => void;
     
    const f1: voidFunc = () => {
      return true;
    };
     
    const f2: voidFunc = () => true;
     
    const f3: voidFunc = function () {
      return true;
    };
    

    And when the return value of one of these functions is assigned to another variable, it will retain the type of void:

    const v1 = f1();
     
    const v2 = f2();
     
    const v3 = f3();