how come () => void is a subtype of () => a if void isn't a subtype of a and a isn't subtype of void?

[zpdDG4gta8XKpMCd]

declare var a: () => void;
declare var b: () => PromiseLike<void>;

a = b; // <-- why!!!
b = a;


declare var c: void;
declare var d: PromiseLike<void>;

c = d;
d = c;

[vladima]

tl;dr; the assumption is that it is safe to assign something that returns some value to something that returns void so result is ignored, however the opposite is not true.

see 3.11.3 Subtypes and Supertypes

• S is an object type, an intersection type, an enum type, or the Number, Boolean, or String primitive type, T is an object type, and for each member M in T, one of the following is true:
  • M is a property and S has an apparent property N where
    • M and N have the same name,
    • the type of N is a subtype of that of M,
    • if M is a required property, N is also a required property, and
    • M and N are both public, M and N are both private and originate in the same declaration, M and N are both protected and originate in the same declaration, or M is protected and N is declared in a class derived from the class in which M is declared.
  • M is a non-specialized call or construct signature and S has an apparent call or construct signature N where, when M and N are instantiated using type Any as the type argument for all type parameters declared by M and N (if any),
    • the signatures are of the same kind (call or construct),
    • M has a rest parameter or the number of non-optional parameters in N is less than or equal to the total number of parameters in M,
    • for parameter positions that are present in both signatures, each parameter type in N is a subtype or supertype of the corresponding parameter type in M, and
    • the result type of M is Void, or the result type of N is a subtype of that of M.

[zpdDG4gta8XKpMCd]

it's not a safe assumption at all, ugh....

[zpdDG4gta8XKpMCd]

I have a function that returns a promise: () => Promise<void>, it is passed around as a callback, after refactoring unintentionally it end up being an argument of a parameter of type () => void. Right here I need the compiler to help me and say it can't be used there, because Promise<void> just can't be treated the same way as void, and what does compiler do? nothing, exactly

[zpdDG4gta8XKpMCd]

anything that has to be serialized can't be passed round safely because there is a chance it will get swallowed by () => void and then end up being passed to the standard JSON.stringify which will then crash

function sendMessageWhenReadyNoValuesIntended(doMyStuff: () => void) {
    window.postMessage(JSON.stringify({ message: doMyStuff() }));
}
interface RandomCrazyObject {
   self: RandomCrazyObject;
}

function processMyInput() : RandomCrazyObject {
    let result = <RandomCrazyObject>{ };
    result.self = result;
    return result;
}
sendMessageWhenReadyNoValuesIntended(processMyInput())

[RyanCavanaugh]

The alternative is much worse -- imagine not being able to write this code:

let arr = [1, 2];
// Error, can't convert 'number' return type to 'void'
[1, 2, 3].forEach(x => arr.push(x));

[zpdDG4gta8XKpMCd]

much worse, I agree, you sound more convincing than ever

[1, 2, 3].forEach(x => { arr.push(x); });

[RyanCavanaugh]

If that's sarcasm I'm missing it 😛

[zpdDG4gta8XKpMCd]

not much sarcasm in 3 extra characters