An obscure built-in interface in TypeScript is VoidFunction, meant to represent functions that take no parameters and return void. Even more obscure is NewableFunction, which supposedly should represent functions that are "newable" or "constructable," but using this interface can lead to unexpected results.
For instance, when you try to access the parameters, Parameters<VoidFunction> returns []:
// Makes sense
type VoidParams1 = Parameters<VoidFunction>;
// ^? []
However, attempting to use ConstructorParameters with VoidFunction will result in an error since it is not constructible. Strangely, both Parameters and ConstructorParameters do not work with NewableFunction:
//@ts-expect-error Is newable, not a regular function
type CtorParams1 = Parameters<NewableFunction>;
// What?
type CtorParams2 = ConstructorParameters<NewableFunction>;
// Error: ^^^^^^^^^^^^^^^
// Type 'NewableFunction' does not satisfy the constraint 'abstract new (...args: any) => any'.
This behavior becomes even weirder when used as a type within code:
function newfn(f: NewableFunction) {}
// This line is OK
newfn(class {});
// Error on the following line
newfn(Date); // `new Date()`...?
// ^^^^
// Argument of type 'DateConstructor' is not assignable to parameter of type 'NewableFunction'.
// Types of property 'apply' are incompatible.
// ...
// Type 'new () => any' is not assignable to type '(this: any) => any'.
It raises the question of why NewableFunction exists if it cannot be properly utilized as intended. Why isn't there a built-in type like new (...args) => any already available? It's a commonly seen/used pattern. Issue ms/TS#44337 fails to address these concerns...
It appears that NewableFunction only accepts functions that can exclusively be invoked with new. An attempt to use newfn(Date) results in an error, while newfn(Float32Array) does not. But why is this restriction in place? How does this limitation serve any practical purpose?