Variadic generic types in TypeScript?

Question

I\'d like to make a function in TypeScript that takes an array of constructor functions and returns a corresponding array of instances. See code below.

Note that th

Answer 1

You can do this in TS3.1 and above using mapped array/tuple types. It's easier to get tuples inferred for rest parameters than it is for array parameters, so I'll show that instead:

function getVariadic<T extends Array<AnyCon>>(...cons: T): {
  [K in keyof T]: T[K] extends AnyCon ? InstanceType<T[K]> : never
};
function getVariadic(...cons: AnyCon[]): any[] {
  return cons.map((c: AnyCon) => new c());
}

let [t2, t3]: [T2, T3] = getVariadic(T2, T3);

This behaves as you expect, I think. Hope that helps. Good luck!

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EDIT: if you really need to do it with an array it's easy to type the function but a little harder to call it in such a way that the order of the array is preserved as a tuple:

function getArray<T extends Array<AnyCon>>(cons: T): {
  [K in keyof T]: T[K] extends AnyCon ? InstanceType<T[K]> : never
};
function getArray(cons: AnyCon[]): any[] {
  return cons.map((c: AnyCon) => new c());
}

// error, getArray([T2, T3]) returns (T2 | T3)[]!
let [t2, t3]: [T2, T3] = getArray([T2, T3]); //