If the input meets the requirements of the generic type, then it will be returned; otherwise, it

Currently, I am working on creating a filter function that takes a generic parameter and ensures that the return type of the given function is either the input if it matches the generic or null if it does not.

type A = { type: 'A' };
type B = { type: 'B' };

type Union = A | B;
const filter = <T extends Union>(filter: <E extends Union>(item: E) => E extends T ? E : null) => {
  // something
};

filter<A>((item) => { 
// Argument of type '<E extends Union>(item: E) => E | null' is not assignable to parameter of type '<E extends Union>(item: E) => E extends A ? E : null'. 
// Type 'E | null' is not assignable to type 'E extends A ? E : null'. 
// Type 'null' is not assignable to type 'E extends A ? E : null'.(2345)
  if (item.type === 'A') return item;

  return null;
});

I am perplexed as to why this code snippet is not functioning as intended. The error message seems misleading, as I believe TypeScript fails to recognize that 

if (item.type === 'A') return item;
actually satisfies the E extends A ? E condition?

One possible alternative could be using a type guard, although this approach would not guarantee that all types of T will be returned.

Typescript playground

typescript

Answer №1

I'm finding it puzzling why the return type of the filter function needs to be E extends T ? E : null. It seems there's a condition within the return type, and I'm uncertain if that's permissible.

This version is more straightforward and compiles successfully:

type A = { type: 'A' };
type B = { type: 'B' };

type Union = A | B;
const filter = (filter: <E extends Union>(item: E) => E | null) => {
  // something
};



filter((item) => {
  if (item.type === 'A') return item;

  return null;
});

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