没有 null 或 undefined 的 Typescript 函数返回值

Question

I'm trying to write a function which returns a value or if the value is null or undefined it should return a default value.

function test<A, B>(input: A, fallbackValue: B): NonNullable<A> | B {
 if (input == null || input == undefined) {
   return fallbackValue;
 } else {
   return input;
 }
}

I get the error

Type 'A' is not assignable to type 'B | NonNullable<A>'.
  Type 'A' is not assignable to type 'NonNullable<A>'.

NonNullable should be A without null or undefined and that's what I checked in the if?

Here is the code in ts playground.

Answer 1

Conditional type (of which NonNullable is) usually don't provide a good implementation in generic functions. You could use a type assertion to get it to work ( return input as any );

A safer approach might be to switch up the types a bit:

function test<A, B>(input: A | undefined | null, fallbackValue: B): A | B {
    if (input == null || input == undefined) {
        return fallbackValue;
    } else {
        return input;
    }
}

declare var s: string | null;
let a = test(s, "") // string