Function should return type of a not-null parameter

user_1065780 0
Viewed 229

I have a function that accepts x and y. One of the coordinates is always null. Another one is always number. The function returns the value of a not-null coordinate: either x or y. The function always returns a number.

function test({ x, y }: { x: null; y: number } | { x: number; y: null }): number {
  if (x === null) {
    // y should be recognized as 'number', but it is 'number | null'
    return y; // ERROR
  }
  return x;
}


test({x: null, y: 42 }); // 42

TypeScript Playground

Is it possible to keep the return type of this function always as a number?

typescript
2 Answers

If you are sure about your requirements you can try the following:

function test({ x, y }: { x: null; y: number } | { x: number; y: null }): number {
  return x === null && y !== null ? y : x!;
}

! is the so called Non-null assertion operator:

A new ! post-fix expression operator may be used to assert that its operand is non-null and non-undefined in contexts where the type checker is unable to conclude that fact.

user_24366550

Currently, TypeScript infers number | null for both x and y as it won't take into account any dependencies between them that follow from the union type. You simplify the problem a bit by not destructuring the argument object, which will allow TypeScript to identify it as a discriminating union and narrow its type based on checks on x (or y):

function test(arg: { x: null; y: number } | { x: number; y: null }): number {
  if (arg.x === null) {
    return arg.y;
  }
  return arg.x;
}

TypeScript playground

user_57701320
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