Typescript: Infer existence of a property by using a helper function-CodePudding

Given the following interfaces and type:

interface A {
  foo: string;
}

interface B {
  bar: string;
}

type AorB = A | B;

The following works:

function getFooOrBar(obj: AorB): string {
  if ('foo' in obj) {
    return obj.foo;
  }
  return obj.bar
}

But the following won't:

function getFooOrBar(obj: AorB): string {
  if (isA(obj)) {
    return obj.foo;
  }
  return obj.bar;
}

function isA(obj: AorB): boolean{
  return 'foo' in obj;
}

because I guess the compiler is not smart enough to infer the property's existence based on the implementation of a function.

Is there a way to extract 'foo' in obj into a helper function so the compiler can still infer the existence of obj.foo?

CodePudding user response：

You need to use typeguards:

interface A {
    foo: string;
}

interface B {
    bar: string;
}

type AorB = A | B;


function isA(obj: AorB): obj is A { // change is here
    return 'foo' in obj;
}

function getFooOrBar2(obj: AorB): string {
    if (isA(obj)) {
        return obj.foo; // ok
    }
    return obj.bar; // ok
}

Playground