angular - Typescript: Get Exclusive Members from Union Class Type

Question

How do I get the Exclusive Member from a Union Typescript?

selectedQueueItems: Array< TestA | TestB > = [];

TestA has a class member called Food, that TestB does not have. However most of the other class members are similar between each.

Receiving Error:

Property 'Food' does not exist on type 'TestA | TestB'.

Property 'Food' does not exist on type 'TestB'

Currently working with existing design in our code base.

Answer 1

You'd need a runtime check to ensure the method you are calling exists.

In this case, you would check to see if each object is an instance of the class that you know has a Food member:

for (const item of selectedQueueItems) {
  if (item instanceof TestA) {
    console.log(item.Food)
  }
}

If the objects are created with new TestA() or new TestB() then this will work fine. If you are creating those object other ways, then you would need to test that in different ways. That really depends on what those objects are and how you build them. But it may involve discriminated unions.