Tips on assigning array union as the return type of a function

I am working with a function parameter that accepts an array union, like this: (ClassA|ClassB)[].
How can I return either ClassA[] or ClassB[] from the function?

When attempting to return type (ClassA|ClassB)[], I encounter the following error:

Assigned expression type (ClassA|ClassB)[] is not compatible with type ClassA[].

classA: ClassA[];
classB: ClassB[];

this.classA = this.function(this.classA, classAObject);
this.classB = this.function(this.classB, classBObject);

function(array: (ClassA|ClassB)[], item: ClassA|ClassB): any {
   // some code...
   return array;
}
typescript

Answer №1

collection: ClassA[] | ClassB[]

Please make sure to specify if it is either a collection of ClassA OR a collection of ClassB, not a mix of items that could be either ClassA or ClassB.

Answer №2

Implementing function overloading can help achieve the desired type signature:

class X {
  x() {}
}
class Y {
  y() {}
}

const x = new X();
const y = new Y();

const arrayOfX: X[] = [];
const arrayOfY: Y[] = [];

function performAction(array: X[], item: X): X[];
function performAction(array: Y[], item: Y): Y[];
function performAction(array: X[] | Y[],item: X | Y): X[] | Y[] {
  return array;
}

const resultOfX = performAction(arrayOfX, x);
const resultOfY = performAction(arrayOfY, y);

It can be observed that resultOfX is inferred to be of type X[], and resultOfY as type Y[].

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