I am writing mocha tests.

I have 2 interfaces(TestInterface1, TestInterface2). They have functions described on it such as:

• TestInterface1 - Func1, Func2
• TestInterface2 - Func1, Func2, Func3

In my tests, I have a class Test that sometimes ends up to be of TestInterface1 but might still have a function Func3. It's like in run-time, can't decide which interface it belongs to.

Let's say it ended up to be TestInterface1. Now, it still could have Func3, but because of the interface, when I call function, it says that Function doesn't exist even though it does. How can I still figure out that Func3 still exists on the Test which is of TestInterface1 ?

CodePudding user response：

// ------
// stub declaration - you have defined those interfaces already
interface TestInterface1 {
  Func1: Function
  Func2: Function
}

interface TestInterface2 {
  Func1: Function
  Func2: Function
  Func3: Function
}

// assuming you have value already
declare const yourInterface: TestInterface1 | TestInterface2;
// ------

// and the IMPORTANT PART:
if ("Func3" in yourInterface) {
    // in this block, yourInterface is coerced to TestInterface2
} else {
    // coerced to TestInterface1
}

CodePudding user response：

If (as stated in your question) you have already narrowed the type of the instance to be TestInterface1, then you'll need to assert that the Func3 property is a function type in order to use it, because the compiler doesn't know it exists.

You can do this using a type predicate, like in the commented example below:

TS Playground

// Because you didn't supply the type of your function,
// I'll provide a template for a generic one here:
type Fn<
  Params extends readonly unknown[] = any[],
  Result = any,
> = (...params: Params) => Result;

interface TestInterface1 {
  Func1: Fn;
  Func2: Fn;
}

interface TestInterface2 extends TestInterface1 {
  Func3: Fn;
}

function hasFunc3 <T extends TestInterface1>(instance: T): instance is T & Pick<TestInterface2, 'Func3'> {
  return typeof (instance as any).Func3 === 'function';
}

// Your question states that the instance is already narrowed to TestInterface1:
declare const instance: TestInterface1;

// Expected error:
instance.Func3(); /*
         ^^^^^
Property 'Func3' does not exist on type 'TestInterface1'. Did you mean 'Func1'?(2551) */

// Use the type predicate:
if (hasFunc3(instance)) {
  instance.Func3(); // ok
}