Advanced type validation using a factory method and pre-defined input parameters

Apologies for the vague title, I'm struggling to find the right words to explain this:

I have two interfaces A and B:

interface A {
    prop1: string
    prop2: object
}

interface B {
    prop3: number
}

There is also interface C, which extends A:

interface C extends A {
    prop2: {
        objectProperty: string
    }
}

Next, I have a function that returns objects of type A:

function func1(arg1: string, arg2: object): A {
    return {
        prop1: arg1,
        prop2: arg2
    }
}

And another function that returns objects of type B:

function func2(arg1: number): B {
    return {
        prop3: arg1
    }
}

Finally, there is a function that should return objects of type C or B, created using the previous two functions:

function func3(): C | B {
    if(...) {
        return func1('some string', { objectProperty: 'another string' });
    } else {
        return func2(100);
    }
}

My goal is to ensure that the returned value from the first branch aligns with type C and receive a warning if it doesn't. For instance:

        return func1('some string', { someOtherProperty: 'another string' });

would trigger a warning as someOtherProperty is not part of type C.

I hope this explanation makes sense... I intend to streamline the process of constructing different types extending A with a single factory method while enforcing the correct output. It might seem like overkill, but I believe there should be a way to achieve this?

Answer №1

Modifying the sample code to emphasize the issue, we encounter an interface A with two different extensions: B and C, where one property from A is narrowed:

interface A {
    prop1: string
    prop2: object
}

interface B extends A {
    prop2: {
        bProp: string;
    }
}

interface C extends A {
    prop2: {
        cProp: string
    }
}

The function makeA takes parameters corresponding to prop1 and prop2 of A and returns a value of type A:

function makeA(arg1: string, arg2: object): A {
    return {
        prop1: arg1,
        prop2: arg2
    }
}

However, when attempting to create strongly typed instances of B and C using this function, errors occur:

function makeBOrC(): B | C {
    if (Math.random() < 0.5) {
        return makeA('', { bProp: '' }); // error! A is not assignable to B | C
    } else {
        return makeA('', { cProp: '' }); // error! A is not assignable to B | C
    }
}

This issue arises because the compiler perceives the return type of makeA() as only A. The type of the argument passed in for the second parameter gets widened to object, which leads to these conflicts with B and C.


To address this problem, we have two potential solutions:

Option #1: Leveraging Type Assertions

In circumstances where the compiler lacks information about the type of a value, type assertions can be employed to specify a certain type. While using type assertions invokes manual verification, it can help in resolving issues like the ones encountered here:

function makeBOrC(): B | C {
    if (Math.random() < 0.5) {
        return origMakeA('', { bProp: '' }) as B; // asserting as B
    } else if (Math.random() < 0.5) {
        return origMakeA('', { cProp: '' }) as C; // asserting as C
    } else {
        return origMakeA('', { oops: 123 }) as B; // no error due to assertion
    }
}

Using as B and as C informs the compiler to view what it considers merely as type A as a B or C respectively, effectively eliminating the errors.


Option #2: Implementing Generics

In this scenario, making makeA() generic based on the type T of the second argument proves to be a viable solution:

function makeA<T extends object>(arg1: string, arg2: T) {
    return {
        prop1: arg1,
        prop2: arg2
    }
}

By removing the explicit return type annotation and allowing inference from the implementation, TypeScript's structural typing enables maintaining the necessary type associations without explicitly declaring A as the output type.


With either approach, adjustments allow makeBOrC() to work seamlessly, ensuring proper assignment to B and C types while alerting about any mismatches that might compromise type safety.

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