Establish characteristics of a class according to a general parameter

class Component { }
class ShallowWrapper { }

// creating a TestContainer class with generic types T and P
class TestContainer<T extends Component, P extends object> 
{
    constructor(reactElement: T, selectors: P) {
        // iterating over selectors to define object properties
    }
    initialize(): void { }
    [elem: keyof P]: any 
    // need to set class properties based on keys of P
    // however, encountering a compiler error:
    // An index signature parameter type cannot be a union type. 
    // Suggested solution: use a mapped object type instead.
}

const p = new TestContainer(new Component, { test: 'a' });

const v = p.test // results in a type error (property does not exist)

The code above attempts to dynamically create object properties using generic parameter P. Unfortunately, the compiler throws an error

An index signature parameter type cannot be a union type. Consider using a mapped object type instead.

How can I overcome this issue?

Answer №1

The compiler is throwing this error because you are attempting to blend syntax for indexed signature parameters with syntax for mapped types. Indexed signature parameters can only have a type of string or number.

To achieve your desired outcome with strong typing, you could utilize a factory method:

class Component { }

class TestContainer<T extends Component> {
    static create<T extends Component, P extends object>(reactElement: T, selectors: P) {
        return Object.assign(new TestContainer(reactElement), selectors);
    }

    private constructor(reactElement: T) {}
}

const p = TestContainer.create(new Component(), { test: 'a' });

const v = p.test;

On another note, if you intended Component to be empty for demonstration purposes, it's worth mentioning that using an empty class to limit types is not recommended. Due to TypeScript's structural type system, an empty class is essentially equivalent to Object. This implies that T extends Component would be the same as T extends Object, which in turn is the same as just T. Experimenting with this will confirm that the following code is valid:

TestContainer.create(42, { test: 'a' })

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