Errors occur when trying to utilize an enum as a generic type in Typescript that are not compatible

Take a look at the code snippet provided. The concept here is that I have different provider implementations that extend a base provider. Each provider requires a settings object that is an extension of a base settings object. Additionally, each provider contains a static method to test these settings before they are passed to the provider (this method must remain static due to legacy reasons and cannot be changed to an instance method at this time)

enum ProviderType {
    TypeA = 'typeA',
    TypeB = 'typeB',
}

interface Settings {
    commonProperty: string;
}

interface SettingsTypeA extends Settings {
    propertyA: string;
}

interface SettingsTypeB extends Settings {
    propertyB: string;
}

type SettingsMap = {
    [ProviderType.TypeA]: SettingsTypeA,
    [ProviderType.TypeB]: SettingsTypeB,
}

interface TestSettingsOptions<T extends ProviderType> {
    settings: SettingsMap[T];
}

abstract class BaseProvider<T extends ProviderType> {
    constructor(protected settings: SettingsMap[T]) {}

    static testSettings<T extends ProviderType>(opts: TestSettingsOptions<T>) {
        throw new Error('Method not implemented');
    }
}

class ProviderA extends BaseProvider<ProviderType.TypeA> {
    constructor(protected settings: SettingsTypeA) {
        super(settings); // Settings has the correct type here: SettingsTypeA
    }

    static testSettings(opts: TestSettingsOptions<ProviderType.TypeA>) {
        // do some testing
    }
}

class ProviderB extends BaseProvider<ProviderType.TypeB> {
    constructor(protected settings: SettingsTypeB) {
        super(settings); // Settings has the correct type here: SettingsTypeB
    }

    static testSettings(opts: TestSettingsOptions<ProviderType.TypeB>) {
        // do some testing
    }
}

Although the basic classes, interfaces, and mapped types are inferred correctly, there seem to be issues with the static methods. Refer to the image below for reference:

https://i.sstatic.net/wyHA7.png

I'm uncertain about what mistake I might be making or why TypeScript is rejecting it as a valid type. Can someone provide guidance on this matter?

Answer №1

When dealing with the static side of a class, it's important to note that it has no access to any of the instance side's generic type parameters. This is because a single constructor needs to be able to create an instance for any possible type parameter, without having a specific value for it at the constructor level.

There is a feature request at microsoft/TypeScript#34665 to allow access within the type signature for abstract static methods in TypeScript, but currently, neither such methods nor static access to instance type parameters are allowed directly.

If making testSettings() an instance method is not feasible, another approach is to implement a generic factory function that generates non-generic classes. An example solution can be found in this Stack Overflow answer.

function BaseProvider<T extends ProviderType>(type: T) {

    abstract class BaseProvider {
        constructor(protected settings: SettingsMap[T]) {

        }

        static testSettings?(opts: TestSettingsOptions<T>) {
            throw new Error('Method not implemented');
        }
    }
    return BaseProvider;
}

This implementation allows the type parameter T to be accessible throughout the BaseProvider function, including the statically declared class returned by the function. It's worth noting that the passed-in type parameter aids in inferring the type T and isn't used otherwise in this scenario.

Subclasses will now extend the output of BaseProvider using an enum type:

class ProviderA extends BaseProvider(ProviderType.TypeA) {
    constructor(protected settings: SettingsTypeA) {
        super(settings); 
    }

    static testSettings(opts: TestSettingsOptions<ProviderType.TypeA>) {
        // implement testing functionality
    }
}

class ProviderB extends BaseProvider(ProviderType.TypeB) {
    constructor(protected settings: SettingsTypeB) {
        super(settings); 
    }

    static testSettings(opts: TestSettingsOptions<ProviderType.TypeB>) {
        // implement testing functionality
    }
}

It's important to keep in mind that while this approach is valid, there are some limitations to consider. For example, the uniqueness of class constructors may impact certain operations like checking with instanceof. Additionally, exporting declarations of function-local classes can require additional annotations for clean exportation in .d.ts files.

For further exploration and testing of the code provided, you can visit the Playground link provided.

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