Provide `declare` keyword support for class methods

Search Terms

declare, class, method

Suggestion

I know that declare keyword already support class properties. I’ve wonder if it is possiable to bring the support to class methods as well?

Use Cases

You override a method a, and there are some other methods that return the return value of a.

Examples

This is just a simple example describing my idea.

class BaseTag
{
    constructor(tag: string) {...}

    static fromString(tag: string): BaseTag
    {
        return new BaseTag(tag);
    }

    static fromNumber(tag: number): BaseTag
    {
        return this.fromString(tag.toString());
    }
}

class ExtendedTag extends BaseTag
{
    static fromString(tag: string): ExtendedTag
    {
        return new BaseTag(`Tag: ${ tag}`);
    }

    declare static fromNumber(tag: number): ExtendedTag;

    // // Without `declare` support
    // static fromNumber(tag: number): ExtendedTag
    // {
    //     return super.fromString(tag) as unknown as ExtendedTag;
    // }
}

Checklist

My suggestion meets these guidelines:

• This wouldn’t be a breaking change in existing TypeScript/JavaScript code
• This wouldn’t change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn’t a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript’s Design Goals.

[JasonHK]

What you’re asking for is #5863

@RyanCavanaugh No, this is a different idea. Let me give you another example:

class BaseClass
{
    declare private version: string;

    public toJSON(): BaseInterface
    {
        return {
            version: this.version,
        };
    }

    protected toObject(): ObjectMixin<BaseInterface>
    {
        return Object.assign(this.toJSON(), { hello: true });
    }
}

interface BaseInterface
{
    version: string;
}

class ExtendedClass extends BaseClass
{
    declare private count: number;

    public toJSON(): ExtendedInterface
    {
        return Object.assign(super.toJSON(), { count: this.count });
    }

    declare protected toObject(): ObjectMixin<ExtendedInterface>;

    // // Without `declare` support
    // protected toObject(): ObjectMixin<ExtendedInterface>
    // {
    //     return super.toObject() as unknown as ObjectMixin<ExtendedInterface>;
    // }
}

interface ExtendedInterface extends BaseInterface
{
    count: number;
}

type ObjectMixin<T> = T & { hello: boolean; };

TypeScript Playground

Although it seems that the example can be replaced by a generic, what if there are multiple return types that had to replace?

I know I can use an interface with the name of the class to override the method, but this only works for public members, not protected or private ones.

More importantly, if declare keyword already supports class properties, why don’t add support to class methods as well?

[JasonHK]

declare protected toObject: () => ObjectMixin<ExtendedInterface>; should work?

@whzx5byb Well… There’s a problem with this approach though, you cannot use a method to override a property, which means this will not work:

class SuperClass
{
    declare protected toObject: () => ObjectMixin<ExtendedInterface>;
}

class SubClass extends SuperClass
{
    protected toObject(): ObjectMixin<...>
    {
        ...
    }
}