扩展打字稿界面+优化功能签名

Question

Is it possible to define an object interface and then extend the interface, refining method signatures?

I'd like this to to work with strictFunctionTypes set to true .

A simple example:

interface Plugin {
    addListener: (eventName: string) => void;
}

interface FooPlugin extends Plugin {
    addListener: (eventName: 'fooChanged') => void;
}

Here I get the following error:

Interface 'FooPlugin' incorrectly extends interface 'Plugin'.
    Types of property 'addListener' are incompatible.
        Type '(eventName: "fooChanged") => void' is not assignable to type '(eventName: string) => void'.
            Types of parameters 'eventName' and 'eventName' are incompatible.
                Type 'string' is not assignable to type '"fooChanged"'.

Here's a more complex example showing what'd I'd eventually like to be able to do:

export interface Plugin {
    addListener: (eventName: string, fn: Function) => void;
}

export interface FooPlugin extends Plugin {
    addListener: (eventName: 'fooInit', fn: (n: { i: boolean }) => void) => void;
    addListener: (eventName: 'fooDestroy', fn: (n: { d: number }) => void) => void;
}

Answer 1

I don't like this solution very much but I suppose that you need this for typing a not-typed library so you can do something like this:

1) Add generics in Plugin interface

export interface Plugin<T, P> {
   addListener: (eventName: T, fn: (value: P) => void) => void;
}

2) Then create a Type with all your listener typed correctly

export type FooPlugin = Plugin<'fooInit', { i: boolean }> & Plugin<'fooDestroy', { d: number 
}>

3) Finally use it in your code

let plugin: FooPlugin = require("someLib");
plugin.addListener('fooInit', ({i}) => ...); 
plugin.addListener('fooDestroy', ({d}) => ...); 

Answer 2

For better or for worse, this will work if you declare methods (which are exempt from strictFunctionTypes ) instead of function-valued properties:

interface Plugin {
    addListener(eventName: string): void;
}

interface FooPlugin extends Plugin {
    addListener(eventName: 'fooChanged'): void;
}