是否可以在打字稿的可索引类型接口中定义函数？

Question

I'd like to define the following:

export interface IValidationContextIndex {
    [validationContextKey: string]: ValidationContext;
    getValidationContexts(): Array<ValidationContext>;
}

This way I can get an object that adheres to the IValidationContextIndex interface and call getValidationContexts on it.

However VSCode is NOT HAPPY with this type of definition. Is it possible to do this in an interface or do I need something like:

class cache {
    constructor(public index: IValidationContextIndex ) {};
    getValidationContexts() {return Object.value(<any> index)}
} 

Answer 1

The problem is that typescripts would prefer that the index signature be consistent will ALL declared members. Since you can use the index to access the getValidationContexts member as well and it's type will not be ValidationContext .

You can make the index signature consistent will the defined members, but it's not ideal as you will need to narrow the return type:

export interface IValidationContextIndex {
    [validationContextKey: string]: ValidationContext | (() => Array<ValidationContext>);
    getValidationContexts(): Array<ValidationContext>;
}

A way to get around this restriction is to use an intersection type:

type IValidationContextIndex = {
    [validationContextKey: string]: ValidationContext;
} & {
    getValidationContexts(): Array<ValidationContext>;
}

But objects of this type can't be created directly, for the same index incompatibility reason. You would need to create the objects using Object.assign :

let o : IValidationContextIndex = Object.assign({ a: new ValidationContext() }, {
    getValidationContexts(): Array<ValidationContext> {
        return Object.values(this)as any
    }
});

Or using a type assertion:

let o : IValidationContextIndex = {
    getValidationContexts(): Array<ValidationContext> {
        return Object.values(this)as any
    }
} as IValidationContextIndex;