TypeScript - extract interface members only - possible?

Question

Is there a way to dynamically extract members from an object belonging to an interface (i.e. not specifying them again explicitly), like this:

let subset = { ...someObject as ISpecific }; 

Currently I get all members that someObject happens to have. So the spread operator does not work here. Are there any ways to do that yet?

Example:

interface ISpecific { A: string; B: string; }
class Extended implements ISpecific { public A: string = '1'; public B: string = '2'; public C: string = '3'; }

let someObject = new Extended(); 
let subset = { ...someObject as ISpecific }; 
console.log(subset);  // -> { A, B, C } but want { A, B }

TypeScript casts are merely hints for the compiler, not real conversions at runtime.

Answer 1

It can be achieved using decorators (see requirements at the end). It can only be used with methods (copying a property get/set accessor yields its momentary return value only, not the accessor function).

// define a decorator (@publish) for marking members of a class for export: 
function publish(targetObj: object, memberKey: string, descriptor: PropertyDescriptor) { 
    if (!targetObj['_publishedMembers']) 
        targetObj['_publishedMembers'] = []; 
    targetObj['_publishedMembers'].push(memberKey); 
}

// this function can return the set of members of an object marked with the @publish decorator: 
function getPublishedMembers(fromObj: object) {
    const res = {}; 
    const members = fromObj['_publishedMembers'] || []; 
    members.forEach(member => { res[member] = fromObj[member].bind(fromObj); }); 
    return res; 
}

// this is for making sure all members are implemented (does not make sure about being marked though): 
interface IPublishedMembers {
    A(): string; 
    B(): number; 
    C(): void; 
}

// this class implements the interface and has more members (that we do NOT want to expose): 
class Full implements IPublishedMembers {
    private b: number = 0xb; 

    @publish public A(): string { return 'a'; }
    @publish public B(): number { return this.b; }
    @publish public C(): boolean { return true; }
    public D(): boolean { return !this.C(); } 
    public E(): void { } 
}

const full = new Full(); 
console.log(full);  // -> all members would be exposed { A(), B(), b, C(), D(), E() }

const published = getPublishedMembers(full) as IPublishedMembers; 
console.log(published);  // -> only sanctioned members { A(), B(), C() }
console.log(published.B());  // -> 11 = 0xb (access to field of original object works)

(This requires the compilerOption "experimentalDecorators":true in your tsconfig.json and an ES5 target, more info at http://www.typescriptlang.org/docs/handbook/decorators.html)