JavaScript - Copy Method of Instance

Question

I want a possibility to copy all properties/methods of a class instance:

class A {
 get prop1() { return 1; }
 get prop2() { return 2; }
 doStuff() {
 return this.prop1 + this.prop2;
 }
}
class B extends A {
 get prop1() { return 5; }
}

class AWrapper {
 constructor(a) {
 // [1] Copy all methods/propertys of a
 this.doStuff = () => a.doStuff() + 42;
 }
}
const a = new A();
const b = new B();
const wA = new AWrapper(a);
const wB = new AWrapper(b);
console.log(a.prop1(), wA.prop1(), wB.prop1()); // 1, 1, 5
console.log(a.doStuff(), wA.doStuff()); // 3, 45

I could copy each method/property by hand, but is there a simple command for [1], such that wA has the same signature as a?

• Titus: You can extend that class (eg: class AWrapper extends A { ... })
• Chris Li: this? developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/…
• random314: Well I want to only work with the instance a. I.e. assume a is just an instance of A, not A directly. I've edit the example

Answer 1

Usually, Proxy is a tool of choice when working with mixins or decorators:

class A {
 get prop1() {
 return 1;
 }
 get prop2() {
 return 2;
 }
 doStuff() {
 return this.prop1 + this.prop2;
 }
}
class B extends A {
 get prop1() {
 return 5;
 }
}
function decorate(target) {
 let mixin = {
 doStuff() {
 return target.doStuff() + 42;
 }
 }
 return new Proxy(target, {
 get(_, prop) {
 return (prop in mixin) ? mixin[prop] : target[prop];
 }
 });
}
const a = new A();
const b = new B();
const wA = decorate(a)
const wB = decorate(b)
console.log(a.prop1, wA.prop1, wB.prop1); // 1, 1, 5
console.log(a.doStuff(), wA.doStuff()); // 3, 45

Answer 2

Use the extends keyword and call the parent's (A) doStuff with : this.doStuff = () => super.doStuff() + 42;

class A {
 get prop1() { return 1; }
 get prop2() { return 2; }
 doStuff() {
 return this.prop1 + this.prop2;
 }
}
class AWrapper extends A {
 constructor(...args) {
 super(...args); 
 this.doStuff = () => super.doStuff() + 42;
 }
}
const a = new A();
const w = new AWrapper(a);
console.log(a.prop1, w.prop1); // 1, 1
console.log(a.doStuff(), w.doStuff()); // 3, 45

Answer 3

Below should do it.

class A {
 get prop1() { return 1; }
 get prop2() { return 2; }
 doStuff() {
 return this.prop1 + this.prop2;
 }
}
class AWrapper extends A{
 constructor(a) {
 super(a);
 this.doStuff = () => a.doStuff() + 42;
 }
}
const a = new A();
const wA = new AWrapper(a);
const wB = new AWrapper(a);
console.log(a.prop1, wA.prop1, wB.prop1); // 1, 1, 1
console.log(a.doStuff(), wA.doStuff()); // 3, 45

Answer 4

You need to make a BWrapper class too - other than that, extends and super are all you need:

class A {
 get prop1() {
 return 1;
 }
 get prop2() {
 return 2;
 }
 doStuff() {
 return this.prop1 + this.prop2;
 }
}
class B extends A {
 get prop1() {
 return 5;
 }
}
class AWrapper extends A {
 constructor(a) {
 super();
 this.doStuff = () => a.doStuff() + 42;
 }
}
class BWrapper extends B {
 constructor(b) {
 super();
 this.doStuff = () => b.doStuff() + 42;
 }
}
const a = new A();
const b = new B();
const wA = new AWrapper(a);
const wB = new BWrapper(b);
console.log(a.prop1, wA.prop1, wB.prop1); // 1, 1, 5
console.log(a.doStuff(a), wA.doStuff(wA)); // 3, 4

• random314: Hm. In my code I have many instances of A. Creating a Wrapper for every one is no option. I just have the concrete instance (a or b) and want to feed it into a function (in here: AWrappers constructur).