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?

You can extend that class (eg: class AWrapper extends A { ... }) — Titus
– Titus, Commented Aug 28, 2019 at 13:30
this? developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/… — Chris Li
– Chris Li, Commented Aug 28, 2019 at 13:30
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 — random314
– random314, Commented Aug 28, 2019 at 13:33

georg · Accepted Answer · 2019-08-28 13:54:31Z

2

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

edited Aug 28, 2019 at 13:54

answered Aug 28, 2019 at 13:45

georg

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Comments

Taki · Accepted Answer · 2019-08-28 13:37:31Z

0

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

answered Aug 28, 2019 at 13:37

Taki

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Comments

Mahendra Pratap · Accepted Answer · 2019-08-28 13:37:32Z

0

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

answered Aug 28, 2019 at 13:37

Mahendra Pratap

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Comments

Jack Bashford · Accepted Answer · 2019-08-28 13:47:48Z

0

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

edited Aug 28, 2019 at 13:47

answered Aug 28, 2019 at 13:39

Jack Bashford

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1 Comment

random314 Over a year ago

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).

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