Can get parent object from child object in javascript?

Question

As the title said, I have two class like below:

class A {
    constructor() {
       this.a = 1
       this.b = 2
    }
}
Class B extends A {
    constructor() {
       super()
       this.c = 3
    }
}

Now, I instance class B

const b1 = new B()

Can I use some method to get the { a:1, b:2 } from b1. Because I need to post some data through API. But my model is something like class B. but the interface of the API is like class A. So I just want to get the parent object from the child object. Thanks in advance.

if B extends A then .... b1.a would be 1, or did I miss something important? — Jaromanda X
– Jaromanda X, Commented Sep 21, 2022 at 10:47
but the interface of the API is like class A - an aspect of inheritance in OOP is that every single B object is an A object too, and can be used where A objects are expected. — tevemadar
– tevemadar, Commented Sep 21, 2022 at 10:53
Practical check: try console.log(b1 instanceof B, b1 instanceof A);. — tevemadar
– tevemadar, Commented Sep 21, 2022 at 10:55
@JaromandaX Yes, you are right. But if the interface of API have a lot of property should be passed. I don't think b1.a b1.b ...... b1.z is a good code. — Courage
– Courage, Commented Sep 21, 2022 at 14:00
@Courage why is it bad code? This is how JS works, prototypical inheritance. — evolutionxbox
– evolutionxbox, Commented Sep 21, 2022 at 14:01

jsejcksn · Accepted Answer · 2022-09-21 11:01:19Z

1

Can I use some method to get the { a:1, b:2 } from b1?

I interpret this to mean that you want an object with only properties a and b from b1. If that's your meaning, then you can simply pick those properties from b1 and post the new object:

function pick (obj, keys) {
  const result = {};
  for (const key of keys) result[key] = obj[key];
  return result;
}

class A {
  constructor () {
     this.a = 1;
     this.b = 2;
  }
}

class B extends A {
  constructor () {
     super();
     this.c = 3;
  }
}

const b1 = new B();

// Post this:
const data = pick(b1, ['a', 'b']);

console.log(data); // { a: 1, b: 2 }

answered Sep 21, 2022 at 11:01

jsejcksn

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2 Comments

Courage Over a year ago

Thanks for your response. But what I want is some decent way. Like downcasting or upcasting in JAVA

evolutionxbox Over a year ago

@Courage do not expect JavaScript to work like Java, they're not the same. JS doesn't have those concepts. Also have a look at stackoverflow.com/questions/44291524/…

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