How to access private fields from parent class in JavaScript?

Question

I have a Heap class that has a private values field. This is good since I do not want anyone to be able to directly modify values.

class Heap {
  
  #values;
  
  constructor(array = []) {
    this.#values = array;
  }

    insert(item) {
    this.#values.push(item);
    this.#bubbleUp(this.#values.length - 1);
  }
}

But now I would like to extend my Heap class into a PriorityQueue subclass. In this class I need to change the signature of some of the methods (for example insert) so that it assigns a priority to a value. However, I cannot figure out how to access the values field from the base class. For instance, given the following PriorityQueue class:

class PriorityQueue extends Heap {
  
  constructor(array = []) {
    super(array);
  }
  
  insert(item, priority) {

    // Error: Private field '#values' must be declared in an enclosing class
    this.#values.push({ item, priority }); 
    this.#bubbleUp(this.#values.length - 1);
  }
}

I get an error when trying to push a value onto values.

Is there any way around this problem? I want to make a field private in the base class, but still accessible to the subclass.

Thanks!

Just call the parent's insert method? super.insert({ item, priority }). — Felix Kling
– Felix Kling, Commented Oct 31, 2021 at 20:23
"I want to make a field private in the base class, but still accessible to the subclass." that is what protected access is. However, JS doesn't have that for now. So you whatever you want to use in subclasses has to be public. — VLAZ
– VLAZ, Commented Oct 31, 2021 at 20:25

Bergi · Accepted Answer · 2021-10-31 22:48:15Z

7

I cannot figure out how to access the values field from the base class.

This is not possible. Private really means private in JS. Don't use it if you want the field to be accessible outside of the class.

I want to make a field private in the base class, but still accessible to the subclass.

That would be a protected member, which JS doesn't support.

Is there any way around this problem?

Well, you don't actually need to access the .#values. All you need to do is call the base class insert method:

class PriorityQueue extends Heap { 
  insert(item, priority) {
    super.insert({ item, priority });
  }
}

I would like to extend my Heap class into a PriorityQueue subclass. In this class I need to change the signature of some of the methods (for example insert) so that it assigns a priority to a value.

This is a bad idea. A subclass should not change the method signatures, otherwise it will violate the Liskov substitution principle. Instead, use composition over inheritance, where a priority queue has or contains a heap:

class PriorityQueue {
  #heap = new Heap(v => v.priority); // assuming it takes a custom comparator?
  
  push(item, priority) {
    this.#heap.insert({ item, priority });
  }
  pop() {
    return this.#heap.…
  }
}

answered Oct 31, 2021 at 22:48

Bergi

671k162 gold badges1k silver badges1.5k bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Espresso Over a year ago

Thank you for your answer, your post helps a lot. My Heap class does use a comparator. I never even considered structuring the PriorityQueue to have a heap instance rather than inheriting from the Heap class. I clearly have much to learn. On a side note, do you recommend any books to learn topics such as composition over inheritance?

Collectives™ on Stack Overflow

How to access private fields from parent class in JavaScript?

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related