trekhleb/javascript-algorithms · error · Error

You have to implement heap pair comparison method

Error message

      You have to implement heap pair comparison method
      for ${firstElement} and ${secondElement} values.
    

What it means

Heap is an abstract base class: pairIsInCorrectOrder(firstElement, secondElement) is the ordering hook that heapifyUp, heapifyDown and remove consult to decide whether a parent/child pair may stay in place, and the base implementation deliberately throws because no default ordering exists. Concrete subclasses define it — MinHeap returns compare.lessThanOrEqual(first, second), MaxHeap returns compare.greaterThanOrEqual(first, second). Seeing this error means the raw Heap class (or an incomplete subclass) is being used instead of a concrete heap.

Source

Thrown at src/data-structures/heap/Heap.js:281

      }

      this.swap(currentIndex, nextIndex);
      currentIndex = nextIndex;
    }
  }

  /**
   * Checks if pair of heap elements is in correct order.
   * For MinHeap the first element must be always smaller or equal.
   * For MaxHeap the first element must be always bigger or equal.
   *
   * @param {*} firstElement
   * @param {*} secondElement
   * @return {boolean}
   */
  /* istanbul ignore next */
  pairIsInCorrectOrder(firstElement, secondElement) {
    throw new Error(`
      You have to implement heap pair comparison method
      for ${firstElement} and ${secondElement} values.
    `);
  }
}

View on GitHub (pinned to 85293e3e2b)

Solutions

1. If you need a standard heap, instantiate MinHeap or MaxHeap instead of Heap.
2. If you subclass Heap, implement pairIsInCorrectOrder(firstElement, secondElement) returning true when the pair is correctly ordered (e.g. firstElement.priority <= secondElement.priority for a min-style heap).
3. Check the import path — it must point at src/data-structures/heap/MinHeap.js or MaxHeap.js, not Heap.js.
4. If you only need custom ordering of primitives, pass a custom Comparator to MinHeap/MaxHeap instead of subclassing Heap.

Example fix

// before
import Heap from './data-structures/heap/Heap';
const heap = new Heap(comparator);
heap.add(3); // throws: heap pair comparison not implemented

// after (option 1: use a concrete heap)
import MinHeap from './data-structures/heap/MinHeap';
const heap = new MinHeap();
heap.add(3);

// after (option 2: subclass and implement the hook)
class MinPriorityHeap extends Heap {
  pairIsInCorrectOrder(a, b) {
    return a.priority <= b.priority;
  }
}

import Heap from './data-structures/heap/Heap';
import MinHeap from './data-structures/heap/MinHeap';
import MaxHeap from './data-structures/heap/MaxHeap';

const isConcreteHeap = (heap) =>
  heap instanceof MinHeap ||
  heap instanceof MaxHeap ||
  (heap instanceof Heap && heap.pairIsInCorrectOrder !== Heap.prototype.pairIsInCorrectOrder);

if (!isConcreteHeap(heap)) {
  throw new TypeError('Heap is abstract: use MinHeap/MaxHeap or override pairIsInCorrectOrder');
}

const isConcreteHeap = (heap) =>
  heap instanceof MinHeap ||
  heap instanceof MaxHeap ||
  (heap instanceof Heap && heap.pairIsInCorrectOrder !== Heap.prototype.pairIsInCorrectOrder);

When it happens

Trigger: Instantiating Heap directly and calling add(), remove(), or anything that triggers heapifyUp/heapifyDown (e.g. new Heap(); heap.add(3)). Writing a custom class that extends Heap (for example to order objects by a priority field) without overriding pairIsInCorrectOrder. Importing Heap via editor autocomplete instead of MinHeap/MaxHeap, or copying Heap into a new structure and dropping the subclass.

Common situations: Building a priority queue directly on the raw Heap class; adapting the heap to custom objects by only overriding the compare function instead of the pair method; porting example code that starts from Heap; refactors where the concrete subclass import is accidentally replaced with the base class.

Related errors

• Cannot construct Heap instance directly
• sort method must be implemented
• Item not found in the tree
• Position is out of allowed range
• Left index can not be greater than right one

AI-assisted analysis of trekhleb/javascript-algorithms@85293e3e2b (2026-08-24). Data as JSON: /api/errors/7ecf6e167b3a39aa. Report an issue: GitHub.