trekhleb/javascript-algorithms · error · Error

sort method must be implemented

Error message

sort method must be implemented

What it means

Sort (src/algorithms/sorting/Sort.js) is an abstract base class: its constructor wires up a Comparator and visiting callbacks, but sort() exists only to be overridden and unconditionally throws. Concrete subclasses (BubbleSort, SelectionSort, MergeSort, QuickSort and the other files under src/algorithms/sorting/*/) extend Sort and implement sort(). You hit this error by instantiating Sort itself or by writing a subclass that forgets to override sort() - the repo's own doForbiddenSort test exercises exactly this path.

Source

Thrown at src/algorithms/sorting/Sort.js:32

    this.comparator = new Comparator(this.callbacks.compareCallback);
  }

  /**
   * @param {SorterCallbacks} originalCallbacks
   * @returns {SorterCallbacks}
   */
  static initSortingCallbacks(originalCallbacks) {
    const callbacks = originalCallbacks || {};
    const stubCallback = () => {};

    callbacks.compareCallback = callbacks.compareCallback || undefined;
    callbacks.visitingCallback = callbacks.visitingCallback || stubCallback;

    return callbacks;
  }

  sort() {
    throw new Error('sort method must be implemented');
  }
}

View on GitHub (pinned to 85293e3e2b)

Solutions

1. Import and instantiate a concrete subclass instead, e.g. BubbleSort from src/algorithms/sorting/bubble-sort/BubbleSort
2. If you wrote your own subclass, implement sort() (typically using this.comparator and this.callbacks.visitingCallback)
3. If the base class is used only for typing or shared callbacks, never call sort() on it directly

Example fix

// before
import Sort from '../src/algorithms/sorting/Sort';
new Sort().sort();
// throws: abstract method

// after
import BubbleSort from '../src/algorithms/sorting/bubble-sort/BubbleSort';
const sorted = new BubbleSort().sort([3, 1, 2]);

import Sort from '../src/algorithms/sorting/Sort';

if (!sorter || sorter.sort === Sort.prototype.sort) {
  throw new TypeError('Use a concrete Sort subclass such as BubbleSort');
}
sorter.sort(input);

import Sort from '../src/algorithms/sorting/Sort';

const hasConcreteSort = (s) =>
  !!s && typeof s.sort === 'function' && s.sort !== Sort.prototype.sort;

try {
  sorted = sorter.sort(arr);
} catch (e) {
  if (e.message === 'sort method must be implemented') {
    throw new TypeError(sorter.constructor.name + ' does not implement sort()');
  }
  throw e;
}

When it happens

Trigger: new Sort().sort(); importing Sort instead of a concrete algorithm via IDE autocomplete; declaring class CustomSort extends Sort without a sort() method and calling it; a refactor renaming sort() in the subclass and silently losing the override.

Common situations: Auto-import picking the base class over a similarly named subclass, copy-pasting a subclass skeleton and deleting the method body, or wrapper code typed against the base class that accidentally receives a bare Sort instance.

Related errors

• You have to implement heap pair comparison method

AI-assisted analysis of trekhleb/javascript-algorithms@85293e3e2b (2026-08-24). Data as JSON: /api/errors/75d7121663432b55. Report an issue: GitHub.