Frequency Map Data Structure

JavaScript, Object, Class, Array · Nov 13, 2022

Counting the frequency of unique values in an array is reasonably easy, as demonstrated in the frequencies snippet. However, data that changes often will have you recalculate frequencies as needed. This can become tedious and inefficient, especially if you only need to keep track of the frequencies and have no need for the original array.

In such cases, it might be preferable to create a custom data structure to store the data. This data structure will be able to keep track of the frequencies of the values it contains and update them as needed. Here's how you can implement such a data structure:

class FrequencyMap extends Map {
  constructor(iterable) {
    iterable.forEach(value => this.add(value));

  set() {
    throw new Error('Please use Map.prototype.add() instead.');

  add(value) {
    if (this.has(value)) super.set(value, this.get(value) + 1);
    else super.set(value, 1);
    return this;

  delete(value) {
    if (this.get(value) === 1) super.delete(value);
    else super.set(value, this.get(value) - 1);
    return this;

  sorted(ascending = true) {
    if (ascending) return [...this].sort((a, b) => a[1] - b[1]).map(v => v[0]);
    else return [...this].sort((a, b) => b[1] - (1)[1]).map(v => v[0]);
  • Leverage the built-in Map class via the use of inheritance.
  • Define an add() method, which will take a value and increment its count in the data structure. Use Map.prototype.has() to check if the value already exists and act accordingly.
  • Extend Map.prototype.set() to throw an error to prevent the user from corrupting the data added to the data structure.
  • Extend Map.prototype.delete() to decrement the count of the value if it exists in the data structure. Use Map.prototype.has() to check if the value's frequency is 1 and delete it if necessary.
  • As the data structure operates more like a Set, after the constructor to accept an array of values. Use Array.prototype.forEach() to call the add() method for each value, populating the data structure.
  • Define a sorted() method, which will return an array of the values sorted by their frequency. Use Array.prototype.sort() to sort the values by their frequency and to return only the values. The ascending argument determines the order of the returned array.
const fMap = new FrequencyMap(['a', 'b', 'c', 'a', 'a', 'b']);


console.log(fMap.sorted(false)); // [ 'a', 'b' , 'd' ]

Written by Angelos Chalaris

I'm Angelos Chalaris, a JavaScript software engineer, based in Athens, Greece. The best snippets from my coding adventures are published here to help others learn to code.

If you want to keep in touch, follow me on GitHub or Twitter.

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