Map sub-class that defines a default value for missing keys

default-map.js

/* jshint asi: true, esnext: true */

(() => {
  'use strict'

  // --------------------------------------------------------------------------

  class DefaultMap extends Map {
    get(key) {
      return super.has(key)
        ? super.get(key)
        : this.getDefault(key)
    }

    getDefault(key) {}
  }

  // --------------------------------------------------------------------------

  class Counter extends DefaultMap {
    getDefault(key) {
      return 0
    }

    sortedItems() {
      return Array.from(this.entries())
        .sort((a, b) => (b[1] - a[1]) || a[0].localeCompare(b[0]))
    }
  }

  // --------------------------------------------------------------------------

  class Collector extends DefaultMap {
    getDefault(key) {
      return []
    }

    count(key) {
      return this.get(key).length
    }
  }

  // --------------------------------------------------------------------------

  const c = new Counter()

  Array.from(document.querySelectorAll('*')).forEach(elem => {
    [...elem.classList].forEach(cls => {
      c.set(cls, c.get(cls) + 1)
    })
  })

  console.table(c.sortedItems())

})()