Histogram.js

/**
 * A Set-like class that keeps track of how many times a value has
 * been added. Call add() and remove() like you would for a Set, and
 * call count() to find out how many times a given value has been added.
 * The default iterator yields the values that have been added at least
 * once. Use entries() if you want to iterate [value, count] pairs.
 */
class Histogram {
    // To initialize, we just create a Map object to delegate to
    constructor() { this.map = new Map(); }

    // For any given key, the count is the value in the Map, or zero
    // if the key does not appear in the Map.
    count(key) { return this.map.get(key) || 0; }

    // The Set-like method has() returns true if the count is non-zero
    has(key) { return this.count(key) > 0; }

    // The size of the histogram is just the number of entries in the Map.
    get size() { return this.map.size; }

    // To add a key, just increment its count in the Map.
    add(key) { this.map.set(key, this.count(key) + 1); }

    // Deleting a key is a little trickier because we have to delete
    // the key from the Map if the count goes back down to zero.
    delete(key) {
        let count = this.count(key);
        if (count === 1) {
            this.map.delete(key);
        } else if (count > 1) {
            this.map.set(key, count - 1);
        }
    }

    // Iterating a Histogram just returns the keys stored in it
    [Symbol.iterator]() { return this.map.keys(); }

    // These other iterator methods just delegate to the Map object
    keys() { return this.map.keys(); }
    values() { return this.map.values(); }
    entries() { return this.map.entries(); }
}