A Map whose values are Sets

lookup.js

class Lookup extends Map {
  get [Symbol.toStringTag] () { return 'Lookup' }

  constructor (iterable = []) {
    super(
      [...iterable].map(
        ([key, value]) => [
          key,
          value instanceof Set
            ? value
            : new Set([value])
        ]
      )
    )
  }

  add (key, value) {
    if (this.has(key)) {
      this.get(key).add(value)
    } else {
      this.set(key, value)
    }

    return this
  }

  get (key) {
    return super.get(key) || new Set()
  }

  set (key, value) {
    return super.set(key, new Set([value]))
  }
}