Serialize an ES2015 Map object to a JSON object

to-json.js

/**
 * Serialize map to object. Goal is to be able to use 'Map' over object when performing
 * large algorithms over a hash map - but then serializing it to an object for sending
 * over a wire.
 */
Map.prototype.toJSON = function() {
  const o = Object.create(null)
  
  for (const k of this.keys()) {
    if (typeof k !== 'string' && typeof k !== 'number') {
      throw new Error(`Cannot serialize a map with a key of type: ${typeof k}`)
    }

    o[k] = this.get(k)
  }

  return o
}