challenge.js

function HashMap () {
  this._items = {};
  this.length = 0;
}

HashMap.prototype.get = function (key) {
  if (key === null || typeof this._items[this._hash(key)] === undefined) {
    return null;
  }
  return this._items[this._hash(key)];
};

HashMap.prototype.put = function (key, value) {
  this._items[this._hash(key)] = value;
  this.length++;
};

HashMap.prototype._hash = function (value) {
  if (value instanceof Object) {
    return (typeof value) + " " + (value._hash || (value._hash = ++this._counter));
  } else {
    return (typeof value) + " " + value.toString();
  }
};

HashMap.prototype._counter = 0;

function pairWithSum(sum, randoms) {
  var deltas = new HashMap(), delta, i, l;
  for (i = 0, l = randoms.length; i < l; i++) {
    if (deltas.get(randoms[i])) {
      return [ randoms[i], deltas.get(randoms[i]) ];
    }
    deltas.put(sum - randoms[i], randoms[i]);
  }
}

pairWithSum(98, [100, 20, -2, 30, 4]); // => [-2, 100]

function HashMap () {
  this._items = {};
  this.length = 0;
}

HashMap.prototype = {
  constructor: HashMap,
  _counter: 0,

  get: function (key) {
    if (key === null || typeof this._items[this._hash(key)] === undefined) {
      return null;
    }
    return this._items[this._hash(key)];
  },

  put: function (key, value) {
    this._items[this._hash(key)] = value;
    this.length++;
  },

  _hash: function (value) {
    if (value instanceof Object) {
      return (typeof value) + " " + (value._hash || (value._hash = ++this._counter));
    } else {
      return (typeof value) + " " + value.toString();
    }
  }
}