Collections that promise for single appearance of object in collection as well as use hashing based on key (if defined) for collection to speed up checks. In small collections that slows down (nearly unmeasurable) but in 'big' collections (32+) it is a great win to prevent iterations for searches.

collection.js

'use strict';

var Collection = this.Collection = exports.Collection = function Collection(key) {
  this.list = [ ];
  this.length = 0;

  this.key = key || null;
  this.map = { };
}

// get if collection has item by function or item
Collection.prototype.has = function has(fnOrItem) {
  if (typeof(fnOrItem) === 'function') { // function
    for(var i = 0, len = this.list.length; i < len; ++i) {
      if (fnOrItem(this.list[i])) {
        return true;
      }
    }
  } else { // item
    if (this.key) {
      return this.map[fnOrItem[this.key]] !== undefined;
    } else {
      for(var i = 0, len = this.list.length; i < len; ++i) {
        if (this.list[i] === fnOrItem) {
          return true;
        }
      }
    }
  }
  return false;
};

// add item if not in list
Collection.prototype.push = function push(item) {
  if (item instanceof Array) { // array
    if (this.key) {
      for(var i = 0, len = item.length; i < len; i++) {
        if (this.map[item[i][this.key]] === undefined) {
          this.list.push(item[i]);
          this.map[item[i][this.key]] = item[i];
        }
      }
    } else {
      for(var n = 0, lenP = item.length; n < lenP; n++) {
        for(var i = 0, len = this.list.length; i < len; i++) {
          if (this.list[i] !== item) {
            this.list.push(item[n]);
          }
        }
      }
    }
  } else { // single
    if (this.key) {
      if (this.map[item[this.key]] !== undefined) {
        return;
      }
      this.list.push(item);
      this.map[item[this.key]] = item;
    } else {
      for(var i = 0, len = this.list.length; i < len; i++) {
        if (this.list[i] === item) {
          return;
        }
      }
      this.list.push(item);
    }
  }
  this.length = this.list.length;
};

// removes item from collection and returns it based on filter function or item
Collection.prototype.pull = function pull(fnOrKey) {
  if (typeof(fnOrKey) === 'function') { // function
    for(var i = 0, len = this.list.length; i < len; i++) {
      item = this.list[i];
      if (fnOrKey(item)) {
        if (this.key) {
          delete this.map[item[this.key]];
        }
        this.list.splice(i, 1);
        this.length = this.list.length;
        return item;
      }
    }
  } else { // item
    if (this.key && this.map[fnOrKey] !== undefined) {
      var item = this.map[fnOrKey];
      for(var i = 0, len = this.list.length; i < len; i++) {
        if (this.list[i] === item) {
          this.list.splice(i, 1);
          this.length = this.list.length;
          delete this.map[fnOrKey];
          return item;
        }
      }
    }
  }
  return null;
};

// remove by item or filter function
Collection.prototype.remove = function remove(fnOrItem) {
  if (typeof fnOrItem === 'function') { // by function
    for(var i = 0, len = this.list.length; i < len; i++) {
      if(fnOrItem(this.list[i])) {
        if (this.key) {
          delete this.map[this.list[i][this.key]];
        }
        this.list.splice(i, 1);
        this.length = this.list.length;
        return true;
      }
    }
  } else { // by object
    for(var i = 0, len = this.list.length; i < len; i++) {
      if ((this.key && this.list[i][this.key] == fnOrItem[this.key]) || (this.list[i] === fnOrItem)) {
        if (this.key) {
          delete this.map[this.list[i][this.key]];
        }
        this.list.splice(i, 1);
        this.length = this.list.length;
        return true;
      }
    }
  }
  return false;
};

// get array (optionally filtered by function)
Collection.prototype.toArray = function toArray(fn) {
  var res = [ ];
  if (fn) {
    for(var i = 0, len = this.list.length; i < len; ++i) {
      if (fn(this.list[i])) {
        res.push(this.list[i]);
      }
    }
  } else {
    for(var i = 0, len = this.list.length; i < len; ++i) {
      res.push(this.list[i]);
    }
  }
  return res;
};

// clear
Collection.prototype.clear = function clear() {
  this.list = [ ];
  this.map = { };
  this.length = 0;
};

// returns item by filter function or key
Collection.prototype.get = function get(fnOrKey) {
  if (typeof(fnOrKey) === 'function') { // function
    for(var i = 0, len = this.list.length; i < len; i++) {
      if (fnOrKey(this.list[i])) {
        return this.list[i];
      }
    }
  } else { // key
    if (this.key && this.map[key] !== undefined) {
      return this.map[key];
    }
  }
  return null;
};

// return filtered list
Collection.prototype.filter = function filter(fn) {
  return this.list.filter(fn);
};

// for each call function
Collection.prototype.forEach = function forEach(fn) {
  for(var i = 0, len = this.list.length; i < len; ++i) {
    fn(this.list[i]);
  }
};