Mapper.js - A small design pattern to avoid m x n loop back while finding some object from object array

Mapper.js

/*
 *
 The MIT License (MIT)

 Copyright (c) 2014 deepakshrma

 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal
 in the Software without restriction, including without limitation the rights
 to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 copies of the Software, and to permit persons to whom the Software is
 furnished to do so, subject to the following conditions:

 The above copyright notice and this permission notice shall be included in all
 copies or substantial portions of the Software.

 THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 SOFTWARE.
 */
class Mapper {
  constructor(array, key) {
    this.map = array.reduce((map, item) => {
      const val = item[key];
      if (!map[val]) {
        map[val] = [];
      }
      map[val].push(item);
      return map;
    }, {});
  }

  find(key) {
    return this.map[key] && this.map[key][Mapper.FIRST_INDEX]; //return blank array
  }

  findAll(key, returnUndefined) {
    //return blank array
    return this.map[key] ? this.map[key] : returnUndefined ? undefined : [];
  }
}

Mapper.FIRST_INDEX = 0;
const users = [
  {
    _id: "57e88c892727ab7f4828c015",
    isActive: true,
    balance: "$1,807.06",
    picture: "http://placehold.it/32x32",
    age: 21,
    name: "Schmidt"
  },
  {
    _id: "57e88c897a849644b79d6555",
    isActive: false,
    balance: "$1,487.94",
    picture: "http://placehold.it/32x32",
    age: 40,
    name: "Griffin"
  },
  {
    _id: "57e88c89075380040077b307",
    isActive: false,
    balance: "$2,939.40",
    picture: "http://placehold.it/32x32",
    age: 22,
    name: "Nellie"
  },
  {
    _id: "57e88c89cbfe5ec2c41ae27f",
    isActive: true,
    balance: "$2,392.49",
    picture: "http://placehold.it/32x32",
    age: 36,
    name: "Nellie"
  },
  {
    _id: "57e88c89ace3c83d92a0de7f",
    isActive: true,
    balance: "$2,161.58",
    picture: "http://placehold.it/32x32",
    age: 40,
    name: "Payne"
  },
  {
    _id: "57e88c895cec7aa54f7409cb",
    isActive: false,
    balance: "$2,109.69",
    picture: "http://placehold.it/32x32",
    age: 37,
    name: "Sandoval"
  }
];
//How to use

var userMapper = new Mapper(users, "name"); // create a map on names
console.log(userMapper.find("Schmidt")); //will return only user with name 'Schmidt'
const userToFind = ["Payne", "Schmidt"];
const reqUsers = userToFind.map(name => userMapper.find(name));
console.log(reqUsers);
var userMapper = new Mapper(users, "isActive"); // create a map on isActive
console.log(userMapper.find(true)); //will return only first active user
console.log(userMapper.findAll(true)); //will return all active user
var userMapper = new Mapper(users, "name"); // create a map on names
console.log(userMapper.find("Nellie")); //will return only first user with name 'Nellie'
console.log(userMapper.findAll("Nellie").map(({ _id }) => _id));
//will return all user _id whose name is 'Nellie'
console.log(userMapper.map); //will return map based on name
if(typeof window !== 'undefined') window.Mapper = Mapper;