theZieger / arrayToObject

 * Turns an Array into an associative Object (while keeping reference!)

 *

 * @param {Array}                 arr    Array of Objects to turn into an

 *                                       associative Object

 * @param {String|Array|Function} mapBy  optional mapping key, can be a

 *                                       simple string (property name in

 *                                       the abjects of arr), a list of

 *                                       property names (which are

 *                                       combined) or a function which

 *                                       returns a unique id to use

 *

 * @throws {TypeError} if arr is not an Array or mapBy is set but not of

 *                     correct type (String, Array, Function)

 *

 * @returns {Object}

 */

  var obj = {};

  if (!Array.isArray(arr)) {

    throw new TypeError('arr argument is not of type Array');

  }

  if (mapBy !== undefined

    && typeof mapBy !== 'string'

    && !Array.isArray(mapBy)

    && typeof mapBy !== 'function'

  ) {

    throw new TypeError(

      'mapBy argument is not of type {String|Array|Function}'

    );

  }

  var methods = {

    string: function(val) {

      this.undefined(val, val[mapBy]);

    },

    object: function(val) {

      var newKey = mapBy.map(function(propertyName){

        return val[propertyName];

      }).join('_');

      this.undefined(val, newKey);

    },

    function: function(val, i, arr) {

      this.undefined(val, mapBy(val, i, arr));

    },

  /**

   * run the designated method by mapBy type from the methods object

   * it binds the methods object so we can use the undefined setter method

   * for different mapBy types and don't have to maintain multiple but

   * same conditions

   */

  arr.forEach(

    methods[(typeof mapBy)].bind(methods)

  );

  return obj;

};