theZieger / arrayToObject

module.exports = function(arr, mapBy) {

  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));

    },

    undefined: function(val, newKey) {

      if (typeof newKey === 'string'

        || typeof newKey === 'number'

      ) {

        obj[newKey] = val;

      }

    }

  };

  /**

   * 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;

};