code: function arrToMap(arr) {
      return arr.reduce((prev, curr) => {
          prev[curr] = true;
          return prev;
      }, {});
  }
docs: Convert simple array type to object map type.
  E.g., ['a', 'b'] => {a: true, b: true}
  
  @inner
  @param {Array.<string|number>} arr the array that has unique element
  @return {Object}