code: function findIndex(array, callback, thisArg) {
        var index = -1,
            length = array ? array.length : 0;
  
        callback = lodash.createCallback(callback, thisArg);
        while (++index < length) {
          if (callback(array[index], index, array)) {
            return index;
          }
        }
        return -1;
      }
docs: This method is similar to `_.find`, except that it returns the index of
  the element that passes the callback check, instead of the element itself.
  
  @static
  @memberOf _
  @category Arrays
  @param {Array} array The array to search.
  @param {Function|Object|String} [callback=identity] The function called per
  iteration. If a property name or object is passed, it will be used to create
  a "_.pluck" or "_.where" style callback, respectively.
  @param {Mixed} [thisArg] The `this` binding of `callback`.
  @returns {Mixed} Returns the index of the found element, else `-1`.
  @example
  
  _.findIndex(['apple', 'banana', 'beet'], function(food) {
  return /^b/.test(food);
  });
  // => 1