var baseFlatten = require('./_baseFlatten'),
    baseRest = require('./_baseRest'),
    baseUniq = require('./_baseUniq'),
    isArrayLikeObject = require('./isArrayLikeObject'),
    last = require('./last');

/**
 * This method is like `_.union` except that it accepts `comparator` which
 * is invoked to compare elements of `arrays`. Result values are chosen from
 * the first array in which the value occurs. The comparator is invoked
 * with two arguments: (arrVal, othVal).
 *
 * @static
 * @memberOf _
 * @since 4.0.0
 * @category Array
 * @param {...Array} [arrays] The arrays to inspect.
 * @param {Function} [comparator] The comparator invoked per element.
 * @returns {Array} Returns the new array of combined values.
 * @example
 *
 * var objects = [{ 'x': 1, 'y': 2 }, { 'x': 2, 'y': 1 }];
 * var others = [{ 'x': 1, 'y': 1 }, { 'x': 1, 'y': 2 }];
 *
 * _.unionWith(objects, others, _.isEqual);
 * // => [{ 'x': 1, 'y': 2 }, { 'x': 2, 'y': 1 }, { 'x': 1, 'y': 1 }]
 */
var unionWith = baseRest(function(arrays) {
  var comparator = last(arrays);
  if (isArrayLikeObject(comparator)) {
    comparator = undefined;
  }
  return baseUniq(baseFlatten(arrays, 1, isArrayLikeObject, true), undefined, comparator);
});

module.exports = unionWith;
