/**
 * lodash 3.2.0 (Custom Build) <https://lodash.com/>
 * Build: `lodash modern modularize exports="npm" -o ./`
 * Copyright 2012-2015 The Dojo Foundation <http://dojofoundation.org/>
 * Based on Underscore.js 1.8.3 <http://underscorejs.org/LICENSE>
 * Copyright 2009-2015 Jeremy Ashkenas, DocumentCloud and Investigative Reporters & Editors
 * Available under MIT license <https://lodash.com/license>
 */
var baseAssign = require('lodash._baseassign'),
    createAssigner = require('lodash._createassigner'),
    keys = require('lodash.keys');

/**
 * A specialized version of `_.assign` for customizing assigned values without
 * support for argument juggling, multiple sources, and `this` binding `customizer`
 * functions.
 *
 * @private
 * @param {Object} object The destination object.
 * @param {Object} source The source object.
 * @param {Function} customizer The function to customize assigned values.
 * @returns {Object} Returns `object`.
 */
function assignWith(object, source, customizer) {
  var index = -1,
      props = keys(source),
      length = props.length;

  while (++index < length) {
    var key = props[index],
        value = object[key],
        result = customizer(value, source[key], key, object, source);

    if ((result === result ? (result !== value) : (value === value)) ||
        (value === undefined && !(key in object))) {
      object[key] = result;
    }
  }
  return object;
}

/**
 * Assigns own enumerable properties of source object(s) to the destination
 * object. Subsequent sources overwrite property assignments of previous sources.
 * If `customizer` is provided it is invoked to produce the assigned values.
 * The `customizer` is bound to `thisArg` and invoked with five arguments:
 * (objectValue, sourceValue, key, object, source).
 *
 * **Note:** This method mutates `object` and is based on
 * [`Object.assign`](https://people.mozilla.org/~jorendorff/es6-draft.html#sec-object.assign).
 *
 * @static
 * @memberOf _
 * @alias extend
 * @category Object
 * @param {Object} object The destination object.
 * @param {...Object} [sources] The source objects.
 * @param {Function} [customizer] The function to customize assigned values.
 * @param {*} [thisArg] The `this` binding of `customizer`.
 * @returns {Object} Returns `object`.
 * @example
 *
 * _.assign({ 'user': 'barney' }, { 'age': 40 }, { 'user': 'fred' });
 * // => { 'user': 'fred', 'age': 40 }
 *
 * // using a customizer callback
 * var defaults = _.partialRight(_.assign, function(value, other) {
 *   return _.isUndefined(value) ? other : value;
 * });
 *
 * defaults({ 'user': 'barney' }, { 'age': 36 }, { 'user': 'fred' });
 * // => { 'user': 'barney', 'age': 36 }
 */
var assign = createAssigner(function(object, source, customizer) {
  return customizer
    ? assignWith(object, source, customizer)
    : baseAssign(object, source);
});

module.exports = assign;