internal/baseAssign.js

define(['./baseCopy', '../object/keys'], function(baseCopy, keys) {

  /**
   * The base implementation of `_.assign` 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 assigning values.
   * @returns {Object} Returns the destination object.
   */
  function baseAssign(object, source, customizer) {
    var props = keys(source);
    if (!customizer) {
      return baseCopy(source, object, props);
    }
    var index = -1,
        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) ||
          (typeof value == 'undefined' && !(key in object))) {
        object[key] = result;
      }
    }
    return object;
  }

  return baseAssign;
});