lodash/internal/baseAssign.js

import baseCopy from './baseCopy';
import keys from '../object/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;
}

export default baseAssign;