code: function extend(target, source) {
    let key;
    for (key in source) {
      if (isPlainObject(source[key]) || isArray(source[key])) {
        if (isPlainObject(source[key]) && !isPlainObject(target[key])) {
          target[key] = {};
        }
        if (isArray(source[key]) && !isArray(target[key])) {
          target[key] = [];
        }
        extend(target[key], source[key]);
      } else if (source[key] !== undefined) {
        target[key] = source[key];
      }
    }
  }
docs: https://github.com/madrobby/zepto/blob/master/src/zepto.js