How to use the extend.apply function in extend

.then(function (objects) {
      return promise(extend.apply(null, objects));
    });
}, {

});
    } else {
      lines = this.scroll.lines(index, length);
      leaves = this.scroll.descendants(LeafBlot, index, length);
    }
    const formatsArr = [lines, leaves].map(blots => {
      if (blots.length === 0) return {};
      let formats = bubbleFormats(blots.shift());
      while (Object.keys(formats).length > 0) {
        const blot = blots.shift();
        if (blot == null) return formats;
        formats = combineFormats(bubbleFormats(blot), formats);
      }
      return formats;
    });
    return extend.apply(extend, formatsArr);
  }

Config.prototype.mergeChain = function(chain) {
  var args = [true, {}].concat(chain);
  args.push(this.data);

  this.data = extend.apply(null, args);
};

extend(deep, obj) {
    return extend.apply({}, arguments);
  },

var extend = function () {
  var objs = [], len = arguments.length;
  while ( len-- ) objs[ len ] = arguments[ len ];

  return _extend.apply(void 0, [ true ].concat( objs ));
};

var execCallback = function() {
    if (--len > 0) {
      return;
    }
    if (isJson) {
      result = result.map(parseJSON).filter(noop);
      if (result.length > 1) {
        result.reverse();
        if (typeof result[0] !== 'object') {
          result[0] = {};
        }
        callback(extend.apply(null, result));
      } else {
        callback(result[0]);
      }
    } else {
      if (isJsHtml) {
        result = wrapTag(result, isBin, charset, wrapJs);
      } else if (isCssHtml) {
        result = wrapTag(result, isBin, charset, wrapCss);
      }
      if (rule.isRawList) {
        callback(result);
      } else {
        callback(fileMgr.joinData(result, !isBin));
      }
    }
  };

export function merge(target, ...source) {

	return extend.apply(null, [true, target].concat(source));
}

function flattenAnnotations(annotations) {
        return extend.apply(null, annotations);
      }