如何在Express4.x中愉快地使用async的方法

 const express = require('express');
 const app = express();
 const PORT = process.env.PORT || 3000;

 app.get('/', (req, res) => {
  const message = doSomething();
  res.send(message);
 });

 // 错误处理中间件
 app.use(function (err, req, res, next) {
  return res.status(500).send('内部错误！');
 });

 app.listen(PORT, () => console.log(`app listening on port ${PORT}`));

 app.get('/', async (req, res) => {
  const message = doSomething();
  res.send(message);
 });

 app.get('/', async (req, res, next) => {
  try {
   const message = doSomething();
   res.send(message);
  } catch(err) {
   next(err);
  }
 });

 function asyncUtil(fn) {
  return function asyncUtilWrap(...args) {
   const fnReturn = fn(args);
   const next = args[args.length - 1];
   return Promise.resolve(fnReturn).catch(next);
  }
 }

 app.use(asyncUtil(async (req, res, next) => {
  const message = doSomething();
  res.send(message);
 }));

app.use = function use(fn) {
 var path = '/';

 // 省略参数处理逻辑
 ...

 // 初始化内置中间件
 this.lazyrouter();
 var router = this._router;

 fns.forEach(function (fn) {
  // non-express app
  if (!fn || !fn.handle || !fn.set) {
   return router.use(path, fn);
  }

  ...

 }, this);

 return this;
};

proto.use = function use(fn) {
 var offset = 0;
 var path = '/';

 // 省略参数处理逻辑
 ...

 var callbacks = flatten(slice.call(arguments, offset));

 for (var i = 0; i < callbacks.length; i++) {
  var fn = callbacks[i];

  ...

  // add the middleware
  debug('use %o %s', path, fn.name || '<anonymous>')

  var layer = new Layer(path, {
   sensitive: this.caseSensitive,
   strict: false,
   end: false
  }, fn);

  layer.route = undefined;

  this.stack.push(layer);
 }

 return this;
};

// 部分核心代码
proto.handle = function handle(req, res, out) {
 var self = this;
 var idx = 0;
 var stack = self.stack;

 next();

 function next(err) {
  var layerError = err === 'route'
   ? null
   : err;
  
  if (idx >= stack.length) {
   return;
  }

  var path = getPathname(req);

  // find next matching layer
  var layer;
  var match;
  var route;

  while (match !== true && idx < stack.length) {
   layer = stack[idx++];
   match = matchLayer(layer, path);
   route = layer.route;

   if (match !== true) {
    continue;
   }

  }

  // no match
  if (match !== true) {
   return done(layerError);
  }

  // this should be done for the layer
  self.process_params(layer, paramcalled, req, res, function (err) {
   if (err) {
    return next(layerError || err);
   }

   if (route) {
    return layer.handle_request(req, res, next);
   }

   trim_prefix(layer, layerError, layerPath, path);
  });
 }

 function trim_prefix(layer, layerError, layerPath, path) {

  if (layerError) {
   layer.handle_error(layerError, req, res, next);
  } else {
   layer.handle_request(req, res, next);
  }
 }
};

Layer.prototype.handle_error = function handle_error(error, req, res, next) {
 var fn = this.handle;

 if (fn.length !== 4) {
  // not a standard error handler
  return next(error);
 }

 try {
  fn(error, req, res, next);
 } catch (err) {
  next(err);
 }
};

Layer.prototype.handle_request = function handle(req, res, next) {
 var fn = this.handle;

 if (fn.length > 3) {
  // not a standard request handler
  return next();
 }

 try {
  fn(req, res, next);
 } catch (err) {
  next(err);
 }
};

Layer.prototype.handle_request = function handle(req, res, next) {
 var fn = this.handle;

 if (fn.length > 3) {
  // not a standard request handler
  return next();
 }
 // 针对 async 语法函数特殊处理
 if (Object.prototype.toString.call(fn) === '[object AsyncFunction]') {
  return fn(req, res, next).catch(next);
 }

 try {
  fn(req, res, next);
 } catch (err) {
  next(err);
 }
};

app.use(async function () {
 const message = doSomething();
 res.send(message);
})

const originAppUseMethod = app.use.bind(app);
app.use = function (fn) {
 if (Object.prototype.toString.call(fn) === '[object AsyncFunction]') {
  const asyncWrapper = function(req, res, next) {
   fn(req, res, next).then(next).catch(next);
  }
  return originAppUseMethod(asyncWrapper);
 }
 return originAppUseMethod(fn);
}