我也来凑个热闹，看看编译后async/await

在koa 2里使用async

'use strict';

var http = require('http');
var koa = require('koa');
var app = new koa();


// number of middleware

var n = parseInt(process.env.MW || '1', 10);
console.log('  %s async middleware', n);

while (n--) {
  app.use(async (ctx, next) => {
    await next();
  });
}

var body = new Buffer('Hello World');

app.use(async (ctx, next) => {
  await next();
  ctx.body = body;
});


// var apprequire('./koa2-async.js')
app.listen(3333);

module.exports = app

使用babel编译后，如下

'use strict';

function _asyncToGenerator(fn) { return function () { var gen = fn.apply(this, arguments); return new Promise(function (resolve, reject) { function step(key, arg) { try { var info = gen[key](arg); var value = info.value; } catch (error) { reject(error); return; } if (info.done) { resolve(value); } else { return Promise.resolve(value).then(function (value) { return step("next", value); }, function (err) { return step("throw", err); }); } } return step("next"); }); }; }

var http = require('http');
var koa = require('koa');
var app = new koa();

// number of middleware

var n = parseInt(process.env.MW || '1', 10);
console.log('  %s async middleware', n);

while (n--) {
  app.use((() => {
    var ref = _asyncToGenerator(function* (ctx, next) {
      yield next();
    });

    return function (_x, _x2) {
      return ref.apply(this, arguments);
    };
  })());
}

var body = new Buffer('Hello World');

app.use((() => {
  var ref = _asyncToGenerator(function* (ctx, next) {
    yield next();
    ctx.body = body;
  });

  return function (_x3, _x4) {
    return ref.apply(this, arguments);
  };
})());

// var apprequire('./koa2-async.js')
app.listen(3333);

module.exports = app;

看一下_asyncToGenerator函数，格式化后

function _asyncToGenerator(fn) {
    return function() {
        var gen = fn.apply(this, arguments);
        return new Promise(function(resolve, reject) {
            function step(key, arg) {
                try {
                    var info = gen[key](arg);
                    var value = info.value;
                } catch(error) {
                    reject(error);
                    return;
                }
                if (info.done) {
                    resolve(value);
                } else {
                    return Promise.resolve(value).then(function(value) {
                        return step("next", value);
                    },
                    function(err) {
                        return step("throw", err);
                    });
                }
            }
            return step("next");
        });
    };
}

很明显，把async函数通过高阶函数转为promise里嵌入的generator来执行的。

整体来说，随着中间件越多，它的性能越差，可以参见 koa、koa2、koa2-async和express的压测性能比较

再看await

app.use(async (ctx, next) => {
  await next();
  ctx.body = body;
});

翻译后

app.use((() => {
  var ref = _asyncToGenerator(function* (ctx, next) {
    yield next();
    ctx.body = body;
  });

  return function (_x3, _x4) {
    return ref.apply(this, arguments);
  };
})());

很明显，await只是yield的别名

结论

• async/await不需要generator，自己带执行器
• 翻译后的await = yield

目前Thunk和Promise是一样，都可以接到yield后面，但await的native里的具体实现里还不好说。

另外据mikeal说，10月份可能会有async/await的native实现，拭目以待吧

如有不对的地方，请指正