原文地址，中文地址，koa文档
1 Introduction
Express.js无疑是当前Node.js中最流行的Web应用程序框架。它几乎成为了大多数Node.js web应用程序的基本的依赖，甚至一些例如Sails.js这样的流行的框架也是基于Express.js。然而你还有一些其他框架的选择，可以给你带来“sinatra”一样的感觉（译注：sinatra是一个简单的Ruby的Web框架，可以参考这篇博文）。另外两个最流行的框架分别是Koa和Hapi。
这篇文章不是打算说服你哪个框架比另外一个更好，而是只是打算让你更好地理解每个框架能做什么，什么情况下一个框架可以秒杀另外一个。
2 Framework backgrounds
我们将要探讨的两个框架看起来都非常相似。每一个都能够用几行代码来构建一个服务器，并都可以非常轻易地构建REST API。我们先瞧瞧这几个框架是怎么诞生的。
2.1 Express
2009年6月26日，TJ Holowaychuk提交了Express的第一次commit，接下来在2010年1月2日，有660次commits的Express 0.0.1版本正式发布。TJ和Ciaron Jessup是当时最主要的两个代码贡献者。在第一个版本发布的时候，根据github上的readme.md，这个框架被描述成：
Insanely fast (and small) server-side JavaScript web development framework built on node.js and V8 JavaScript engine.
差不多5年的时间过去了，Express拥有了4,925次commit，现在Express的最新版本是4.10.1，由StrongLoop维护，因为TJ现在已经跑去玩Go了。
2.2 Koa
大概在差不多一年前的2013年8月17日，TJ Holowaychuk（又是他！）只身一人提交了Koa的第一次commit。他描述Koa为“表现力强劲的Node.js中间件，通过co使用generators使得编写web应用程序和REST API更加丝般顺滑”。Koa被标榜为只占用约400行源码空间的框架。Koa的目前最新版本为0.13.0，拥有583次commits。
2.3 Hapi
2011年8月5日，WalmartLabs的一位成员Eran Hammer提交了Hapi的第一次commit。Hapi原本是Postmile的一部分，并且最开始是基于Express构建的。后来它发展成自己自己的框架，正如Eran在他的博客里面所说的：
hapi was created around the idea that configuration is better than code, thatbusiness logic must be isolated from the transport layer...
Hapi最新版本为7.2.0，拥有3,816次commits，并且仍然由Eran Hammer维护。
所有开发者要开发Node.js web应用程序的第一步就是构建一个基本的服务器。所以我们来看看用这几个框架构建一个服务器的时候有什么异同。
3 Creating a server
The first step for any developer when working on a Node.js web application is to create a basic server. So lets create a server using each framework to see their similarities and differences.
4 Routes
Now lets dig into one of the most important features of a server, routing. First lets create the cliche "Hello world" application for each framework and then move on to something a little more useful, REST API.
5 The Good and The Bad
5.1 Express
5.1.1 The Good
Express has the biggest community not only out of the three frameworks compared here but out of all the web application frameworks for Node.js. It is the most matured framework out of the three, with almost 5 years of development behind it and now has StrongLoop taking control of the repository. It offers a simple way to get a server up and running and promotes code reuse with it's built in router.
5.1.2 The Bad
There is a lot of manual tedious tasks involved in Express. There is no built in error handling, it is easy to get lost in all of the middleware that could be added to solve a solution, and there are many ways to do one thing. Express describes itself as being opinionated, this could be good or bad but for beginning developers who must likely chose Express, this is a bad thing. Express also has a larger foot print compared to the other frameworks.
5.2 Koa
5.2.1 The Good
Koa has a small footprint, it is more expressive and it makes writing middleware a lot easier than the other frameworks. Koa is basically a barebone framework where the developer can pick (or write) the middleware they want to use rather than compromising to the middleware that comes with Express or Hapi. It's the only framework embracing ES6, for instance its using ES6 generators.
5.2.2 The Bad
Koa is still unstable and heavily in development. Using ES6 is still ahead of the game for example version 0.11.9+ of Node.js needs to be used to run Koa and right now the latest stable version on Node.js is version 0.10.33. One thing that is in the good but could also be in the bad much like Express is the option of selecting multiple middlewares or writing your own middleware. Such as the router we looked at earlier, there are a lot of middleware routers to handle a variety of options.
5.3 Hapi
5.3.1 The Good
Hapi is proud to say that their framework is based on configuration over code, and a lot of developers would argue that this is a good thing. This is very usual in large teams to add consistency and reusability. Also with the framework being backed by WalmartLabs as well as many other big name companies using Hapi in production, it has been battle tested and companies are confident enough to run their applications off of it. So all signs point towards this project continuing to mature in to a great framework.
5.3.2 The Bad
Hapi definitely seems to be more tailored towards bigger or more complex applications. It is probably a little too much boilerplate code to throw together a simple web app and there is also a lot less examples or open source applications that use hapi. So choosing it might involve a little more part on the developer rather than using third party middleware.
6 Summary
We have seen some good but practical examples of all three frameworks. Express is definitely the most popular and most recognized framework of the three. It is almost a reaction to first create a server using Express when starting new development on an application but hopefully now there might be some thought involved whether to use Koa or Hapi as an alternative. Koa shows real promise for the future and is ahead of the pack with embracing ES6 and the web component ideology that the web development community is moving towards. Hapi should be the first consideration for large teams and large projects. It pushes for configuration over code which almost always benefits teams and the re-usability most teams strive towards. Now go out and try a new framework, maybe you'll love it, maybe you'll hate, but you will never know and in the end, it will make you a better developer.