June 2021
Intermediate to advanced
290 pages
9h 21m
Chinese
Content preview from Node与Express开发（第2版）
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中间件
｜
99
那么在管线中，一个请求是如何“终结”的呢？这就轮到传入每个中间件的
next
函数发挥
作用了：如果你没有调用
next()
，请求就终结于这个中间件。
10.1
基本原理
要理解
Express
如何工作，关键是要学习灵活地思考中间件和路由处理函数。以下几点需
要牢记。
•
路由处理函数（
app.get
、
app.post
等，常常统称为
app.METHOD
）可以被认为是只处理
特定
HTTP
动
词（
GET
，
POST
等）的中间件。反过来，中间件也可以被认为是处理所有
HTTP
动词的路由处理函数
（基本上等同于
app.all
，它也处理所有
HTTP
动词。对于
某些少见的动词，如
PURGE
，中间件跟
app.all
的处理效果有微小的差异，但对于常用
的动词，二者处理效果一样）。
•
路由处理函数需要一个路径作为第一个参数。如果你想让这个路径匹配任何路由，就使
用
/*
。
中间件也可以接收一个路径作为第一个参数，但这是可选的（如果略去，它将匹
配任何路径，就像指定了
*
一样）
。
•
路由处理函数和中间件接收一个回调函数，这个回调函数可以接收
2
个、
3
个或
4
个参
数（从技术上来说，你也可以让它没有或只有一个参数，但这种形式没有什么用）。如
果有
2
个或
3
个参数
，前
2
个参数就是
request
和
response
对象，第三个参数就是
next
函数。如果有
4
个参数，这个回调函数就成了
错误处理
中间件，第一个参数就成了错误
对象，紧接着是
request
、
response
和
next
这些对象。
•
如果你
没有
调用
next()
，管线就会终结，不再有路由处理函数和中间件的处理 ...
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I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.