Javascript 理解Express.js中的中间件和路由处理程序
我试图了解中间件和路由处理程序在Express中是如何工作的。在中，作者给出了一个有趣的路由和中间件的示例，但没有给出实际的细节 有人能帮我理解下例中每一步都发生了什么，这样我就可以确定我的想法是正确的吗？下面是一个例子（我的理解和评论中的问题）：Javascript 理解Express.js中的中间件和路由处理程序,javascript,node.js,express,web,middleware,Javascript,Node.js,Express,Web,Middleware,我试图了解中间件和路由处理程序在Express中是如何工作的。在中，作者给出了一个有趣的路由和中间件的示例，但没有给出实际的细节 有人能帮我理解下例中每一步都发生了什么，这样我就可以确定我的想法是正确的吗？下面是一个例子（我的理解和评论中的问题）： 您应该查看Express文档。它充满了你会发现有用的资源，特别是如果你是新手的话。以下是与您的问题相关的文档相关部分的一些链接 希望这能有所帮助。在express中，中间件注册的顺序会产生很大的不同，当express收到请求时，它只执行中间
您应该查看Express文档。它充满了你会发现有用的资源，特别是如果你是新手的话。以下是与您的问题相关的文档相关部分的一些链接
希望这能有所帮助。在express中，中间件注册的顺序会产生很大的不同，当express收到请求时，它只执行中间件注册，并且与请求的url匹配 express中间件具有以下签名
函数（req，res，next）{}函数（err、req、res、next）{} //get the express module as app
var app = require('express')();
//1. when this *middleware* is called, this is printed to the console, always.
//As this is first registered middleware, it gets executed no matter what because no url match were provided. This middleware does not stop the middleware chain execution as it calls next() and executes next middleware in chain.
app.use(function(req, res, next){
console.log('\n\nALLWAYS');
next();
});
//2. when a route /a is called, a response 'a' is sent and
//the app stops. There is no action from here on
//chain stops because this middleware does not call next()
app.get('/a', function(req, res, next){
console.log('/a: route terminated');
res.send('a');
});
//3. this never gets called as a consequence from above
//because (2) never calls next.
app.get('/a', function(req, res){
console.log('/a: never called');
});
//4. this will be executed when GET on route /b is called
//it prints the message to the console and moves on to the next function
//question: does this execute even though route /a (2) terminates abruptly?
//app.get('/b' ... does not depend in any way on (2). as url match criteria are different in both middleware, even if /a throws an exception, /b will stay intact. but this middleware will get executed only at /b not /a. i.e. if /a calls next(), this middleware will not get executed.
app.get('/b', function(req, res, next){
console.log('/b: route not terminated');
next();
});
//5. question: this gets called after above (4)?
//Yes, as (4) calls next(), this middleware gets executed as this middleware does not have a url filter pattern.
app.use(function(req, res, next){
console.log('SOMETIMES');
next();
});
//6. question: when does this get executed? There is already a function to handle when GET on /b is called (4)
//As (4) calls next(), (5) gets called, again (5) calls next() hence this is called. if this was something other '/b' like '/bbx' this wouldn't get called --- hope this makes sense, url part should match.
app.get('/b', function(req, res, next){
console.log('/b (part 2): error thrown' );
throw new Error('b failed');
});
//7. question: I am not sure when this gets called... ?
// This happens (4) calls next() -> (5) calls next() -> (6) throws exception, hence this special error handling middleware that catches error from (6) and gets executed. If (6) does not throw exception, this middleware won't get called.
//Notice next(err) this will call (10). -- as we are passing an error
app.use('/b', function(err, req, res, next){
console.log('/b error detected and passed on');
next(err);
});
//8. this is executed when a GET request is made on route /c. It logs to the console; throws an error.
app.get('/c', function(res, req){
console.log('/c: error thrown');
throw new Error('c failed');
});
//9. question: this catches the above error and just moves along?
//Yes, as this middleware calls next(), it will move to next matching middleware. so it will call (11) and not (10) because (10) is error handling middleware and needs to be called like next(err)
app.use('/c', function(err, req, res, next) {
console.log('/c: error deteccted but not passed on');
next();
});
//10. question: this follows the above and prints an error based on above?
//Which ever middleware from above calls next(err) will end up calling this one. ie. (7) does so.
//This also sends a 500 response?
//This just sends text as '500 - server error'
//in order to set status code you'll need to do res.status(500).send ...
app.use(function(err, req, res, next){
console.log('unhandled error detected: ' + err.message);
res.send('500 - server error');
//Also set status code
//res.status(500).send('500 - server error');
});
//11. question: this is the catch all for something that falls through and sends a 404?
//No, this does not catch error, as in (7). This route will get elected when non of the above middleware were able to respond and terminate the chain. So this is not an error handling route, this route just sends 404 message if non of the above routes returned a response and stopped chain of execution
app.use(function(req, res){
console.log('route not handled');
res.send('404 - not found');
//Also set status code
//res.status(400).send('404 - not found');
});
//12. This app listens on the 3000 port.
app.listen(3000, function(){
console.log('listening on 3000');
});
如果您需要更多的说明，请告诉我。中间件可以很好地使用，而且一开始有点难以理解。 中间件中要记住的最重要的事情是
正如前面的回答中提到的，排序在中间件中非常重要。由于中间件是一个接一个地执行的，所以请尝试从上到下严格理解代码
app.use(function(req,res,next){
app.use(function(err,req,res,next){
因此#11不是一个错误处理中间件。它只是停止中间件链，因为它没有next（）函数，并且是序列中的最后一个中间件。
现在，您还应该了解#7是一个错误处理中间件，它从#6获取/b路由的错误#7处理在err中传递的错误，然后将错误参数传递给next（）函数。
next（）
next（）只是沿着链传递控制的函数。如果您觉得一个中间件不足以满足该特定路由（甚至不满足任何路由），则可以调用next（）函数，该函数将控制权传递给下一个有效的中间件。
您可以使用路由指定有效性，或使其具有通用性，例如#9和#10。来自#9的错误将不会传递给#10。这是因为#9中的next（）没有传递err参数，因此作为错误处理中间件的#10将无法捕获它#9到11，你需要比这更具体。你到底不明白什么？在代码中添加了我的理解和问题。这就像试图一口吃掉整个比萨饼。首先切块（app.get做什么？app.use做什么？下一步是什么？？等等），然后从每个切片中取一小块（2个app.get调用使用同一路径意味着什么？）你有时间读我的答案吗？
//get the express module as app
var app = require('express')();
//1. when this *middleware* is called, this is printed to the console, always.
//As this is first registered middleware, it gets executed no matter what because no url match were provided. This middleware does not stop the middleware chain execution as it calls next() and executes next middleware in chain.
app.use(function(req, res, next){
console.log('\n\nALLWAYS');
next();
});
//2. when a route /a is called, a response 'a' is sent and
//the app stops. There is no action from here on
//chain stops because this middleware does not call next()
app.get('/a', function(req, res, next){
console.log('/a: route terminated');
res.send('a');
});
//3. this never gets called as a consequence from above
//because (2) never calls next.
app.get('/a', function(req, res){
console.log('/a: never called');
});
//4. this will be executed when GET on route /b is called
//it prints the message to the console and moves on to the next function
//question: does this execute even though route /a (2) terminates abruptly?
//app.get('/b' ... does not depend in any way on (2). as url match criteria are different in both middleware, even if /a throws an exception, /b will stay intact. but this middleware will get executed only at /b not /a. i.e. if /a calls next(), this middleware will not get executed.
app.get('/b', function(req, res, next){
console.log('/b: route not terminated');
next();
});
//5. question: this gets called after above (4)?
//Yes, as (4) calls next(), this middleware gets executed as this middleware does not have a url filter pattern.
app.use(function(req, res, next){
console.log('SOMETIMES');
next();
});
//6. question: when does this get executed? There is already a function to handle when GET on /b is called (4)
//As (4) calls next(), (5) gets called, again (5) calls next() hence this is called. if this was something other '/b' like '/bbx' this wouldn't get called --- hope this makes sense, url part should match.
app.get('/b', function(req, res, next){
console.log('/b (part 2): error thrown' );
throw new Error('b failed');
});
//7. question: I am not sure when this gets called... ?
// This happens (4) calls next() -> (5) calls next() -> (6) throws exception, hence this special error handling middleware that catches error from (6) and gets executed. If (6) does not throw exception, this middleware won't get called.
//Notice next(err) this will call (10). -- as we are passing an error
app.use('/b', function(err, req, res, next){
console.log('/b error detected and passed on');
next(err);
});
//8. this is executed when a GET request is made on route /c. It logs to the console; throws an error.
app.get('/c', function(res, req){
console.log('/c: error thrown');
throw new Error('c failed');
});
//9. question: this catches the above error and just moves along?
//Yes, as this middleware calls next(), it will move to next matching middleware. so it will call (11) and not (10) because (10) is error handling middleware and needs to be called like next(err)
app.use('/c', function(err, req, res, next) {
console.log('/c: error deteccted but not passed on');
next();
});
//10. question: this follows the above and prints an error based on above?
//Which ever middleware from above calls next(err) will end up calling this one. ie. (7) does so.
//This also sends a 500 response?
//This just sends text as '500 - server error'
//in order to set status code you'll need to do res.status(500).send ...
app.use(function(err, req, res, next){
console.log('unhandled error detected: ' + err.message);
res.send('500 - server error');
//Also set status code
//res.status(500).send('500 - server error');
});
//11. question: this is the catch all for something that falls through and sends a 404?
//No, this does not catch error, as in (7). This route will get elected when non of the above middleware were able to respond and terminate the chain. So this is not an error handling route, this route just sends 404 message if non of the above routes returned a response and stopped chain of execution
app.use(function(req, res){
console.log('route not handled');
res.send('404 - not found');
//Also set status code
//res.status(400).send('404 - not found');
});
//12. This app listens on the 3000 port.
app.listen(3000, function(){
console.log('listening on 3000');
});
app.use(function(req,res,next){
app.use(function(err,req,res,next){