# express
## Basic
const express = require('express')
const app = express()\
Add a new route method to handle requests for the path "/". The first argument specifies the path or URL, the next argument is the route handler. Inside the route handler, let's use the response object to send a status code 200 (OK) and text to the client
app.get('/', (request, response, nextHandler) => {
response.status(200).send('Hello from ExpressJS')
})
Finally, use the listen method to accept new connections on port 1337:
app.listen(1337, () => console.log('Web Server running on port 1337'),)
## Initial
create new
```
var express = require('express');
var app = express();
app.get('/', function(req, res){
res.send('hello world');
});
app.listen(3000);
```
## Router :
We can use app.route()
```
app.route('/book')
.get(function(req, res) {
res.send('Get a random book');
})
.post(function(req, res) {
res.send('Add a book');
})
.put(function(req, res) {
res.send('Update the book');
});
```
Routing refers to determining how an application responds to a client request to a particular endpoint, which is a URI (or path) and a specific HTTP request method (GET, POST, and so on).
```
app.METHOD(PATH, HANDLER);
```
A `router` object is an isolated instance of middleware and routes. You can think of it as a “mini-application,” capable only of performing middleware and routing functions. Every Express application has a built-in app router
CRUD
```
// GET method route
app.get('/', function (req, res) {
res.send('GET request to the homepage');
});
// POST method route
app.post('/', function (req, res) {
res.send('POST request to the homepage');
});
```
```
app.put('/user', function(req, res) {
res.send('Got a PUT request at /user');
});
```
```
app.delete('/user', function(req, res) {
res.send('Got a DELETE request at /user');
});
```
Respond to all METHOD to the /user route:
```
app.all('/user', function(req, res) {
res.send('Got a request at /user');
});
```
```
const express = require('express');
const router = express.Router();
// middleware that is specific to this router
router.use(function timeLog(req, res, next) {
console.log('request for books - Time: ', Date.now());
next();
});
// define the home page route
router.get('/', function(req, res) {
res.send('Books home page');
});
// define the about route
router.get('/about', function(req, res) {
res.send('About books');
});
module.exports = router;
```
## Advanced Routing
regular expression
* `[abc]` Matches either an a, b or c character
* `[a-z]` Matches any characters between a and z, including a and z.
* `.` Matches any character other than newline
* `\d` Matches any decimal digit. Equivalent to \[0-9].
* `a?` Matches an `a` character or nothing.
* `a+` Matches one or more consecutive `a` characters.
* `a*` Matches zero or more consecutive `a` characters.
```
app.get('/ab?cd', function(req, res) {
res.send('ab?cd');
});
```
#### Routing Parameter
```
Route path: /users/:userId/books/:bookId
Request URL: http://localhost:3000/users/34/books/8989
req.params: { "userId": "34", "bookId": "8989" }
app.get('/users/:userId/books/:bookId', function(req, res) {
res.send(req.params);
});
```
#### Request Body
```
const app = require('express')();
const bodyParser = require('body-parser');
app.use(bodyParser.json()); // for parsing application/json
app.post('/users', function(req, res) {
res.send(req.body);
});
```
## app.use(\[path], function)]
```
var express = require('express');
var app = express();
// 一个简单的 logger
app.use(function(req, res, next){
console.log('%s %s', req.method, req.url);
next();
});
// 响应
app.use(function(req, res, next){
res.send('Hello World');
});
app.listen(3000);
```
static:, ./public static service
```
app.use(express.static(__dirname + '/public'));
```
## app.all(path, \[callback...], callback)
match all HTTP active
## Handling error
In Express, 404 responses are not the result of an error, so the error-handler middleware will not capture them. This behavior is because a 404 response simply indicates the absence of additional work to do; in other words, Express has executed all middleware functions and routes, and found that none of them responded. All you need to do is add a middleware function at the very bottom of the stack (below all other functions) to handle a 404 response:
```
app.use(function(req, res, next) {
res.status(404).send("Sorry can't find that!");
});
app.use(function(err, req, res, next) {
console.error(err.stack);
res.status(500).send('Something broke!');
});
app.get('/error', function (req, res) {
throw new Error('BROKEN') // Express will catch this on its own.
})
app.get('/', function (req, res, next) {
fs.readFile('/file-does-not-exist', function (err, data) {
if (err) {
next(err) // Pass errors to Express.
} else {
res.send(data)
}
})
})
```
## Express Middleware
1. app.use `app.METHOD()`
2. router.use `router.METHOD()`
3. err app.use(function(err, req,res,next))
4. import, third party middleware
```
router.use('url',function(req,res,next){next()})
```
```
app.use('url', function(req,res,next){next()})
```
Middleware functions are functions that have access to the request object (req), the response object (res), and the next middleware function in the application’s request-response cycle. The next middleware function is commonly denoted by a variable named next.
* [应用层中间件](https://expressjs.com/zh-cn/guide/using-middleware.html#middleware.application)
* [路由器层中间件](https://expressjs.com/zh-cn/guide/using-middleware.html#middleware.router)
* [错误处理中间件](https://expressjs.com/zh-cn/guide/using-middleware.html#middleware.error-handling)
* [内置中间件](https://expressjs.com/zh-cn/guide/using-middleware.html#middleware.built-in)
* [第三方中间件](https://expressjs.com/zh-cn/guide/using-middleware.html#middleware.third-party)
Middleware functions can perform the following tasks:
* Execute any code.
* Make changes to the request and the response objects.
* End the request-response cycle.
* Call the next middleware function in the stack.
middleware e.g.
function: redirect('/login');
```
app.use(function(req, res, next) {
if (req.isAuthenticated) {
next();
} else {
res.redirect('/login');
}
});
```
```
// define the home page route
router.get('/', function(req, res) {
res.send('Books home page');
});
```
**Example - Error Handling**
You define error-handling middleware in the same way as other middleware, except with four arguments instead of three; specifically with the signature (err, req, res, next):
```
app.use(function(err, req, res, next) {
console.error(err.stack);
res.status(500).send('Something broke!');
});
```
### third part :
```
npm install cookie-parser
var cookieParser = require('cookie-parser');
// load the cookie-parsing middleware
app.use(cookieParser());
```
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