Express の Hello World

Write it down to yourself Express の Hello World

Express Is based on Node.js platform , Fast 、 to open up 、 minimalist Web Development framework . If you use Node.js Development backend , Probably can't leave Express. Although there are many excellent frameworks , Such as Egg.js appear , Study Express Understand some basic underlying HTTP Knowledge is necessary .

Hello World

To use Express You need to install Express. Create folder node, Run the following command to install Express.

npm install [email protected]

stay node Create under folder express Folder , stay express Create helloworld.js

Here is helloworld.js The content of

let express = require('express');
let app = express();

app.get('/', function (req, res) {
    
  console.log('hello, world');
  res.send('hello, world');
});

app.listen(3000, function () {
    
  console.log('server started.');
});

after , stay express Run on path node helloworld.js. And access http://localhost:3000,

understand Hello World

express()

What does the above code mean . I didn't bother to remember the first line

let app = express();

express() Create a Express application , This is a result of express Top level functions exposed by the module . What does that mean , This means that if you change the imported module name to springBoot Then the code can still run

let springBoot = require('express');
let app = springBoot();

Ha ha ha , This is because express What the module exposes is a function . Here is node_modules/express/lib/express.js Code for

/** * Expose `createApplication()`. */
exports = module.exports = createApplication;

get()

app.get('/', function (req, res) {
    
  console.log('hello, world');
  res.send('hello, world');
});

get() Methods will GET The request is routed to the specified path , And call the specified callback function . The prototype of this method is
app.get(path, callback [, callback ...]). The first parameter is the address of the route , namely GET The request path for .🤨 You may ask , What I am asking is not http://localhost:3000 Do you ？ Why is it in the code / Well ？

Open the control panel , At a glance . What seems to be visited is http://localhost:3000, It's actually http://localhost:3000/
get() The first parameter of the method is path. This parameter can be a string , It could be regular , It can also be arrays . The default is /, Represents the root path . Even if you ask for the root path , The first parameter cannot be left blank ！

If the user accesses , No path is lost (http://localhost:3000) Or lose (http://localhost:3000/index) We all want to go back hello, word What shall I do? ？ Try this

app.get(['/', '/index'], function (req, res) {
    
  console.log('hello, world');
  res.send('hello, world');
});

More about regular and so on , You can refer to * here

Then look get() The second argument to the method callback Callback function . The callback function receives 2 Parameters （3 Parameters, wait ） this 2 The first parameter is well-known request and response. What appears in the code send() Method means to send HTTP Respond to .

res.send('hello, world');

What is the third parameter that pops up ？next
express Modules allow us to request the same , Set different callback functions , These callback functions pass through next call , Next, let's take the example above . Although the access root path / and /index All return the same content , But we do something special like accessing the root path , You can write like this

app.get('/', function (req, res, next) {
    
  console.log(' I am the root path ');
  next(); //  Next callback function 
});
app.get(['/', '/index'], function (req, res) {
    
  console.log('hello, world');
  res.send('hello, world');
});

listen()

What this function specifies is express Port on which the program listens . If the port is ignored or 0, The operating system will assign any unused port .
You can also specify the host that the city permits to listen to , as follows

app.listen(3000, '127.0.0.1', function () {
    
  console.log('server started.');
});

You can also receive error messages in the callback function . because 3306 Has been used by this computer mysql Occupy , So startup failed listen EACCES: permission denied 127.0.0.1:3306

app.listen(3306, '127.0.0.1', function (err) {
    
  if (err) {
    
    console.log(' Start exception ', err);
  }
  console.log('server started.');
});

Okay , This is it. express Of Hello World The whole thing . The next plan is to write get The requested part . come on. ！！！！