探索 Express 中间件中'下一个”函数的功能及其在重定向到另一个函数中的作用。

Question

我是Nodejs新手，正在研究Express js，现在我正在研究特定路由的“中间件函数”，我想知道“next有什么用”，意味着在验证“next”函数可以做什么之后做 ？如果我们想移动/重定向到另一个函数那么我们该怎么做？什么是“checkAuthentication”？这是我当前的代码

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

// Custom middleware function
const authMiddleware = (req, res, next) => {
  // Check if user is authenticated
  const isAuthenticated = checkAuthentication(req);
  
  if (isAuthenticated) {
    next();
  } else {
    // User is not authenticated, send an unauthorized response
    res.status(401).send('Unauthorized');
  }
};

// Middleware function is applied to specific routes
app.get('/protected', authMiddleware, (req, res) => {
  res.send('Protected Route');
});

// Route handler
app.get('/', (req, res) => {
  res.send('Home Page');
});

// Start the server
app.listen(3000, () => {
  console.log('Server is listening on port 3000');
});

Answer 1

接下来是传递给中间件函数的回调函数。您可以在不同的框架中找到它的不同名称，但概念保持不变。

我将尝试通过您的代码本身来解释中间件。

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

function checkAuthentication(req) {
  /*
  I am considering this as my authentication function. 
  When the user logged in, server has sent him a token, which now will act as a validator. 
  */

  if (req.headers.token) {
    const user = someFunctionToFetchUser(req.header.token)
    return user
  } else {
    return false
  }
}

/*
Now I want a function to be there on each protected api routes
to make user if user has a valid token then only allow them to interact with the data base otherwise throw an error 
*/


const authMiddleware = (req, res, next) => {
  // Check if user is authenticated
  const isAuthenticated = checkAuthentication(req);

  if (isAuthenticated) {
    // Now you have verified the has valid a token. So allow user 
    // to reach to the controller or any other middleware, if any.
    next();
  } else {
    // User is not authenticated, send an unauthorized response
    res.status(401).send('Unauthorized');
  }
};

// Middleware function is applied to specific routes
app.get('/protected', authMiddleware, (req, res) => {
  res.send('Protected Route');
});

// You can have any number of middlewares
app.get('/protected', authMiddleware, someOtherMiddleware, (req, res) => {
  res.send('Protected Route');
});

// And also you can pass data from middleware to next middleware/controller also by attahching it with request

function newMiddleware(req, res, next) {
  req.foo = "bar" //Now you can access this foo variable in next function.
}