Question 184493 in Javascript Question posted in
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Javascript – What is the parameter "next" used for in Express -Nodejs?

I am working with Nodejs and i am using express js,And right now i am using middleware function,In middleware
function we have three main parameters "req,res,next", In "next" parameter…go to next "middleware function"

  1. I have three middleware function but i call/use/execute only "first middleware function", so after execute first middleware ( reach to "next parameter" in first middleware) if "next parameter" go to "next middleware function" then why not "middleware2" and "middleware3" execute automatically ? Here is my current code
    const express = require('express');
    const app = express();
    
    // First middleware function
    const middleware1 = (req, res, next) => {
      console.log('This is middleware 1');
      next(); // Calling next() passes control to the next middleware function
    };
    
    // Second middleware function
    const middleware2 = (req, res, next) => {
      console.log('This is middleware 2');
      next(); // Calling next() passes control to the next middleware function
    };
    
    // Third middleware function
    const middleware3 = (req, res, next) => {
      console.log('This is middleware 3');
      next(); // Calling next() passes control to the next middleware function
    };
    
    app.use(middleware1);
    
    // Route handler
    app.get('/', (req, res) => {
      res.send('Hello, World!');
    });
    
    app.listen(3000, () => {
      console.log('Server listening on port 3000');
    });
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2

Answers


  1. 0

    You’ve just defined these middlewares but haven’t added them to the express pipeline like the first middleware.

    Once you add them using app.use() they’ll be executed in the order they are added (given that next() is called in the preceding middleware)

    update

    A middleware should either explicitly end the request (for example send a response back to client) or pass control to next middleware in the pipeline. If not the request will be hanging.

    From expressjs website:

    If the current middleware function does not end the request-response cycle, it must call next() to pass control to the next middleware function. Otherwise, the request will be left hanging.

    So by calling next we inform express that this middleware has finished processing the request and wants to pass control to the next middleware.

    The next function is a function in the Express router which, when invoked, executes the middleware succeeding the current middleware.

    Example

    Say you have 3 middlewares for logging, authenticating and authorizing a request.

    1- logging middleware gets the request and logs its info into a file then calls next to pass control to authentication middleware.

    2- authentication middleware checks if the request is from a logged in user or not. If user is logged in then it calls next to pass control to authorization middleware. But if user in anonymous this middleware does not allow the request to proceed to next middleware and ends the request by sending an error back to client.

    Hope this clarifies the concept.

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  2. 0

    if "next parameter" go to "next middleware function" then why not "middleware2" and "middleware3" execute automatically ? Here is my current codestrong text

    next calls the next thing the server knows about, which you have registered, whether that’s a middleware or the HTTP handling function. Its purpose is to simply move the request through the server, not to magically jump around to your other functions "automatically".

    You still have to actually tell the server about each middleware you intend to use, next can’t jump away to code that the server has never seen and has no idea exists. If you don’t tell the server about middleware2, and middleware1 doesn’t directly call middleware2, then how could the flow of execution possibly reach that function?

    okay but still my point about "next" if we use "next" then what will be effect and if we will not use "next" then what will happen ? kindly explain only about "next" parameter

    If you don’t call next, then the server simply doesn’t work. It’s not a convienence thing for moving through middleware, it’s required to pass the flow of execution through the server. If you don’t call it, your middelware1 function will be the only thing that the server calls to handle a request, which does nothing. Your HTTP handling function is never reached, and the browser will eventually time-out without receiving a response.

    You must call next so that the next thing in line to process a request can receive control, whether that’s a subsequent middleware or the handlers you’ve registered with app.get('/', ...).

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