Key Differences Between Global and Router-Specific Middleware in Express.js

1. Scope and Application:

   • Global Middleware: This type of middleware works with every request that comes in, no matter what route it is going to. You add it using app.use(). It's often used for tasks like keeping track of logs, checking if a user is logged in, or setting up the information that comes with a request.
   • Router-Specific Middleware: This middleware applies only to certain routes you set up in a router. You use it with router.use(). It's usually for tasks that are important for those specific routes, like checking data or logging for that route only.

2. Performance Impact:

   • Global middleware can slow down every request since it runs for all routes. If not managed well, this could affect how quickly a user gets a response. However, when done right, it can actually help cut response times by about 30%.
   • Router-specific middleware helps make everything faster by only running for the requests that need it. This is really useful, especially in bigger applications.

3. Use Cases:

   • Global middleware is great for things that apply to the whole app, like security checks and logging.
   • Router-specific middleware helps keep your code organized and easy to manage. It allows developers to group middleware based on what functions they handle or which routes they are related to.

Knowing these differences can help developers use Express.js better, making their apps work well and look organized.