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Express JS app.use() Function

Express JS app.use() Function

By Apoorv Dwivedi
2 mins read
Last updated: 30 Jan 2024
184 views
Topics Covered

Overview

Server-side frameworks are essential in web development because they make creating reliable and practical web applications easier. Among these frameworks, Express.js stands out as a well-liked option because of its adaptability, simple aesthetic, and extensive feature set. App.use() is one of the foundational methods in the Express.js framework. The syntax, parameters, return value, exceptions, functionality, and in-depth examples of the app.use() function will all be covered in detail in this article.

Syntax of Express JS app.use() Function

Express.js has a middleware function called app.use(). Middleware operations provide a link between received requests and delivered responses. Before handing up control to the subsequent middleware or routing handler, they can perform various functions like authentication, logging, data processing, and more.

The syntax of the app.use() function is as follows:

app:

refers to an instance of the Express application.

path (optional):

specifies the base URL or route where the middleware should be applied. If omitted, the middleware is executed for every request.

middlewareFunction:

is the actual middleware function that processes the request and response.

Parameters of app.use()

path:

A string identifying the base URL or route where the middleware should be used, which is optional. If it is not given, the middleware will handle all requests.

middlewareFunction:

A function that accepts three arguments: req (the object representing the request), res (the object representing the answer), and next (the function to call to transfer control to the following middleware).

Return Value of app.use()

The function app.use() doesn't yield anything. It is utilized to configure middleware features for the Express application.

Exceptions of app.use()

There are no inherent exceptions in the app.use() function. The request can hang and cause timeout problems if the middleware's next function isn't called.

How does app.use() Work?

An Express server request is routed via a series of middleware functions that each handle the request or response in a particular way. Middleware is registered by the app.use() function in the order that they are defined. Before calling the next() function within another middleware, which subsequently transfers control to the next middleware in line, middleware functions are executed sequentially.

Examples of app.use()

1. Applying Middleware Globally:

2. Applying Middleware to a Specific Route:

Conclusion

In summary, the Express.js app.use() function is a cornerstone of middleware handling in web applications, offering:

  • Easily incorporate functionality like authentication and logging.
  • Manipulate incoming data and shape outgoing responses.
  • Execute middleware in the defined order, enhancing control flow.
  • Apply middleware globally or to specific routes for tailored handling.
  • Add features like error handling and data preprocessing.
  • Attention to next(): Ensure next() is invoked to pass control, preventing bottlenecks.