Express.js is a popular framework for building web applications in Node.js, but even seasoned developers encounter errors that can be tricky to debug. This guide will cover some of the most common Express.js errors, explain why they occur, and provide practical fixes to get your application back on track.
This error usually occurs when you attempt to send multiple responses for the same request. For example, you might accidentally call res.send() or res.json() more than once in a route handler.
Example:
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Fix:
Ensure you only send one response per request. Use conditional logic or return statements to prevent further execution after sending a response.
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This happens when middleware is not properly linked or next() is not called within it. Middleware functions must explicitly pass control to the next middleware or route handler.
Example:
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Fix:
Call next() unless the middleware ends the response.
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If req.body is undefined, it's likely because you forgot to use a body-parsing middleware, such as express.json() or express.urlencoded().
Example:
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Fix:
Include the body-parsing middleware in your app initialization.
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This error occurs when no route matches the incoming request. By default, Express doesn’t provide a 404 handler.
Fix:
Add a catch-all middleware at the end of your route definitions to handle 404 errors.
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This happens when another process is already using the port your app is trying to bind to.
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Fix:
Find and terminate the conflicting process or use a different port. You can also handle the error programmatically:
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Express.js errors can be frustrating, but understanding their root causes makes them easier to solve. With these common fixes, you’ll be better equipped to debug your applications and keep your projects running smoothly.
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