This is an HTML response.
');\n});\n\nconst PORT = 3000;\napp.listen(PORT, () => {\n console.log(`Server listening on port ${PORT}`);\n console.log(`Test with: curl http://localhost:${PORT}/user`);\n console.log(`Test with: curl http://localhost:${PORT}/html`);\n});","lang":"typescript","description":"This quickstart demonstrates how to integrate `modify-response-middleware` into an Express application to intercept and modify JSON response bodies before they are sent to the client."},"warnings":[{"fix":"Ensure `app.use(modifyRes(...))` is declared early in your Express application's middleware stack, ideally before route definitions. If using router-level middleware, place it before route-specific handlers within that router.","message":"Middleware order is critical. `modify-response-middleware` must be applied *before* any route handlers or other middleware that send responses (e.g., `res.send()`, `res.json()`). Placing it after these handlers will result in it never being invoked for those responses, as headers might have already been sent.","severity":"breaking","affected_versions":">=1.0.0"},{"fix":"Profile your application with and without the middleware in production scenarios. Consider applying the middleware only to specific routes (using `app.use('/path', modifyRes(...))`) or conditionally based on response size if performance becomes an issue.","message":"Modifying response bodies, especially after decompression and before re-compression, can introduce performance overhead. For very large responses or high-traffic applications, this can significantly impact server throughput and latency due to CPU usage for (de)compression and memory for buffering.","severity":"gotcha","affected_versions":">=1.0.0"},{"fix":"If `noCache` is not suitable, ensure your modified response handler explicitly removes or recalculates `Content-Length` and `ETag` headers if the body size changes significantly. Monitor network responses to ensure headers align with modified content.","message":"When modifying response content, `Content-Length` and `ETag` headers can become invalid. The `noCache: true` option helps mitigate `ETag` issues by setting `Cache-Control: no-cache`, but if `Content-Length` is present and incorrect, it can lead to client-side issues or truncated responses. The middleware attempts to handle this, but explicit verification is sometimes needed.","severity":"gotcha","affected_versions":">=1.0.0"},{"fix":"Always validate the content type (e.g., `res.get('Content-Type')`) within your `modifyRes` callback before attempting type-specific parsing (e.g., `JSON.parse()`). Wrap parsing logic in `try...catch` blocks to handle malformed or unexpected data gracefully. Return the original `content` buffer if parsing/modification fails.","message":"The callback function provided to `modifyRes` expects `content` to be a `Buffer`. If your modification logic assumes a specific content type (e.g., JSON) and attempts parsing without proper checks, it can throw errors for other content types (e.g., HTML, plain text, images).","severity":"gotcha","affected_versions":">=1.0.0"}],"env_vars":null,"last_verified":"2026-04-23T00:00:00.000Z","next_check":"2026-07-22T00:00:00.000Z","problems":[{"fix":"Before parsing, check `res.get('Content-Type')` within your `modifyRes` callback to ensure it's `application/json`. Add a `try...catch` block around `JSON.parse()` to handle non-JSON or malformed responses gracefully.","cause":"Attempting to `JSON.parse()` a non-JSON response, often an HTML error page or another content type, because the `Content-Type` header was not checked.","error":"SyntaxError: Unexpected token < in JSON at position 0"},{"fix":"Always add a check `if (content && Buffer.isBuffer(content))` before calling `content.toString()` or attempting to process `content`. For empty responses, `content` might legitimately be `null` or an empty buffer, in which case it should be returned as is.","cause":"The `content` parameter passed to your callback is `null`, `undefined`, or not a `Buffer` instance, often for empty responses or if upstream middleware unexpectedly altered the body format.","error":"TypeError: content.toString is not a function"},{"fix":"When the response body is modified, the middleware should ideally reset or remove the `Content-Length` header. If issues persist, ensure the middleware or your callback removes `res.removeHeader('Content-Length')` or explicitly sets `res.setHeader('Content-Length', Buffer.byteLength(newContent))` for non-compressed content.","cause":"The `Content-Length` header sent by the server does not match the actual byte length of the modified response body, causing client-side errors or incomplete downloads.","error":"(Client-side) ERR_CONTENT_LENGTH_MISMATCH or truncated responses"}],"ecosystem":"npm","meta_description":null,"install_score":null,"install_tag":null,"quickstart_score":null,"quickstart_tag":null}