Debugging Express Applications
1. Using Debug Module
Example: DEBUG env
DEBUG=express:* node app.js
DEBUG=express:router,express:application node app.js
DEBUG=app:* node app.js
2. Enabling Express Debug Output
| Namespace | What it logs |
|---|
| express:application | App init / settings |
| express:router | Route matching |
| express:router:layer | Middleware execution |
| express:view | View rendering |
3. Using Node.js Debugger
Example: --inspect
node --inspect app.js
node --inspect-brk app.js # break on first line
node --inspect=0.0.0.0:9229 app.js # remote
4. Setting Breakpoints
Example: debugger statement
app.get("/users/:id", (req, res) => {
debugger; // pauses when devtools attached
res.json(getUser(req.params.id));
});
5. Using console.log() Statements
Example: Tagged logs
console.log("[users]", { id: req.params.id, query: req.query });
console.error("[users] failed", err);
Warning: Remove or downgrade console.log in production — use logger.debug() with a level switch instead.
6. Installing morgan for Logging
Example: morgan dev
app.use(morgan("dev")); // GET /users 200 4.231 ms - 432
7. Using VS Code Debugger
Example: launch.json
{
"version": "0.2.0",
"configurations": [
{
"type": "node",
"request": "launch",
"name": "Express",
"skipFiles": ["<node_internals>/**"],
"program": "${workspaceFolder}/src/server.js",
"envFile": "${workspaceFolder}/.env"
}
]
}
8. Inspecting Request/Response Objects
Example: console.dir
console.dir(req, { depth: 1, colors: true });
console.dir({ headers: req.headers, body: req.body }, { depth: null });
Example: chrome://inspect
# 1. node --inspect app.js
# 2. open chrome://inspect
# 3. click "inspect" under Remote Target
10. Analyzing Stack Traces
Example: source-map-support
// node 20+ has built-in source maps with --enable-source-maps
// node --enable-source-maps dist/server.js
| Flag | Effect |
|---|
--enable-source-maps | Map stack traces back to TS/source |
--stack-trace-limit=50 | Show more frames |
Error.captureStackTrace | Skip wrapper frames in custom errors |
11. Using nodemon for Auto-Restart
Example: node native watch
# Node 20+ — no nodemon needed:
node --watch --env-file=.env src/server.js
# Or nodemon:
npx nodemon src/server.js