Debugging Express

console.log solves most bugs. For the rest, attach a real debugger.

Chrome DevTools

node --inspect-brk src/index.js

Visit chrome://inspect → click “inspect” → full DevTools: set breakpoints, watch variables, profile.

--inspect-brk pauses on the first line. --inspect doesn’t.

VS Code

Create .vscode/launch.json:

{
  "version": "0.2.0",
  "configurations": [
    {
      "type": "node",
      "request": "launch",
      "name": "Express",
      "program": "${workspaceFolder}/src/index.js",
      "envFile": "${workspaceFolder}/.env",
      "runtimeArgs": ["--watch"],
      "skipFiles": ["<node_internals>/**"]
    }
  ]
}

Press F5. Set breakpoints in the gutter of any file.

Inline debugger;

Drop a debugger; statement anywhere in your code:

app.post("/transfer", (req, res) => {
  if (req.body.amount > 10000) {
    debugger;   // pauses here when running with --inspect
  }
  // ...
});

The debug Package

Express uses this internally. Enable verbose logging:

DEBUG=express:* node src/index.js

Express logs every middleware execution, route match, error path. Invaluable when you don’t understand why a route isn’t matching.

To add it to your own code:

npm install debug

import debug from "debug";

const log = debug("myapp:auth");

export function requireAuth(req, res, next) {
  log("auth check for %s", req.url);
  // ...
}

Enable with DEBUG=myapp:* node src/index.js. Disabled = zero overhead.

Production Debugging

You can’t run --inspect on production. Use:

• Logs — structured, with request IDs, queryable
• Sentry — full stack traces with request context
• Tracing — see the path through your services
• Feature flags — toggle logging without redeploy

Building observability into the app is how you debug production.

Common Express Issues