Debugging Express

To see all the internal logs used in Express, set the DEBUG environment variable to express:* when launching your app.

$ DEBUG=express:* node index.js

On Windows, use the corresponding command.

> set DEBUG=express:* & node index.js

Running this command on the default app generated by the express generator prints the following output:

$ DEBUG=express:* node ./bin/www
  express:router:route new / +0ms
  express:router:layer new / +1ms
  express:router:route get / +1ms
  express:router:layer new / +0ms
  express:router:route new / +1ms
  express:router:layer new / +0ms
  express:router:route get / +0ms
  express:router:layer new / +0ms
  express:application compile etag weak +1ms
  express:application compile query parser extended +0ms
  express:application compile trust proxy false +0ms
  express:application booting in development mode +1ms
  express:router use / query +0ms
  express:router:layer new / +0ms
  express:router use / expressInit +0ms
  express:router:layer new / +0ms
  express:router use / favicon +1ms
  express:router:layer new / +0ms
  express:router use / logger +0ms
  express:router:layer new / +0ms
  express:router use / jsonParser +0ms
  express:router:layer new / +1ms
  express:router use / urlencodedParser +0ms
  express:router:layer new / +0ms
  express:router use / cookieParser +0ms
  express:router:layer new / +0ms
  express:router use / stylus +90ms
  express:router:layer new / +0ms
  express:router use / serveStatic +0ms
  express:router:layer new / +0ms
  express:router use / router +0ms
  express:router:layer new / +1ms
  express:router use /users router +0ms
  express:router:layer new /users +0ms
  express:router use / <anonymous> +0ms
  express:router:layer new / +0ms
  express:router use / <anonymous> +0ms
  express:router:layer new / +0ms
  express:router use / <anonymous> +0ms
  express:router:layer new / +0ms

When a request is then made to the app, you will see the logs specified in the Express code:

  express:router dispatching GET / +4h
  express:router query  : / +2ms
  express:router expressInit  : / +0ms
  express:router favicon  : / +0ms
  express:router logger  : / +1ms
  express:router jsonParser  : / +0ms
  express:router urlencodedParser  : / +1ms
  express:router cookieParser  : / +0ms
  express:router stylus  : / +0ms
  express:router serveStatic  : / +2ms
  express:router router  : / +2ms
  express:router dispatching GET / +1ms
  express:view lookup "index.pug" +338ms
  express:view stat "/projects/example/views/index.pug" +0ms
  express:view render "/projects/example/views/index.pug" +1ms

To see the logs only from the router implementation, set the value of DEBUG to express:router. Likewise, to see logs only from the application implementation, set the value of DEBUG to express:application, and so on.

Applications generated by express

An application generated by the express command uses the debug module and its debug namespace is scoped to the name of the application.

For example, if you generated the app with $ express sample-app, you can enable the debug statements with the following command:

$ DEBUG=sample-app:* node ./bin/www

You can specify more than one debug namespace by assigning a comma-separated list of names:

$ DEBUG=http,mail,express:* node index.js

Advanced options

When running through Node.js, you can set a few environment variables that will change the behavior of the debug logging:

Name	Purpose
DEBUG	Enables/disables specific debugging namespaces.
DEBUG_COLORS	Whether or not to use colors in the debug output.
DEBUG_DEPTH	Object inspection depth.
DEBUG_FD	File descriptor to write debug output to.
DEBUG_SHOW_HIDDEN	Shows hidden properties on inspected objects.