util.debuglog()

util.debuglog(section)

This is used to create a function which conditionally writes to stderr based on the existence of a NODE_DEBUG environment variable. If the section name appears in that environment variable, then the returned function will be similar to console.error(). If not, then the returned function is a no-op.

For example:

var debuglog = util.debuglog('foo');

var bar = 123;
debuglog('hello from foo [%d]', bar);

If this program is run with NODE_DEBUG=foo in the environment, then it will output something like:

FOO 3245: hello from foo [123]

where 3245 is the process id. If it is not run with that environment variable set, then it will not print anything.

You may separate multiple NODE_DEBUG environment variables with a comma. For example, NODE_DEBUG=fs,net,tls.