The util.debuglog() method is used to create a function that conditionally writes debug messages to stderr based on the existence of the NODE_DEBUG environment variable. If the section name appears within the value of that environment variable, then the returned function operates similar to console.error(). If not, then the returned function is a no-op.
import { debuglog } from 'node:util';
const log = debuglog('foo');
log('hello from foo [%d]', 123);
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.
The section supports wildcard also:
import { debuglog } from 'node:util';
const log = debuglog('foo');
log('hi there, it\'s foo-bar [%d]', 2333);
if it is run with NODE_DEBUG=foo* in the environment, then it will output something like:
FOO-BAR 3257: hi there, it's foo-bar [2333]
Multiple comma-separated section names may be specified in the NODE_DEBUG environment variable: NODE_DEBUG=fs,net,tls.
The optional callback argument can be used to replace the logging function with a different function that doesn't have any initialization or unnecessary wrapping.
import { debuglog } from 'node:util';
let log = debuglog('internals', (debug) => {
// Replace with a logging function that optimizes out
// testing if the section is enabled
log = debug;
});