debuglog
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.
const util = require('node:util');
const debuglog = util.debuglog('foo');
debuglog('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:
const util = require('node:util');
const debuglog = util.debuglog('foo-bar');
debuglog('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.
const util = require('node:util');
let debuglog = util.debuglog('internals', (debug) => {
// Replace with a logging function that optimizes out
// testing if the section is enabled
debuglog = debug;
});
Since
v0.11.3
Return
The logging function
Parameters
A string identifying the portion of the application for which the debuglog
function is being created.
A callback invoked the first time the logging function is called with a function argument that is a more optimized logging function.