A string identifying the portion of the application for which the debuglog function is being created.
Optional callback: ((fn: "util".DebugLoggerFunction) => void)A callback invoked the first time the logging function is called with a function argument that is a more optimized logging function.
The util.debuglog() method is used to create a function that conditionally
writes debug messages to stderr based on the existence of the NODE_DEBUGenvironment 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('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('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_DEBUGenvironment 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('util');
let debuglog = util.debuglog('internals', (debug) => {
// Replace with a logging function that optimizes out
// testing if the section is enabled
debuglog = debug;
});
The logging function
A string identifying the portion of the application for which the debuglog function is being created.
Optional callback: ((fn: "util".DebugLoggerFunction) => void)A callback invoked the first time the logging function is called with a function argument that is a more optimized logging function.
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The
util.debuglog()method is used to create a function that conditionally writes debug messages tostderrbased on the existence of theNODE_DEBUGenvironment variable. If thesectionname appears within the value of that environment variable, then the returned function operates similar toconsole.error(). If not, then the returned function is a no-op.If this program is run with
NODE_DEBUG=fooin the environment, then it will output something like:where
3245is the process id. If it is not run with that environment variable set, then it will not print anything.The
sectionsupports wildcard also:if it is run with
NODE_DEBUG=foo*in the environment, then it will output something like:Multiple comma-separated
sectionnames may be specified in theNODE_DEBUGenvironment variable:NODE_DEBUG=fs,net,tls.The optional
callbackargument can be used to replace the logging function with a different function that doesn't have any initialization or unnecessary wrapping.Returns
The logging function