tiny node.js debugging utility modelled after node core's debugging technique.
$ npm install debug
With debug you simply invoke the exported function to generate your debug function, passing it a name which will determine if a noop function is returned, or a decorated console.error, so all of the console format string goodies you're used to work fine. A unique color is selected per-function for visibility.
var debug = require('debug')('http')
, http = require('http')
, name = 'My App';
// fake app
debug('booting %s', name);
debug(req.method + ' ' + req.url);
// fake worker of some kind
var debug = require('debug')('worker');
debug('doing some work');
The DEBUG environment variable is then used to enable these based on space or comma-delimited names. Here are some examples:
On Windows the environment variable is set using the set command.
Then, run the program to be debugged as usual.
When actively developing an application it can be useful to see when the time spent between one debug() call and the next. Suppose for example you invoke debug() before requesting a resource, and after as well, the "+NNNms" will show you how much time was spent between calls.
When stdout is not a TTY, Date#toUTCString() is used, making it more useful for logging the debug information as shown below:
If you're using this in one or more of your libraries, you should use the name of your library so that developers may toggle debugging as desired without guessing names. If you have more than one debuggers you should prefix them with your library name and use ":" to separate features. For example "bodyParser" from Connect would then be "connect:bodyParser".
The * character may be used as a wildcard. Suppose for example your library has debuggers named "connect:bodyParser", "connect:compress", "connect:session", instead of listing all three with DEBUG=connect:bodyParser,connect:compress,connect:session, you may simply do DEBUG=connect:*, or to run everything using this module simply use DEBUG=*.
You can also exclude specific debuggers by prefixing them with a "-" character. For example, DEBUG=*,-connect:* would include all debuggers except those starting with "connect:".
Debug works in the browser as well, currently persisted by localStorage. Consider the situation shown below where you have worker:a and worker:b, and wish to debug both. Somewhere in the code on your page, include:
window.myDebug = require("debug");
("debug" is a global object in the browser so we give this object a different name.) When your page is open in the browser, type the following in the console:
Refresh the page. Debug output will continue to be sent to the console until it is disabled by typing myDebug.disable() in the console.
a = debug('worker:a');
b = debug('worker:b');
a('doing some work');
b('doing some work');
Web Inspector Colors
Colors are also enabled on "Web Inspectors" that understand the %c formatting
option. These are WebKit web inspectors, Firefox (since version
and the Firebug plugin for Firefox (any version).
Colored output looks something like:
stderr vs stdout
You can set an alternative logging method per-namespace by overriding the log method on a per-namespace or globally:
var debug = require('debug');
var error = debug('app:error');
// by default stderr is used
error('goes to stderr!');
var log = debug('app:log');
// set this namespace tolog via console.loglog.log = console.log.bind(console); // don't forget to bind to console!
log('goes to stdout');
error('still goes to stderr!');
// setall output to go via console.info
// overrides all per-namespace log settings
debug.log = console.info.bind(console);
error('now goes to stdout via console.info');
log('still goes to stdout, but via console.info now');
Save debug output to a file
You can save all debug statements to a file by piping them.
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