JavaScript Debug: A simple wrapper for console.log


This code provides a simple wrapper for the console's logging methods, and was created to allow a very easy-to-use, cross-browser logging solution, without requiring excessive or unwieldy object detection. If a console object is not detected, all logged messages will be stored internally until a logging callback is added. If a console object is detected, but doesn't have any of the debug, info, warn, and error logging methods, log will be used in their place. For convenience, some of the less common console methods will be passed through to the console object if they are detected, otherwise they will simply fail gracefully.

Benefits of using debug over console directly

  • No errors are thrown if console object doesn't exist, and no extra object detection is required, just log away!
  • If the specified logging method doesn't exist, method will degrade to log.
  • If console object doesn't exist, logs are stored internally and can be shown if and when an alternate console is added with debug.setCallback.
  • Console logging can be disabled at any time with debug.setLevel(0). This won't stop debug from storing logs, which allows you to inject a console (like Firebug Lite) or log viewer after the fact, to see already-logged data.
  • In WebKit, logged arguments are wrapped in an array [] for a more visually pleasing visual output.
// Instead of this:
window.console && console.log
  && console.log( this, 'that', { the: 'other' } );

// Just do this:
debug.log( this, 'that', { the: 'other' } );

Supported logging methods, and their degradation path

  • debug.log > console.log
  • debug.debug > console.debug > console.log
  • > > console.log
  • debug.warn > console.warn > console.log
  • debug.error > console.error > console.log

For example:

// If the current console doesn't have any of the four non-log
// methods used here, log will be used.

var a = 0,
  b = 'two',
  c = { foo: 1, bar: 2, baz: 'three' },
  d = false,
  e = [ 3, 4, 5, 6, 7, 8 ];

debug.log( a );
debug.debug( b ); c );
debug.warn( d );
debug.error( e );

(View a live example)

Pass-through console methods

  • assert, clear, count, dir, dirxml, exception, group, groupCollapsed, groupEnd, profile, profileEnd, table, time, timeEnd, trace

These console methods are passed through (but only if both the console and the method exists), so use them without fear of reprisal. Note that these methods will not be passed through if the logging level is set to 0 via debug.setLevel.

For example:

// debug.time and debug.timeEnd only do something if console.time
// and console.timeEnd are defined, otherwise they fail gracefully.

debug.time( 'test' );
for ( var i = 0; i < 10000; i++ ) {
  document.createElement( 'div' );
debug.timeEnd( 'test' );

(View a live example)

So, what do I do when console doesn't exist?

Here's a useful bookmarklet (what's a bookmarklet?): Debug + Firebug Lite

When clicked, if the page doesn't use JavaScript Debug, this bookmarklet will just open Firebug Lite. If the page does use JavaScript Debug, the bookmarklet will open Firebug Lite and pre-populate it with any already-logged items via the debug.setCallback method. View the documentation for more information on debug.setCallback.

This bookmarklet is just one way of viewing debug logs, if you don't have a console. You could just as easily write your own alternative console, and log to that, for example:

// This example uses jQuery, but yours doesn't need to.

function debug_callback( level ) {
  var args = arguments, 1 );
  $('#debug').length || $('<div id="debug"/>').appendTo( 'body' );
    .addClass( 'debug-' + level )
    .html( '[' + level + '] ' + args )
    .appendTo( '#debug' );

debug.setCallback( debug_callback );

(View a live example)

Can I temporarily disable logging?

Yes, very easily. Just add debug.setLevel(0) into your code, and all console logging will be disabled. This is very useful when you need your page or application to go live, but you're not ready to remove all your debugging hooks. Of course, this only disables console logging--since debug is still keeping track of logged messages, you can inject an alternate console (see the bookmarklet above) at any time to view the logs. View the documentation for more information on debug.setLevel.

Thanks again to Paul Irish and Adam Sontag for their help with the API and examples.

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