wp/app/plugins/debug-bar/js/debug-bar-js.dev.js

(function() {
	var count, list, dbjsError,
		rawCount = 0,
		errors = [];

	window.onerror = function( errorMsg, url, lineNumber ) {
		if ( ! document.getElementById( 'debug-bar-js-error-count' ) )
			errors[ errors.length ] = [errorMsg, url, lineNumber];
		else
			dbjsError(errorMsg, url, lineNumber);
	}

	jQuery(document).ready( function(){
		for ( err in errors )
			dbjsError( errors[err][0], errors[err][1], errors[err][2] );

	});

	dbjsError = function( errorMsg, url, lineNumber ) {

		var errorLine, place, button, tab;

		rawCount++;

		if ( !count )
			count = document.getElementById( 'debug-bar-js-error-count' );
		if ( !list )
			list = document.getElementById( 'debug-bar-js-errors' );

		if ( !count || !list )
			return; // threw way too early... @todo cache these?

		if ( 1 == rawCount ) {
			button = document.getElementById( 'wp-admin-bar-debug-bar' );
			if ( !button )
				return; // how did this happen?
			if ( button.className.indexOf( 'debug-bar-php-warning-summary' ) === -1 )
				button.className = button.className + ' debug-bar-php-warning-summary';

			tab = document.getElementById('debug-menu-link-Debug_Bar_JS');
			if ( tab )
				tab.style.display = 'block';
		}

		count.textContent = rawCount;
		errorLine = document.createElement( 'li' );
		errorLine.className = 'debug-bar-js-error';
		errorLine.textContent = errorMsg;
		place = document.createElement( 'span' );
		place.textContent = url + ' line ' + lineNumber;
		errorLine.appendChild( place );
		list.appendChild( errorLine );

	};

})();