Liferay uses AlloyUI for frontend development by default. AlloyUI is a wrapper around YUI. With that said, all of the YUI development niceties are available for debugging AlloyUI code. For instance, the console:

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AUI().ready('aui-base', 'console', function(A) {
    new A.Console({
        height: '250px',
        newestOnTop: false,
        style: 'separate',
        visible: true,
        width: '600px'
    }).render();
});

Sweet. Now, I can do things like:

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AUI().use('aui-base', function(A) {
    A.log("This is some test that will log to the YUI console.");
});