django-pipeline: Why do these JS lines cause yui-compressor to crash?

I have a Django app, and within the app I'm trying to bundle and minify all Javascript files and CSS files.

To do this, I'm using django-pipeline 1.2.6, and I have configured my settings file such that it uses the default YUICompressor to compress JS and CSS files. My settings file is configured like this:

...

# PIPELINE SETTINGS (for compressing/bundling css and js files)
STATICFILES_STORAGE = "pipeline.storage.PipelineCachedStorage"

PIPELINE_YUI_BINARY = "/usr/bin/yui-compressor"

PIPELINE_JS = {
    'min': {
        'source_filenames': (              
            "js/*.js",
        ),
        'output_filename': 'js/min.js'
    }
}

PIPELINE_CSS = {
    ...
}

PIPELINE = True

When I ran the command python src/foo_app/manage.py collectstatic, I got the "broken pipe" exception. I quickly realized that yui-compressor was crashing because of an issue with the JS. So I began the process of isolating the file and line of code that was causing yui-compressor to crash.

I eventually narrowed it down to two lines that define the class attribute for the buttons on a jquery popup:

    $('#some-modal').dialog({
        ...
        buttons: [
            {
                text  : 'Print',
                click : function() {
                    ...
                },
                class : 'foo-class bar-class' // <- this line
            },
            {
                text  : 'Close',
                click : function() {
                    ...
                },
                class : 'foo-class bar-class' // <- this line
            }
        ],
    });

When I remove these class definitions, yui-compressor compresses my JS just fine. However with these lines included, yui-compressor crashes.

Why would these lines cause the compressor to crash? This is valid Javascript, and this does set the class of these buttons correctly.

Maybe this is related with yui-compressor ES6 class keywords processing. Try to wrap class key in quotes:

change this:

class : 'foo-class bar-class' // <- this line

to:

'class' : 'foo-class bar-class' // <- this line