You can just get the individual time components and calculate the total. You seem to be expecting Moment to already have this feature neatly packaged up for you, but it doesn't. I doubt it's something that people have a need for very often.
Example:
var m = moment();_x000D_
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var ms = m.milliseconds() + 1000 * (m.seconds() + 60 * (m.minutes() + 60 * m.hours()));_x000D_
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console.log(ms);
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<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.18.1/moment.min.js"></script>
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