Get last monday in month in moment.js

You're almost there. You just need to add a simple loop to step backward day-by-day until you find a Monday:

result = moment().endOf('month');
while (result.day() !== 1) {
    result.subtract(1, 'day');
}
return result;

you get always monday with isoweek:

moment().endOf('month').startOf('isoweek')