Starting the week on Monday with isoWeekday()

Call startOf before isoWeekday.

var begin = moment(date).startOf('week').isoWeekday(1);

Working demo

You just need to replace begin.startOf('isoWeek'); with begin.startOf('week');.

This way you can set the initial day of the week.

moment.locale('en', {
    week: {
        dow: 6
    }
});
moment.locale('en');

Make sure to use it with moment().weekday(1); instead of moment.isoWeekday(1)

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