algorithm - JavaScript - get the first day of the week from current date - Stack Overflow

Using the getDay method of Date objects, you can know the number of day of the week (being 0=Sunday, 1=Monday, etc).

You can then subtract that number of days plus one, for example:

function getMonday(d) {    d = new Date(d);    var day = d.getDay(),        diff = d.getDate() - day + (day == 0 ? -6:1); // adjust when day is sunday    return new Date(d.setDate(diff));  }    getMonday(new Date()); // Mon Nov 08 2010

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