使用jQuery或JS从给定日期获取上一个星期六

Question

I am trying to get the last Saturday from a given date. If function argument is 9/12/2019 it should return 9/7/2019 .

I have tried two methods below with no success:

 function getLastSaturday(theDate) { debugger var dateToUse = new Date(theDate); var lastSaturday = new Date(new Date().setDate(dateToUse.getDate() - (dateToUse.getDay() == 0 ? 7 : dateToUse.getDay() + 1))); return lastSaturday; } function getLatestSaturday(theDate) { var dateToUse = new Date(theDate); var latestSaturday = new Date(new Date().setDate(dateToUse.getDate() - dateToUse.getDay() + 1)); return latestSaturday; } console.log(getLastSaturday('9/12/2019')); console.log(getLatestSaturday('9/12/2019')); console.log(getLastSaturday('8/5/2019')); 

Update

Modified getLastSaturday(theDate) that works

function getLastSaturday(theDate) {
    var dateToUse = new Date(theDate);
    var start = dateToUse.getDay() == 0 ? 7 : dateToUse.getDay();
    var target = 6; // Saturday

    if (target >= start)
        target -= 7;
    var lastSaturday = dateToUse.addDays(target - start);
    return lastSaturday;
}

Date.prototype.addDays = function(days) {
    var date = new Date(this.valueOf());
    date.setDate(date.getDate() + days);
    return date;
}

Answer 1

Consider the line:

var lastSaturday = new Date(new Date().setDate(dateToUse.getDate() - (dateToUse.getDay() == 0 ? 7 : dateToUse.getDay() + 1)));

This gets the current date, then subtracts the day number of some other date. All you need to do is subtract the date's day number + 1, eg

 function getLastSaturday(date) { // Copy date so don't modify original let d = new Date(date); // Adjust to previous Saturday d.setDate(d.getDate() - (d.getDay() + 1)); return d; } // Samples [new Date(2019,8,23), new Date(2019,8, 1), new Date(2019,8, 7), new Date(2019,8,12) ].forEach(d => console.log(d.toString() + ' => ' + getLastSaturday(d).toString() )); 

Also see get previous saturday's date and next friday's .

Using the built–in parser for unsupported string formats is strongly recommended against, see Why does Date.parse give incorrect results?

The date format "9/7/2019" is ambiguous, it represents 9 July to most people or 7 September to some. The simplest way to avoid confusion is to use the month name rather than number.