How to get timestamp of last Friday using JavaScript?

const t = new Date().getDate() + (6 - new Date().getDay() - 1) - 7 ;
const lastFriday = new Date();
lastFriday.setDate(t);
console.log(lastFriday);

First you need to get a date diff to last friday:

var now = new Date(),
    day = now.getDay();

Depending on what "last friday" means to you, use following

• nearest past friday

  var diff = (day <= 5) ? (7 - 5 + day ) : (day - 5);

• if today is friday, then return today, otherwise nearest past friday

  var diff = (7 - 5 + day) % 7;

• friday of the last week

  var diff = 7 - 5 + day;

Then substract this from the current date, and set result to the date object using setDate function. The setDate function will correctly handle negative numbers, changing month and year respectively:

From mdn:

If the dayValue is outside of the range of date values for the month, setDate() will update the Date object accordingly. For example, if 0 is provided for dayValue, the date will be set to the last day of the previous month.

var date = new Date();
date.setDate(now.getDate() - diff);
date.setHours(0);
date.setMinutes(0);
date.setSeconds(0);

var friday = date.getTime();

Full code

function getLastFridayOf(date) {
    var d = new Date(date),
        day = d.getDay(),
        diff = (day <= 5) ? (7 - 5 + day ) : (day - 5);

    d.setDate(d.getDate() - diff);
    d.setHours(0);
    d.setMinutes(0);
    d.setSeconds(0);

    return d.getTime();
}

Following for example only, it depends on a JS engine, and may produce different results

new Date(getLastFridayOf('2015-05-01')).toDateString() 
-> Fri Apr 24 2015

new Date(getLastFridayOf('2015-05-19')).toDateString() 
-> Fri May 15 2015

new Date(getLastFridayOf('2015-05-16')).toDateString() 
-> Fri May 15 2015

new Date(getLastFridayOf('2015-05-15')).toDateString() 
-> Fri May 08 2015