JSFiddle - React, Tailwind, and code Playground

JavaScript

/* For a given date, get the ISO week number
 *
 * Based on information at:
 *
 *    http://www.merlyn.demon.co.uk/weekcalc.htm#WNR
 *
 * Algorithm is to find nearest thursday, it's year
 * is the year of the week number. Then get weeks
 * between that date and the first day of that year.
 *
 * Note that dates in one year can be weeks of previous
 * or next year, overlap is up to 3 days.
 *
 * e.g. 2014/12/29 is Monday in week  1 of 2015
 *      2012/1/1   is Sunday in week 52 of 2011
 */
function getWeekNumber(d) {
    // Copy date so don't modify original
    d = new Date(+d);
    d.setHours(0, 0, 0);
    // Set to nearest Thursday: current date + 4 - current day number
    // Make Sunday's day number 7
    d.setDate(d.getDate() + 4 - (d.getDay() || 7));
    // Get first day of year
    var yearStart = new Date(d.getFullYear(), 0, 1);
    // Calculate full weeks to nearest Thursday
    var weekNo = Math.ceil((((d - yearStart) / 86400000) + 1) / 7)
    // Return array of year and week number
    return weekNo;
}

var path = "images/week_" + getWeekNumber(new Date()) + "/image_day_" + new Date().getDay();
alert(path);