JS Date Utilities

date-utils.js

/**
 * Given an array of days of week, return in how many days the
 * next candidate day is. If no candidate is found, then -1 is
 * returned, since delta is always positive and non-zero.
 *
 * The following chart expresses the logic, where 'X' represents
 * the `dayOfWeek` and `daysOfWeek` represent the days of next
 * possible occurrence.
 *
 * ```
 *    X  1 2 3 4 5 6 7
 *    1  D   D         => 3 .. +2
 *    1    D           => 1 .. +1
 *    2        D       => 4 .. +2
 *    2  D             => 1 .. +6
 *    4        D       => 4 .. +7
 *    4  D     D       => 1 .. +4
 * ```
 *
 * @param dayOfWeek
 * @param daysOfWeek
 */
function getNextDayDelta(dayOfWeekNow: number, daysOfWeek: number[]) {
    let selectedDelta = 8;

    for (let i = 0; i < daysOfWeek.length; i++) {
        const dayOfWeek = daysOfWeek[i];
        let nextDayDelta = dayOfWeek - dayOfWeekNow;
        if (nextDayDelta === 0) {
            // if same day, then it will be in week's time
            nextDayDelta = 7;
        } else if (nextDayDelta < 0) {
            // if negative then we substract that value (7 + -delta)
            nextDayDelta = 7 + nextDayDelta;
        }

        if (nextDayDelta < selectedDelta) {
            selectedDelta = nextDayDelta;
        }
    }

    if (selectedDelta < 8) {
        return selectedDelta;
    }

    return -1;
}

console.log(getNextDayDelta(1, [1, 3]));
console.log(getNextDayDelta(1, [2]));
console.log(getNextDayDelta(2, [4]));
console.log(getNextDayDelta(2, [1]));
console.log(getNextDayDelta(4, [4]));
console.log(getNextDayDelta(4, [1, 4]));