weekdaysBetween - get number of weekdays occurring between two moment objects (inclusively)

weekdaysBetween.test.ts

import { weekdaysBetween } from './weekdaysBetween';

describe('Report helpers', () => {
    describe('weekdaysBetween', () => {
        it('Returns 0 for a Tuesday on a 1-day Monday range', () => {
            const tuesdayCount = weekdaysBetween(moment('2018-07-02'), moment('2018-07-02'), 2);
            expect(tuesdayCount).toBe(0);
        });
        it('Returns 1 for a Monday on a 1-day Monday range', () => {
            const mondayCount = weekdaysBetween(moment('2018-07-02'), moment('2018-07-02'), 1);
            expect(mondayCount).toBe(1);
        });
        
        const isoWeekDays = Array(7).fill(0).map((_, i) => i + 1);
        it('Works for a four-day range', () => {
            const counts = {};
            isoWeekDays.forEach(isoWeekday => {
                counts[isoWeekday] = weekdaysBetween(moment('2018-04-06'), moment('2018-04-09'), isoWeekday);
            });
            expect(counts['1']).toBe(1);
            expect(counts['2']).toBe(0);
            expect(counts['3']).toBe(0);
            expect(counts['4']).toBe(0);
            expect(counts['5']).toBe(1);
            expect(counts['6']).toBe(1);
            expect(counts['7']).toBe(1);
        });
        it('Returns 1 for each day on a weeklong range', () => {
            isoWeekDays.forEach(isoWeekday => {
                const weekdayCount = weekdaysBetween(
                    moment('2018-01-01'),
                    moment('2018-01-07'),
                    isoWeekday,
                );
                expect(weekdayCount).toBe(1);
            });
        });
        it('Returns 2 for each day on a two week long range', () => {
            isoWeekDays.forEach(isoWeekday => {
                const weekdayCount = weekdaysBetween(
                    moment('2018-05-07'),
                    moment('2018-05-20'),
                    isoWeekday,
                );
                expect(weekdayCount).toBe(2);
            });
        });
        it('Returns 0 for all days for an invalid range', () => {
            isoWeekDays.forEach(isoWeekday => {
                const weekdayCount = weekdaysBetween(
                    moment('2019-12-12'),
                    moment('2009-01-01'),
                    isoWeekday,
                );
                expect(weekdayCount).toBe(0);
            });
        });
        it('Returns 52 for each day for a 52-week long period', () => {
            isoWeekDays.forEach(isoWeekday => {
                const weekdayCount = weekdaysBetween(
                    moment('2018-01-01'),
                    moment('2018-12-30'),
                    isoWeekday,
                );
                expect(weekdayCount).toBe(52);
            });
        });
    });
});

weekdaysBetween.ts

// Adapted from https://stackoverflow.com/a/29091126/5814943
export const weekdaysBetween = (startDate: moment.Moment, endDate: moment.Moment, isoWeekday: number) => {
    if (startDate.isSame(endDate, 'day')) {
        return startDate.isoWeekday() === isoWeekday
            ? 1
            : 0; 
    }
    const daysToAdd = ((7 + isoWeekday) - startDate.isoWeekday()) % 7;
    const nextOccurence = startDate.clone().add(daysToAdd, 'days');
    if (nextOccurence.isAfter(endDate)) {
        return 0;
    }
    const weeksBetween = endDate.diff(nextOccurence, 'weeks');
    return weeksBetween + 1;
};