Code challenge for Chiliz

main.js

/*
 * Your program must print string with the number of years and months and the total number of days between the dates.
 * Dates are provided in dd.mm.yyyy format.
 * You are not allowed to plug in JS libraries such as moment.js or date-fns directly into the code. All code need to be written in this file.
 * 
 * Result must be shown as a string in years, months and total days. If years or months are 0, then it should not be displayed in the output.
 *
 * Example:
 * Input: ['01.01.2000', '01.01.2016']
 * Output:
 * '16 years, total 5844 days'
 *
 * Example 2:
 * Input: ['01.11.2015', '01.02.2017']
 *
 * Output:
 * '1 year, 3 months, total 458 days'
*/
const dates = [
    ['01.01.2000', '01.01.2016'],
    ['01.01.2016', '01.08.2016'],
    ['01.11.2015', '01.02.2017'],
    ['17.12.2016', '16.01.2017'],
    ['01.01.2016', '01.01.2016'],
    ['28.02.2015', '13.04.2018'],
    ['28.01.2015', '28.02.2015'],
    ['17.03.2022', '17.03.2023'],
    ['17.02.2024', '17.02.2025'],
];

const MILLISECONDS_IN_DAY = 86400000;

/**
 * Recieves array of start and end date strings in 'dd.MM.yyyy' format.
 * 
 * @param {String[]} dates 
 * @returns {String}
 */
function outputDate(dates) {
    // Split dates into start and end dates
    const [start, end] = dates;

    // Convert dates from strings into date objects
    const startDate = fromDayMonthYear(start);
    const endDate = fromDayMonthYear(end);

    // Initialise string for output
    let outputString = '';

    // Get difference in years from function below
    const yearDiff = differenceInYears(startDate, endDate);
    // If there is a difference in years, add this to the output string
    if (yearDiff > 0) outputString += `${yearDiff} ${pluralise('year', yearDiff)}, `;

    // Get difference in months from function below
    const monthDiff = differenceInMonths(startDate, endDate);
    // If there is a difference in months, add this to the output string
    if (monthDiff > 0) outputString += `${monthDiff} ${pluralise('month', monthDiff)}, `;

    // Work out days between the two dates using MS in days
    const differenceInDays = Math.round((endDate.getTime() - startDate.getTime()) / MILLISECONDS_IN_DAY);
    // Add days to end of output string
    outputString += `total ${differenceInDays} days`;
    
    // Return the final output string
    return outputString;
}

/**
 * Takes a string and pluralises it if value is larger than 1
 * 
 * @param {String} string 
 * @param {Number} value 
 * @returns {String}
 */
function pluralise(string, value) {
    if (value > 1) string += 's';
    return string;
}

/**
 * Function to take a date string in 'dd.MM.yyyy' format
 * and return a date object using that date
 * 
 * @param {String} date 
 * @returns {Date}
 */
function fromDayMonthYear(date) {
    // Split out values from provided date string
    let [day, month, year] = date.split('.');

    // Parse int values from returned string values
    year = parseInt(year);
    month = parseInt(month) - 1;
    day = parseInt(day);
    
    // Create new date object using those values
    return new Date(year, month, day);
}

/**
 * Gets a value for full months between two provided dates
 * 
 * @param {Date} start 
 * @param {Date} end 
 * @returns {Number} 
 */
function differenceInMonths(start, end) {
    // Clone dates so no original values are changed
    const startDate = new Date(start);
    const endDate = new Date(end);

    // Work out absolute values for year and month differences
    const diffInYears = endDate.getFullYear() - startDate.getFullYear();
    const diffInMonths = endDate.getMonth() - startDate.getMonth();
  
    // Get complete difference in months using years*12
    const difference = diffInYears * 12 + diffInMonths
    // Set end dates month to the current month - difference in months
    endDate.setMonth(endDate.getMonth() - difference);

    // If end date is then before start date, the last month wasnt a full month
    const lastMonthNotFull = isAfter(startDate, endDate) === -1;
    // Subtract 1 if the last month isnt full from the total difference
    const result = difference - lastMonthNotFull;
    
    // Modulo difference over 12 to remove full years from the final value
    return result % 12
}

/**
 * Gets a value for full years between two dates
 * 
 * @param {Date} start 
 * @param {Date} end 
 * @returns {Number} 
 */
function differenceInYears(start, end) {
    // Clone dates so no original values are changed
    const startDate = new Date(start);
    const endDate = new Date(end);

    // Work out absolute value for year difference
    const difference = endDate.getFullYear() - startDate.getFullYear();
    // Set end dates year to the current year - difference in months
    endDate.setFullYear(endDate.getFullYear() - difference)

    // If end date is then before start date, the last year wasnt a full year
    const lastYearNotFull = isAfter(startDate, endDate) === -1

    // Subtract 1 if the last year isnt full from the total difference
    const result = difference - lastYearNotFull
    
    // Return result
    return result
}

/**
 * Function to check if endDate is after startDate
 * 
 * @param {Date} start 
 * @param {Date} end 
 * @returns {Number}
 */
function isAfter(start, end) {
    // Subtract startDate ms from endDate
    const msDiff = end.getTime() - start.getTime()

    // Return value depending on difference
    if (msDiff < 0) {
        return -1
    } else if (msDiff > 0) {
        return 1
    } else {
        return msDiff
    }
}