Finding the nth day from today - JavaScript (JS Date)

We are required to write a JavaScript function that takes in a number n as the only input.

The function should first find the current day (using Date Object in JavaScript) and then the function should return the day n days from today.

For example −

If today is Monday and n = 2,

Then the output should be −

Wednesday

Understanding the Problem

JavaScript's Date.getDay() returns 0 for Sunday, 1 for Monday, and so on. We need to handle the modulo operation correctly to cycle through weekdays.

Method 1: Using Array Index Mapping

const num = 15;

const findNthDay = num => {
    const weekdays = ["Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"];
    const today = new Date().getDay();
    const targetDay = (today + num) % 7;
    return weekdays[targetDay];
};

console.log(`Today: ${new Date().toDateString()}`);
console.log(`${num} days from now: ${findNthDay(num)}`);

Today: Mon Dec 09 2024
15 days from now: Tuesday

Method 2: Using Date Addition

const findNthDayByDate = num => {
    const today = new Date();
    const futureDate = new Date(today);
    futureDate.setDate(today.getDate() + num);
    
    const weekdays = ["Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"];
    return weekdays[futureDate.getDay()];
};

console.log(`5 days from now: ${findNthDayByDate(5)}`);
console.log(`30 days from now: ${findNthDayByDate(30)}`);

5 days from now: Saturday
30 days from now: Wednesday

Complete Example with Current Day Display

const getDayInfo = num => {
    const weekdays = ["Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"];
    const today = new Date();
    const currentDay = weekdays[today.getDay()];
    const targetDay = weekdays[(today.getDay() + num) % 7];
    
    return {
        today: currentDay,
        nDaysLater: targetDay,
        calculation: `(${today.getDay()} + ${num}) % 7 = ${(today.getDay() + num) % 7}`
    };
};

const result = getDayInfo(10);
console.log(`Today is: ${result.today}`);
console.log(`10 days from now: ${result.nDaysLater}`);
console.log(`Calculation: ${result.calculation}`);

Today is: Monday
10 days from now: Thursday
Calculation: (1 + 10) % 7 = 4

Key Points

• getDay() returns 0-6 where Sunday = 0
• Use modulo operator (%) to cycle through days of the week
• Method 2 handles month/year transitions automatically
• Both approaches work for negative numbers (past dates)

Conclusion

Both methods effectively calculate the nth day from today. The array-based approach is simpler for day-name calculations, while the Date addition method handles complex date arithmetic automatically.