Problem Description

The problem asks you to extend the JavaScript Date object by adding a new method called nextDay(). This method should be available on any date object and when called, it should return a string representing the next day's date in the format YYYY-MM-DD.

For example, if you have a date object representing "2014-06-20" and you call nextDay() on it, the method should return "2014-06-21" as a string. The method needs to handle all cases including month boundaries (like going from June 30th to July 1st) and year boundaries (like going from December 31st to January 1st of the next year).

The implementation works by:

1. Creating a new Date object from the current date's value to avoid modifying the original date
2. Adding 1 day to this new date using setDate(getDate() + 1), which automatically handles month and year rollovers
3. Converting the resulting date to ISO string format using toISOString() which gives a format like "2014-06-21T00:00:00.000Z"
4. Extracting just the date portion (first 10 characters) using slice(0, 10) to get "YYYY-MM-DD"

The solution uses TypeScript's global declaration to properly type the new method being added to the Date prototype, ensuring type safety when using this enhanced functionality.

Intuition

When we need to add functionality to all instances of an existing JavaScript object type, the natural approach is to extend its prototype. Since we want every Date object to have a nextDay() method, we add this method to Date.prototype.

The key insight is that JavaScript's Date object already has built-in intelligence for handling date arithmetic. When we use setDate() with a value that exceeds the current month's days, it automatically rolls over to the next month. For example, if we're on June 30th and add 1 day using setDate(31), JavaScript knows June only has 30 days and automatically advances to July 1st. Similarly, it handles year transitions seamlessly.

To preserve the original date object (following good practice of not mutating input), we create a copy using new Date(this.valueOf()). The valueOf() method returns the timestamp, ensuring we get an exact copy of the date.

For formatting, we leverage the toISOString() method which always returns dates in a consistent format: "YYYY-MM-DDTHH:mm:ss.sssZ". Since we only need the date portion (YYYY-MM-DD), we can simply take the first 10 characters using slice(0, 10). This is more elegant and reliable than manually constructing the string with getFullYear(), getMonth(), and getDate(), especially since we'd need to handle zero-padding for single-digit months and days.

The TypeScript declaration at the beginning ensures type safety by informing the compiler that we're intentionally adding a new method to the Date interface, preventing type errors when using the enhanced functionality.

Solution Approach

The implementation extends the native Date prototype to add the nextDay() method. Here's a step-by-step breakdown:

1. TypeScript Declaration

declare global {
    interface Date {
        nextDay(): string;
    }
}

This declaration tells TypeScript that we're adding a new method to the Date interface globally, preventing type errors when calling nextDay() on date objects.

2. Prototype Extension

Date.prototype.nextDay = function () {
    // Implementation here
}

By adding the method to Date.prototype, every Date instance automatically inherits this method.

3. Creating a Copy

const date = new Date(this.valueOf());

• this.valueOf() returns the primitive value (timestamp in milliseconds) of the current date object
• Creating a new Date object from this timestamp ensures we don't modify the original date
• This follows the principle of immutability

4. Adding One Day

date.setDate(date.getDate() + 1);

• getDate() returns the day of the month (1-31)
• Adding 1 to this value and passing it to setDate() advances the date by one day
• JavaScript's Date object automatically handles edge cases:
  • Month boundaries: setDate(32) in January becomes February 1st
  • Year boundaries: December 31st + 1 day becomes January 1st of the next year
  • Leap years: February 28th/29th transitions are handled correctly

5. Formatting the Output

return date.toISOString().slice(0, 10);

• toISOString() returns a string in ISO 8601 format: "YYYY-MM-DDTHH:mm:ss.sssZ"
• slice(0, 10) extracts the first 10 characters, giving us exactly "YYYY-MM-DD"
• This approach guarantees correct formatting with zero-padding for single-digit months and days

The entire solution leverages JavaScript's built-in date handling capabilities rather than manually calculating days, months, and years, making it both concise and robust.

Example Walkthrough

Let's walk through the solution with a concrete example to see how it handles different scenarios:

Example 1: Regular Day Transition

const date1 = new Date('2014-06-20');
console.log(date1.nextDay()); // "2014-06-21"

Step-by-step execution:

1. Start with date object representing June 20, 2014
2. this.valueOf() returns the timestamp (e.g., 1403222400000)
3. Create new date: new Date(1403222400000)
4. getDate() returns 20, so setDate(20 + 1) sets day to 21
5. toISOString() produces "2014-06-21T00:00:00.000Z"
6. slice(0, 10) extracts "2014-06-21"

Example 2: Month Boundary

const date2 = new Date('2014-06-30');
console.log(date2.nextDay()); // "2014-07-01"

Step-by-step execution:

1. Start with June 30, 2014 (last day of June)
2. Create copy of the date
3. getDate() returns 30, so setDate(30 + 1) attempts to set day to 31
4. JavaScript knows June has only 30 days, automatically rolls to July 1st
5. toISOString() produces "2014-07-01T00:00:00.000Z"
6. slice(0, 10) extracts "2014-07-01"

Example 3: Year Boundary

const date3 = new Date('2014-12-31');
console.log(date3.nextDay()); // "2015-01-01"

Step-by-step execution:

1. Start with December 31, 2014 (last day of year)
2. Create copy of the date
3. getDate() returns 31, so setDate(31 + 1) attempts to set day to 32
4. JavaScript knows December has only 31 days, automatically rolls to January 1st of 2015
5. toISOString() produces "2015-01-01T00:00:00.000Z"
6. slice(0, 10) extracts "2015-01-01"

The beauty of this solution is that we don't need to write any special logic for these edge cases - JavaScript's Date object handles all the complex date arithmetic automatically when we use setDate() with values beyond the valid range for the current month.

Solution Implementation

Time and Space Complexity

Time Complexity: O(1)

The nextDay method performs a constant number of operations:

• new Date(this.valueOf()) - creates a new Date object in O(1) time
• date.setDate(date.getDate() + 1) - getting and setting the date are both O(1) operations
• date.toISOString() - converts the date to ISO string format in O(1) time
• slice(0, 10) - extracts the first 10 characters (YYYY-MM-DD) in O(1) time

All operations execute in constant time regardless of the input date value.

Space Complexity: O(1)

The method allocates a fixed amount of memory:

• One new Date object is created
• One string is generated from toISOString()
• One substring is created from slice(0, 10)

The space used does not scale with any input size and remains constant.

Common Pitfalls

1. Timezone Issues with ISO String Conversion

The JavaScript implementation uses toISOString() which always returns UTC time. If the original date is in local time near midnight, the conversion to UTC might shift the date backward or forward by a day.

Example Problem:

// If you're in UTC-5 timezone and create a date for midnight local time
const date = new Date('2014-06-20'); // Interpreted as midnight local time
console.log(date.nextDay()); // Might return "2014-06-20" instead of "2014-06-21"

Solution: Use local date methods instead of ISO string:

Date.prototype.nextDay = function() {
    const date = new Date(this.valueOf());
    date.setDate(date.getDate() + 1);
  
    const year = date.getFullYear();
    const month = String(date.getMonth() + 1).padStart(2, '0');
    const day = String(date.getDate()).padStart(2, '0');
  
    return `${year}-${month}-${day}`;
}

2. Mutating the Original Date Object

Forgetting to create a copy of the date before modification will mutate the original date object, causing unexpected side effects.

Example Problem:

Date.prototype.nextDay = function() {
    this.setDate(this.getDate() + 1); // WRONG: Modifies original date
    return this.toISOString().slice(0, 10);
}

const today = new Date('2014-06-20');
const tomorrow = today.nextDay();
console.log(today); // Date has been changed to 2014-06-21!

Solution: Always create a new Date instance as shown in the correct implementation.

3. Python: Losing Time Information

In the Python implementation, if you need to preserve time information while getting the next day, the current implementation only returns the date string without time.

Example Problem:

date = ExtendedDate(2014, 6, 20, 15, 30, 45)  # 3:30:45 PM
next = date.nextDay()  # Returns "2014-06-21" without time

Solution: Provide an option to return the full datetime object or include time in the string:

def nextDay(self, include_time=False):
    next_date = self + timedelta(days=1)
    if include_time:
        return next_date.strftime('%Y-%m-%d %H:%M:%S')
    return next_date.strftime('%Y-%m-%d')

4. Edge Case: Daylight Saving Time Transitions

During DST transitions, a "day" might be 23 or 25 hours. Using setDate() handles this correctly, but manual hour-based calculations would fail.

Example Problem:

// WRONG approach using hours
Date.prototype.nextDay = function() {
    const date = new Date(this.getTime() + 24 * 60 * 60 * 1000); // Add 24 hours
    return date.toISOString().slice(0, 10);
}

Solution: Stick with setDate() method which properly handles DST transitions by working with calendar days rather than fixed time periods.