Problem Description

In this coding task, the goal is to extend the functionality of the JavaScript Date object by introducing a new method called nextDay(). When called on a Date object, this method should calculate the date for the following day and return it as a string formatted as "YYYY-MM-DD". For example, if the method is invoked on a Date object representing June 20, 2014, it should return the string "2014-06-21" which represents June 21, 2014.

Intuition

To achieve the result of getting the next day's date, we employ a straightforward approach. By using the Date object's built-in methods, we can carry out this task with a few steps:

1. Get the current date from the this reference inside our new nextDay method. The this keyword refers to the Date object that nextDay is called upon.
2. Create a copy of the current date to avoid mutating the original Date object.
3. Increment the date of this copied object by one, using the getDate and setDate methods. This action effectively moves the date forward by one day.
4. Convert the updated Date object into a string formatted as ISO 8601 using toISOString. This will yield a string in the format "YYYY-MM-DDTHH:mm:ss.sssZ".
5. Extract and return only the date portion from this ISO string (i.e., the first 10 characters, "YYYY-MM-DD").

By following these steps, we conform to JavaScript's date handling and ensure correctness while providing a neatly formatted result.

Solution Approach

The solution leverages the native JavaScript Date object's methods to achieve the desired functionality. Here's a step-by-step breakdown of the algorithm, patterns, and data structure used in the implementation:

1. Monkey Patching: The solution uses a pattern known as "monkey patching," where a method is added to an existing built-in object at runtime. By extending the Date object's prototype, we introduce a new method nextDay that becomes available to all instances of Date.

2. Creating a New Date Instance: A new Date object is instantiated using this.valueOf(). The valueOf method returns the primitive value of the specified object. In the case of a Date object, it returns the number of milliseconds since January 1, 1970, 00:00:00 UTC. This number is used to create a new Date object which is a copy of the original, ensuring that the original Date object remains unchanged.

3. Incrementing the Date: By calling getDate on the newly created Date object, we get the day of the month for the specified date according to local time. We then use setDate to advance this value by one, which moves the date to the next day.

4. Formatting the Result: The toISOString method is used, which converts a Date object into a string in ISO format (YYYY-MM-DDTHH:mm:ss.sssZ). Since the problem only requires the date component in "YYYY-MM-DD" format, slice(0, 10) is applied to this string to extract the first ten characters, discarding the time portion.

5. Returning the Result: The string representation of the next day in "YYYY-MM-DD" format is then returned.

The solution doesn't directly employ complex data structures or algorithms as it mainly revolves around date manipulation using built-in Date object methods. The pattern used here is simple and efficient since JavaScript's Date object internally handles all the complexities related to leap years, time zones, daylight saving time changes, and other date-related quirks.

Here's the implementation based on the approach described:

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

Date.prototype.nextDay = function () {
    const date = new Date(this.valueOf());
    date.setDate(date.getDate() + 1);
    return date.toISOString().slice(0, 10);
};

/**
 * Example usage:
 * const date = new Date("2014-06-20");
 * console.log(date.nextDay()); // "2014-06-21"
 */

By following these steps, the nextDay method reliably calculates the next day, formats it appropriately, and integrates seamlessly into the Date object for convenient usage.

Example Walkthrough

To provide a better understanding of the nextDay method implementation, let's walk through a small example where we apply the solution approach to a Date object representing April 30, 2021. This date is of particular interest because the following day is May 1, 2021, which is the start of a new month. Demonstrating the function works across month boundaries ensures it handles more complex cases of date manipulation.

1. Creating the Date Object: We first construct a Date object representing April 30, 2021.

   const date = new Date("2021-04-30");

2. Adding the nextDay Method: We could imagine the nextDay method has already been added to the Date prototype as described in the solution approach.

3. Calling nextDay: When we call the nextDay method on our date object:

   const nextDayStr = date.nextDay();

4. Inside nextDay: Here's what happens within the nextDay method:

   • It creates a new Date object that represents the same moment in time as date by using this.valueOf().
   • It then calls getDate() on the new Date object to retrieve the day of the month, which is 30. It then uses setDate() to increment this value by one. The JavaScript Date object automatically handles the transition to the next month, so the internal date representation becomes May 1, 2021.
   • Finally, it uses toISOString() to convert the Date object to a string in ISO format, followed by slice(0, 10) to extract the year, month, and day parts.

5. Result: The nextDay method returns the string "2021-05-01" which is the formatted string representing the next day, conforming to the "YYYY-MM-DD" format. This result is stored in nextDayStr.

In summary, on invoking date.nextDay(), where date is April 30, 2021, our output will be:

console.log(nextDayStr); // "2021-05-01"

This confirmed our function correctly moves from the last day of April to the first day of May, thereby validating the method's ability to handle month changes seamlessly.

Solution Implementation

Time and Space Complexity

The time complexity of the nextDay method is O(1), indicating that it takes a constant amount of time to compute the next day regardless of the size of the input. This is because the internal operations, such as getDate, setDate, and toISOString, are all constant-time operations provided by the Date object's API.

The space complexity is also O(1) because the additional space used does not grow with the size of the input. A single new Date instance is created, and its use of memory does not depend on the input date.