The goal is to set the deliveryDate object to 9 days from the current date. The Date.prototype.setDate() method is designed for this. It sets the day of the month for a Date object. Crucially, if the provided day value exceeds the number of days in the current month, the method automatically adjusts the month and year accordingly. The expression (new Date()).getDate() + 9 retrieves the current day of the month (e.g., 15) and adds 9 to it. Passing this result to deliveryDate.setDate() correctly calculates and sets the date to 9 days in the future, handling month and year rollovers.

B. Date.current() is not a standard JavaScript function. The correct static method is Date.now(), which returns milliseconds, an invalid value for the setDate() method.

C. A JavaScript Date object does not have a public property named .date for assignment. Also, new Date(+9) creates a date 9 milliseconds after the 1970 epoch.

D. This option has two errors: it attempts to assign to the non-existent .date property and calls the non-existent Date.current() function.

1. Mozilla Developer Network (MDN) Web Docs

Date.prototype.setDate(): "The setDate() method of a Date instance changes the day of the month for this date according to local time... If the dayValue is outside of the range of date values for the month

setDate() will update the Date object accordingly." This confirms the rollover behavior essential for the correct answer.

Source: Mozilla

Date.prototype.setDate()

Accessed on [Current Date]

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/GlobalObjects/Date/setDate

2. Mozilla Developer Network (MDN) Web Docs

Date.prototype.getDate(): "The getDate() method of a Date instance returns the day of the month for this date according to local time." This confirms the method used to retrieve the current day number.

Source: Mozilla

Date.prototype.getDate()

Accessed on [Current Date]

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/GlobalObjects/Date/getDate

3. ECMAScript® 2023 Language Specification (ECMA-262

14th edition): The behavior of setDate is formally defined. The specification details the internal SetDate abstract operation

which correctly calculates the final date when the day argument causes the month or year to change.

Source: ECMA International

ECMAScript Language Specification

14th Edition

June 2023

Section 21.4.4.25 Date.prototype.setDate ( date ).