Why Date() returns wrong date?

I have the following function:

public static addDaysNextPrev(operation: number, date: Date = new Date(), days: number = 7): IDateCalendarNextPrev {
    const currentDate = new Date(date);
    const prevDate = new Date(date);

    currentDate.setDate(currentDate.getDate() + (days * operation));
    prevDate.setDate(currentDate.getDate() - (days * operation));
    console.log(prevDate);

}

I try to add/minus some days from current date. Idea is to add 7 days to current date and return prev date and next.

Parameter operation is number 1 or positive or negative that determine direction to next date or to prev date.

I always get wrong date in line:

console.log(prevDate);

This is my second solution, more obviously:

public static addDaysNextPrev(operation: boolean, date: Date = new Date(), days: number = 7): IDateCalendarNextPrev {
    let currentDate = new Date(date);
    let prevDate = new Date(date);

    if (operation) {
      prevDate = date;
      currentDate.setDate(currentDate.getDate() + days);
    } else {
      currentDate.setDate(date.getDate() - days);
      prevDate.setDate(currentDate.getDate() - days);
    }
}

You're adding 7 days to currentDate at line:

currentDate.setDate(currentDate.getDate() + (days * operation));

and subtracting 7 days of currentDate at the line:

prevDate.setDate(currentDate.getDate() - (days * operation));

So your prevDate will have the same value as currentDate initial value.

Correction here:

let operation = 1;
let date: Date = new Date();
let days: number = 7

const currentDate = new Date(date);
const prevDate = new Date(date);

prevDate.setDate(currentDate.getDate() - (days * operation));

console.log(prevDate);