Javascript for setting the date of each upcoming day of the week?

So I found this little snippet of javascript that sets the date for the very next calendar Saturday

var nextSaturday = new Date();
nextSaturday.setDate(now.getDate() + (6 - 1 - now.getDay() + 7) % 7 + 1);

I can't wrap my head around this math. How would I change this for each day of the week? For instance, what would be changed to get the date for Monday?

Let's consider today, Friday, February 24th, 2023, the 5th day of the week.

const today = new Date('2023-02-24')
console.log(today.getDate())    // 24
console.log(today.getDay())     // 5

Now we want to find the next Saturday (tomorrow, the 25th, the 6th day of the week).

const saturday = new Date('2023-02-25')
console.log(saturday.getDay())     // 6

To advance from Friday to Saturday, we need to increment by 1 day. Naively, we could do this (note use of 6 for saturday.getDay()):

let nextSaturday = new Date()
nextSaturday.setDate(today.getDate() + 6 - today.getDay())
console.log(nextSaturday.getDate())    // 25   

The issue comes when we want to find the next Monday. In this case, we might try (getDay() returns 1 for Monday):

let nextMonday = new Date()
nextMonday.setDate(today.getDate() + 1 - today.getDay())
console.log(nextMonday.getDate())     // 20  

This has gone back in time because Monday's day number is less than Friday's so we end up subtracting 4 from today's date (Monday.getDay() - today.getDay() = 1 - 5 = -4). To work around this, we need to make sure that the number is always positive, which we do by adding 7, and then taking the result modulo 7 (note in some languages modulo always returns a positive number so adding 7 is not necessary):

nextMonday = new Date()
nextMonday.setDate(today.getDate() + (1 - today.getDay() + 7) % 7)
console.log(nextMonday.getDate())     // 27 

Now this all works fine except for one case, when we want to find the next Friday (the 3rd of March). In this case, our issue is that (5 - today.getDay() + 7) % 7 == 0, so we will get today's date again.

let nextFriday = new Date()
nextFriday.setDate(today.getDate() + (5 - today.getDay() + 7) % 7)
console.log(nextFriday.getDate())     // 24

To work around this, we need to change the code so that instead of adding 0 in this case, it adds 7 (i.e. we need a range of 7,1..6 instead of 0..6). The way to do this is to subtract 1 from 5 - today.getDay() + 7 before the modulo and then add it back afterwards:

nextFriday = new Date()
nextFriday.setDate(today.getDate() + (5 - 1 - today.getDay() + 7) % 7 + 1)
console.log(nextFriday.getDate())     // 3 

In general, to adapt this code for any given day of the week, replace the 5 in the above code with the value of getDay() (0 = Sunday, 1 = Monday, ...) for that day of the week.

Full code snippet:

<!-- begin snippet: js hide: true console: true babel: false -->

<!-- language: lang-js -->

const today = new Date('2023-02-24')
console.log(today.getDate())    // 24
console.log(today.getDay())     // 5

const saturday = new Date('2023-02-25')
console.log(saturday.getDay())     // 6

let nextSaturday = new Date()
nextSaturday.setDate(today.getDate() + 6 - today.getDay())
console.log(nextSaturday.getDate())    // 25

let nextMonday = new Date()
nextMonday.setDate(today.getDate() + 1 - today.getDay())
console.log(nextMonday.getDate())     // 20  

nextMonday = new Date()
nextMonday.setDate(today.getDate() + (1 - today.getDay() + 7) % 7)
console.log(nextMonday.getDate())     // 27 

let nextFriday = new Date()
nextFriday.setDate(today.getDate() + (5 - today.getDay() + 7) % 7)
console.log(nextFriday.getDate())     // 24

nextFriday = new Date()
nextFriday.setDate(today.getDate() + (5 - 1 - today.getDay() + 7) % 7 + 1)
console.log(nextFriday.getDate())     // 3 

<!-- end snippet -->