Repeat the Nth elements We haven't had a string question for a while (5 days to be precise), so let's go for one.

Given a string s and a positive integer n, take every nth element of s, repeat it n times, and put it back into s.

For example, if n = 3 and s = "Hello, World!", every third character is Hl r!. You then repeat each character n times to produce HHHlll   rrr!!!. You then replace the original letters with the repeated versions to produce the final product of HHHellllo,   Worrrld!!!

You are to accomplish this task in the shortest code possible in your language!

Rules


This is a code-golf so the shortest code in bytes wins
n is guaranteed to be smaller than the length of s and greater than 0
The first character of s is where the nth characters are taken from, and is always repeated n times
s will only consist of printable ASCII (code points 0x20 (space) to 0x7E (~))


Test cases

s, n =&gt; output

"Hello, World!", 3 =&gt; "HHHellllo,   Worrrld!!!"
"Code golf", 1 =&gt; "Code golf"
"abcdefghijklm", 10 =&gt; "aaaaaaaaaabcdefghijkkkkkkkkkklm"
"tesTing", 6 =&gt; "ttttttesTingggggg"
"very very very long string for you to really make sure that your program works", 4 =&gt; "vvvvery    veryyyy verrrry loooong sssstrinnnng foooor yoooou toooo reaaaally    makeeee surrrre thhhhat yyyyour    proggggram    workkkks"
Jelly, 3 bytes

Ḣs×


Input is taken as s, n.

Try it online!

How it works

Ḣs×  Main link. Argument: s, n

Ḣ    Head; yield s.
     This pops the list, leaving [n] as the main link's argument.
 s   Split s into chunks of length n.
  ×  Multiply each chunk by [n], repeating its first element n times.