How can I join elements of an array in Bash?

Question

If I have an array like this in Bash:

FOO=( a b c )

How do I join the elements with commas? For example, producing a,b,c.

Answer 1

My attempt.

$ array=(one two "three four" five)
$ echo "${array[0]}$(printf " SEP %s" "${array[@]:1}")"
one SEP two SEP three four SEP five