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

$ foo=(a "b c" d)
$ bar=$(IFS=, ; echo "${foo[*]}")
$ echo "$bar"
a,b c,d