How can I join elements of an array in Bash?

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

Rewriting solution by Pascal Pilz as a function in 100% pure Bash (no external commands):

function join_by { local IFS="$1"; shift; echo "$*"; }

For example,

join_by , a "b c" d #a,b c,d
join_by / var local tmp #var/local/tmp
join_by , "${FOO[@]}" #a,b,c

Alternatively, we can use printf to support multi-character delimiters, using the idea by @gniourf_gniourf

function join_by { local d=$1; shift; echo -n "$1"; shift; printf "%s" "${@/#/$d}"; }

For example,

join_by , a b c #a,b,c
join_by ' , ' a b c #a , b , c

join_by ')|(' a b c #a)|(b)|(c
join_by ' %s ' a b c #a %s b %s c
join_by $'\n' a b c #abc
join_by - a b c #a-b-c
join_by '\' a b c #a\b\c