Manipulate zsh arrays with operator j

Question

The following code is taken from here:

function +vi-git-st() {
    local ahead behind remote
    local -a gitstatus

    # Are we on a remote-tracking branch?
    remote=${$(git rev-parse --verify ${hook_com[branch]}@{upstream} \
        --symbolic-full-name 2>/dev/null)/refs\/remotes\/}

    if [[ -n ${remote} ]] ; then
        # for git prior to 1.7
        # ahead=$(git rev-list origin/${hook_com[branch]}..HEAD | wc -l)
        ahead=$(git rev-list ${hook_com[branch]}@{upstream}..HEAD 2>/dev/null | wc -l)
        (( $ahead )) && gitstatus+=( "${c3}+${ahead}${c2}" )

        # for git prior to 1.7
        # behind=$(git rev-list HEAD..origin/${hook_com[branch]} | wc -l)
        behind=$(git rev-list HEAD..${hook_com[branch]}@{upstream} 2>/dev/null | wc -l)
        (( $behind )) && gitstatus+=( "${c4}-${behind}${c2}" )

        hook_com[branch]="${hook_com[branch]} [${remote} ${(j:/:)gitstatus}]"
    fi
}

I do not understand the last line. Variable gitstatus is an array, so what is ${(j:/:)gitstatus} supposed to do? I know that it outputs the string first_array_element/second_array_element but I did not manage to find any documentation about operator j. Is this some specific zsh feature, or is it standard shell programming construct?

Answer 1

That's the parameter expansion flag which joins array elements. See (j:...:) Flag.

In that specific case, it joins the elements within the array using / as the separator. E.g.

zsh% foo=(1 2 3)    
zsh% echo $foo
1 2 3
zsh% echo ${(j:/:)foo}
1/2/3