$join
Takes an array array and a separator string sep and joins the elements
of the array (each cast to string) separated by sep.
Options¶
- array(required) Array of values to be joined.
- sep(optional) Separator string. Default- ''(empty string).
Example¶
{
    "code": {
        "$join": {
            "array": ["foo", "bar", "baz"],
            "sep": "-"
        }
    }
}
{
    "code": "foo-bar-baz"
}