How to write a function that works with both JsonArray and JsonObject?

There is no base class for JsonArray and JsonObject. (Back in version 3.0, they used to derive from Printable, but this inheritance has been removed to reduce the memory footprint.)

However, both JsonArray and JsonObject can be “stored” in a JsonVariant. (I put “stored” in quotes because the JsonVariant only contains a reference, not a copy.)

So here is your function:

void sendJson(JsonVariant json) {
    char buffer[512];
    json.printTo(buffer, sizeof(buffer));
    g_server.send(200, "application/json", buffer);
}

But in that case, you loose some specificities of the JsonObject class. In particular, you don’t have the containsKey() method. If you need this function, you must cast the JsonVariant back to a JsonObject&.

For instance:

void myFunction(JsonVariant variant)
{
    if (variant.is<JsonArray&>())
    {
        JsonArray& array = variant;
        // ...
    }
    else if (variant.is<JsonObject&>())
    {
        JsonObject& object = variant;
        // ...
    }
    else
    {
        // ...
    }
}