Form Field showon:"empty

Question

Using this documentation https://docs.joomla.org/Form_field I have developed a plugin and used the showon attribute within the XML (head to the bottom of that page).

Whilst is shows me I can use

showon="foo:1[AND]bar:1" and

showon="foo:1[OR]bar:1"

I would like to showon based on whether a text field is not empty. Is this a possibility or will I require JS?

I have also heard you can do showon="field1!:1" and showon=field!"

Here's my code which shows both fields.

<field
        name="telephone"
        type="text"
        label="MOD_HEADER_CONTACT_TELEPHONE_LABEL"
        description="MOD_HEADER_CONTACT_TELEPHONE_DESC"
        default=""
        class=""
/>
<field
        name="countrycode"
        type="text"
        label="MOD_HEADER_CONTACT_COUNTRY_CODE_LABEL"
        description="MOD_HEADER_CONTACT_COUNTRY_CODE_DESC"
        default=""
        class=""
        required="required"
        showon="telephone!:"
/>

Answer 1

Yes it is possible to do this:

showon="foo!:"