If JSON contains a syntax error, when you click on “Manage TemplateData” button to load TemplateData editor, you will see
[[https://www.mediawiki.org/wiki/MediaWiki:Templatedata-errormsg-jsonbadformat | MediaWiki:Templatedata-errormsg-jsonbadformat]], but it looks strange; current content:
> Bad JSON format. You can cancel this operation so you can correct it, delete the current <templatedata> tags and try again, or continue to replace the current TemplateData with a new one.
This lets thinking there is 3 possibilities, whereas there is only two buttons: either you manually fix JSON or you let the editor to reset templateData. So, I think it would be clearer to remove “delete the current <templatedata> tags and try again” text, like that:
> Bad JSON format. You can cancel this operation so you can correct it, or continue to replace the current TemplateData with a new one.
Also, “Replace” button ([[https://www.mediawiki.org/wiki/MediaWiki:Templatedata-modal-json-error-replace | MediaWiki:Templatedata-modal-json-error-replace]]) would gain in clarity, in my opinion, to be renamed “Reset” instead (if so, adapting previous message again).