May be because Outsystems wants us to code less the option isnt availabe but at times we can not wait for a feature to be available in the product and need to be coding to get it done for the use case(While a lot of companies are still running legacy applications that does not allow standard integrations).
It would be great power to be able to code inline for the extensions. Developer productivity does contribute to speed of delivery :-)
I prefer to use Codelobster IDE for JS development.