You can now get code completion in your favourite editor when editing Javascript for Intervals.icu custom fields, streams, charts etc..
VS Code:
IntelliJ IDEA:
The instructions are here:
https://www.npmjs.com/package/@intervals-icu/js-data-model
For info on the JS extension points:
That’s nice. 
One questions though. I didn’t work with npm / nodejs yet. If there is an update on the data model, will it get fetched automatically, or must I run an update somehow?
And one minor issue, this is written on the Getting Started page:
Open that project in your IDE and create a Typescript file, Types.js:
but from your screenshot it should be Types.ts.
Tx. I will fix that. To upgrade do:
npm install @intervals-icu/js-data-model@latest --save