Group memberParts Iterator has no map() function?

https://gojs.net/latest/api/symbols/Iterator.html

Is there a js array map() method equivalent ?

How can I obtain a js array from Iterator instances ? ([...myIterator] fails with Uncaught TypeError: siblings is not iterable, [...myIterator.iterator] fails in the same way

Yes, it does. See the go.d.ts file, where it is declared. Have you tried it?

It’s not documented though, because our collections classes were designed and implemented back in 2010 before anyone had proposed what later would become Set and Map in ES2015/ES6. Fortunately our design is mostly compatible with those two JavaScript classes, with the exception of iteration, so we simply added properties and methods that are synonyms for the ES6 properties and methods. In GoJS version 2.0 we made some minor incompatible changes to be more fully compatible, again excepting for iteration. It is the incompatibility of iteration that causes [...iter] to fail.

We’re considering replacing our implementation with augmented versions of the ES6 Set and Map in GoJS version 3.0. But there’s no schedule for that.