CC @cknitt on the version constraints.
Can't remember if we have it documented somewhere, perhaps on the web page.

Applying by spread is ES6(ECMAScript 2015) spec, and has been supported longer than let in most browsers.

The only problem is IE11, which I don't think we support.

I don't think we have documented the requirements on the JS version / spec at the moment.

Starting with v12, ES6 (ECMAScript 2015) is required because we are now making use of the following ES6 features:

• let keyword
• arrow functions
• spread operator

/cc @fhammerschmidt

We should set a threshold for what syntax we support. But I think we should be fairly aggressive there so we can use newer features.

Great stuff! Maybe all/parts of this could be implemented to use real array spreads instead of concats with the array spread syntax.

Right. It could be reused in other places like the array and dict output

@cometkim ready to merge?

Yep