I saw your comment that you don't know why the result is an array. I think it's because the GO method has a dual return type: (output *js.Object, err error)
I'm very new to GO and gopherjs (in fact I found your repo after spending loo long spinning my wheels on trying to do this exact thing: build a HCL2 parser for node.js), but from what I have seen the convention in GO is that errors are not thrown but signaled using this dual-return type pattern. I suspect that if you changed the signature to:

func parseToObject(input string) (output *js.Object)

then the result via JS would not be enclosed in an array. I would hope that there is a way to make the JS version throw the error using the gopherjs lib. If I get some more time, I may try some things out and make a PR, but for now I just wanted to mention this.

But, thanks for making this repo. I was about to give up hope.