I agree; the point of a struct type would be to allow a compact memory representation, and you're not going to get it if your constructor can do if(someArg) { a = 1; } else { a = 1; b = 2; }.

You don't strictly need known/consistent types, but it sure helps, since otherwise everything needs to be 8 bytes.

I don't think a way to read into and out of ArrayBuffers is possible, since these can have pointers in them. I think it needs a StructArray class instead, so there's a way to actually make a compact memory array out of all of this.