o := struct {
    *js.Object
    a []int `js:"a"`
    b js.M  `js:"b"`
}{Object: js.Global.Get("Object").New()}
o.a = []int{1}
o.b = js.M{"x": "x"}
fmt.Printf("%v ", o.a[0])
o.a[0] = 2 // <== this doesn't work
fmt.Printf("%v ", o.a[0])
o.Get("a").SetIndex(0, 3) // <== this works but is awkward
fmt.Printf("%v\n", o.a[0])
fmt.Printf("%v ", o.b["x"])
o.b["x"] = "y" // <== this doesn't work either
fmt.Printf("%v ", o.b["x"])
o.Get("b").Set("x", "z") // <== works but awkward
fmt.Printf("%v ", o.b["x"])