According to the Go spec on slice expressions:

If the sliced operand of a valid slice expression is a nil slice, the result is a nil slice.

var s []int
s = s[0:0]
fmt.Println(s == nil) // should be true

GopherJS doesn't currently implement that. Slicing a nil slice keeps it empty but makes it non-nil:

• https://play.golang.org/p/9U18mKBuQ3L
• https://gopherjs.github.io/playground/#/9U18mKBuQ3L

I discovered this bug as part of the work to support Go 1.11, because this bug started to cause go/doc tests to fail (see #836 (comment)).