You’d need to enclose the entire pipeline in parentheses, like this:

const value$ = (
  range(0, 10)
  |> filter(x => x % 2 === 0)
  |> map(x => x + x)
  |> scan((acc, x) => acc + x, 0)
  |> toArray()
).subscribe(x => console.log(x));

For the draft spec, see this issue and this PR.