When using CAPTURE in the javascript backend, this replaces the return value by $effekt.unit. Some of the existing FFI relies on this behavior (we then don't have to explicitly return unit):

Example program

The following program triggers above behavior via promise.resolve.

import io

extern io def rT[T](run: => T at {io, async, global}): T =
  js "$effekt.runToplevel((ks, k) => ${run}(ks, k))"

def main() = {
  val pr = promise::make[Int]()
  println(rT[Int](box {
    pr.resolve(2);
    12
  }))
}

Playground

Thanks @b-studios for helping to find the underlying cause.