This JsPromiseBinding will cache the result of the thenable. Given a scala.scalajs.js.Thenable that will be delay executed.

import scala.scalajs.js
var upstreamEvaluationCount1 = 0
val delayedThenable = js.Promise.resolve[Unit](()).`then`[Double] { case () =>
  upstreamEvaluationCount1 += 1
  math.random
}

The execution will not be performed right now.

val jsPromiseBinding = JsPromiseBinding(delayedThenable)
upstreamEvaluationCount1 should be(0)

When there are multiple usages of jsPromiseBinding, each usage should be triggered with the same value.

var evaluationCount1 = 0
var evaluationCount2 = 0
var evaluationCount3 = 0

val usage1 = Binding {
  jsPromiseBinding.bind match {
    case Some(Right(value)) =>
      evaluationCount1 += 1
    case _ =>
  }
}

val usage2 = Binding {
  jsPromiseBinding.bind match {
    case Some(Right(value)) =>
      evaluationCount2 += 1

      val usage3 = Binding {
        jsPromiseBinding.bind match {
          case Some(Right(value)) =>
            evaluationCount3 += 1
          case _ =>
        }
      }

      val _ = usage1.bind
      usage3.bind
    case _ =>
  }
}

usage2.watch()

upstreamEvaluationCount1 should be(0)
evaluationCount1 should be(0)
evaluationCount2 should be(0)
evaluationCount3 should be(0)

And each usage should be triggered once and only once.

delayedThenable.toFuture.map { _ =>
  upstreamEvaluationCount1 should be(1)
  evaluationCount1 should be(1)
  evaluationCount2 should be(1)
  evaluationCount3 should be(1)
}