In QJSEngine, we can just write "use strict" once at the top of the file and it will apply for all functions.

I haven't tested QtScript, but I would be surprised if it's different.

One of the reasons we said redirect should have an absolute direction rather than "reverse" was so that we can squash this code into something more like:

fadeWindow(window, direction) {
   if (window.animation) {
       redirect(window.animation, direction);
   } else {
      window.animation = animate(....);
      redirect(window, direction);
   }
}

which IMHO would be a lot easier to read compared to two functions which have to handle 3 possible states of existing animations.

We don't delete window.animation, so we can't just simply redirect the animation.

Also, from the API standpoint, redirect() can return false, which means it couldn't redirect the animation and some further actions have to be taken (e.g. cancel the animation).

In order to delete window.animation, I assume we need some inspection mechanism to figure out whether all animations has finished.

In order to delete window.animation

effect.animationEnded.connect(function(window) {

delete window.animation;

});

(And if we were doing multiple animations in an array then we change the above code to remove the array entry and delete the property if it's empty)

Oh, I forgot that there's the second parameter.

Well, we still need

if (window.animation) {
    if (redirect(window.animation, Effect.SomewhereIBelong) {
        return;
    }
    cancel(window.animation);
}

window.animation = animate({ ... });

because redirect can fail. It most likely won't happen, but we still need to handle such cases.

AFAIK it can only fail if you call redirect inside a slot directly connected to animationEnded

but sure, we can have that check.