Get Animation KeyframeEffect property value for the current time?

Question

How do we get the value of a property of an Animation KeyframeEffect for the current time?

For example, suppose we have the following animation with a naive attempt to get a property's current value, without passing any element to the KeyframeEffect, only wishing to animate a number in memory:

const keyframes = new KeyframeEffect(null, [
    { // from
        someProp: 0,
    },
    { // to
        someProp: 35,
    }
], {duration: 2000, easing: 'ease'})

const anim = new Animation(
    keyframes,
    document.timeline
)

anim.play()

requestAnimationFrame(function loop() {
  console.log('current value of someProp:', keyframes.getComputedTiming().progress * 35)

  if (anim.playState != 'finished') requestAnimationFrame(loop)
}, 100)

The output of current value of someProp is incorrect, as my math does not align with the chosen easing of "ease".

Suppose we are on a ScrollTimeline, or the currentTime of the timeline is modified in arbitrary ways we are not aware of, etc. It would get complicated to attempt to figure the value ourselves using custom math and array handling based on current timing.

Is there a way to get the simply access the current value of a property?

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The reason we would want to do this is to see if it would be viable to use Web Animations to animate things like WebGL scenes, or canvas drawings, or anything that does not animate DOM element CSS properties.