<link href='//fonts.googleapis.com/css?family=Signika+Negative:300' rel='stylesheet' type='text/css'>
<h3>You can only toggle direction when the tween is not active</h3>
<div class="wrapper">
  <div class="box"></div>
</div>
<button class="dark-grey-button club-demo-button" id="tweenBox">toggle tween direction</button> 

body, html {
  font-family: Signika Negative, sans-serif;
  background: #111;
  color:#efefef;
}

.wrapper {
  width:400px;
  height:100px;
  background:#444;
  margin-bottom:10px;
}

.box {
  width:100px;
  height:100px;
  background:#88ce02;
}

button {
  margin:10px 0;
  padding:10px;
}

const tween = gsap.to(".box", {
  duration: 2, 
  x: 300, 
  ease: "none",
  reversed: true
});


document.querySelector("#tweenBox").addEventListener("click", function() {
  if(!tween.isActive()) {
    // Only reverse the direction if the tween is not active
    tween.reversed() ? tween.play() : tween.reverse();
  }
});


/* from the API documentation

isActive() indicates whether or not the animation is currently active (meaning the virtual playhead is actively moving across this instance's time span and it is not paused, nor are any of its ancestor timelines). So for example, if a tween is in the middle of tweening, it's active, but after it is finished (or before it begins), it is not active. If it is paused or if it is placed inside of a timeline that's paused (or if any of its ancestor timelines are paused), isActive() will return false. If the playhead is directly on top of the animation's start time (even if it hasn't rendered quite yet), that counts as "active".

*/