A Javascript if statement for a keyframes in CSS?

I've been searching all over the internet but couldn't find an answer to this.

Basically, I want to make an if statement in javascript... Once a keyframes animation in CSS3 gets to 100%, do some action. I just don't know what syntax to use.

I really appreciate any help you can provide.

CSS3:

@keyframes progress-animation{
0%{
    width: 0%;
}

100%{
    width: 100%;
}
}

@keyframes loader {
0% {
  transform: rotate(0deg);
}

25% {
  transform: rotate(180deg);
}

50% {
  transform: rotate(180deg);
}

75% {
  transform: rotate(360deg);
}

100% {
  transform: rotate(360deg);
}

}

What I want to do (in words):

if (progress-animation == '100%')
    stop animation (loader)

Sorry if it's a quite simple answer; I'm a bit new to Javascript.

You can utilize animationend event.

You can also add a condition using animationName parameter received from the event to only check for particular animations.

Here's a demo:

const element = document.querySelector('.progress')
element.addEventListener("animationend", onAnimationComplete);

function onAnimationComplete(e){
  if(e.animationName==='progress-animation'){
     console.log("progress-animation ended")
  }
}

.progress{
  background:red;
  padding:10px;
  width:100px;
  border:solid 1px black;
  border-radius:16px;
  animation: progress-animation linear 3s;
}
@keyframes progress-animation{
  0%{
  width:0px;
  }
  100%{
   width:100px;
 }
}

<div class='progress'>

</div>