JavaScript and CSS execution order?

JS section: 

function removeTransition(e) {
          if (e.propertyName !== "transform") return;
           e.target.classList.remove("playing");
         }
keys.forEach(key => key.addEventListener("transitionend", removeTransition));

css section: 

.key {
  transition: all .07s ease;
}
.playing {
  box-shadow: 0 0 1rem -sharpffc600;
  transform: scale(1.1);
  border-color: -sharpffc600;
}

the rest of the code is mainly to listen for the keydown event on the keyboard. When a key is pressed, execute: 

key.classList.add("playing");

what I don"t clear is when removeTransition is called? And after .remove ("playing") , when will the new style be rendered?

it is found that when the browser processes css, it will terminate the execution of JS first. When is the transitioned event triggered?

Javascript css
Mar.03,2021
The

transitionend event is triggered after the CSS transition ends.

in your example, transitionend does trigger multiple times.

such as 



</div>

js: 

function fun_transitionend(){
    console.log("transition end");
}
function fun_hover(){
    console.log("start event binding");
    document.getElementById("transition_div").addEventListener('transitionend',fun_transitionend);
}

css: 

.transition_div{
    height:100px;
    width:100px;
    background:red;
    -webkit-transition:width 5s;
    -moz-transition:width  5s;
    -ms-transition:width  5s;
    -o-transition:width  5s;
    transition:width  5s;
}
.transition_div:hover
{
    width:300px;
}
When the

graph begins to transition the width, the event binding code starts to execute, and the transitionend event handler is triggered when the transformation is complete.

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