foo.js

function clearAll(el, func){
  console.log("b");
  el.appendBefore(el.previousElementSibling);
  el.classList.remove("transitioning");
  el.nextElementSibling.style.position = "static";
  el.nextElementSibling.removeEventListener("mousemove", func);
  
}

function elementTransition(element, distX, func){
  document.styleSheets[0].insertRule(".transitioning{ transition: transform 300ms linear;"+
                                     "transform: translate(" + distX + "px, 0px);}");
  element.classList.add("transitioning");
  console.log("a")
  window.setTimeout( clearAll(element, func), 300);
}