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Here you can Sed posuere consectetur est at lobortis. Donec ullamcorper nulla non metus auctor fringilla. Maecenas sed diam eget risus varius blandit sit amet non magna. Donec id elit non mi porta gravida at eget metus. Praesent commodo cursus magna, vel scelerisque nisl consectetur et.

HTML

              
                <div id="box"></div>

              
            
!

CSS

              
                #box {
  width: 100px;
  height: 100px;
  background: lightgreen;
}

body {
  overflow-x: hidden;
}
              
            
!

JS

              
                let box         = document.getElementById('box');
let direction   = 'forward';
let oldPosition = 0;

function moveBox() {
  // 1) Move box
  if (direction === 'forward') {
    newPosition = oldPosition += 5;
  } else {
    newPosition = oldPosition -= 5;
  }

  box.style.marginLeft = `${newPosition}px`;  // ⚠️ write to DOM, invalidating previous snapshot


  // 2) Determine next direction
  let boxPosition = box.getBoundingClientRect().x;  // ⚠️ read from DOM, forcing Layout because snapshot was invalidated
  let boxWidth    = box.offsetWidth;
  let windowWidth = window.innerWidth;
  let boxHitRightBoundary = boxPosition + boxWidth >= windowWidth;
  let boxHitLeftBoundary  = boxPosition <= 0;

  if (boxHitRightBoundary) {
    direction = 'backward';
  } else if (boxHitLeftBoundary) {
    direction = 'forward';
  }

  // 3) Call requestAnimationFrame again to loop
  requestAnimationFrame(moveBox);  // ⚠️ run repeatedly in quick succession
}

requestAnimationFrame(moveBox); 



/**************************************
  Optimized version below
***************************************/

// let box         = document.getElementById('box');
// let direction   = 'forward';
// let oldPosition = 0;
// let boxWidth    = box.offsetWidth;  // ✅ cache values ahead of time
// let windowWidth = window.innerWidth;

// function moveBox() {
//   // 1) Determine next direction
//   let boxPosition = box.getBoundingClientRect().x; // ✅ read from DOM, taking advantage of cached snapshot
//   let boxHitRightBoundary = boxPosition + boxWidth >= windowWidth;
//   let boxHitLeftBoundary  = boxPosition <= 0;

  // if (boxHitRightBoundary) {
  //   direction = 'backward';
  // } else if (boxHitLeftBoundary) {
  //   direction = 'forward';
  // }
  
//   // 2) Move box
//   if (direction === 'forward') {
//     newPosition = oldPosition += 5;
//   } else {
//     newPosition = oldPosition -= 5;
//   }
//   box.style.marginLeft = `${newPosition}px`;  // ✅ write to DOM, browser doesn't execute until Javascript is done

//   // 3) Call requestAnimationFrame again to loop
//   requestAnimationFrame(moveBox);
// }

// requestAnimationFrame(moveBox); 
              
            
!
999px

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