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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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