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HTML

              
                <div id="frame">
  <div id="block"></div>
</div>
              
            
!

CSS

              
                #frame {
  border: 1px solid red;
}

#block {
  width: 20px;
  height: 40px;
  background: red;
  position: relative;
}
              
            
!

JS

              
                // Move the block to the right, all the way to the end of the frame
const moveBlock = () => {
  // Convert the left position of the block (value of the form "XXpx") to a number
  const xBlock = parseFloat(getComputedStyle(blockElement).left);
  // Convert the width of the frame (value of the form "XXpx") to a number
  const xMax = parseFloat(getComputedStyle(frame).width);
  // If the block isn't already to the end of the frame
  if (xBlock + blockWidth <= xMax) {
    // Block movement
    blockElement.style.left = (xBlock + movement) + "px";
    animationId = requestAnimationFrame(moveBlock);
  }
  else {
    // Cancel the animation
    cancelAnimationFrame(animationId);
  }
};

//const frameElement = document.getElementById("frame");
const blockElement = document.getElementById("block");
// Convert the block width (value of the form "XXpx") to a number
const blockWidth = parseFloat(getComputedStyle(block).width);

// Movement value in pixels
const movement = 7;

// Start the animation
let animationId = requestAnimationFrame(moveBlock);
              
            
!
999px

Console