/frontend-js-best-practices

Avoid Layout Thrashing

Avoid interleaving style writes with layout reads. When you read a layout property (like offsetWidth, getBoundingClientRect(), or getComputedStyle()) between style changes, the browser is forced to trigger a synchronous reflow.

This is OK (browser batches style changes):

function updateElementStyles(element: HTMLElement) {
  // Each line invalidates style, but browser batches the recalculation
  element.style.width = "100px";
  element.style.height = "200px";
  element.style.backgroundColor = "blue";
  element.style.border = "1px solid black";
}

Incorrect (interleaved reads and writes force reflows):

function layoutThrashing(element: HTMLElement) {
  element.style.width = "100px";
  let width = element.offsetWidth; // Forces reflow
  element.style.height = "200px";
  let height = element.offsetHeight; // Forces another reflow
}

Correct (batch writes, then read once):

function updateElementStyles(element: HTMLElement) {
  // Batch all writes together
  element.style.width = "100px";
  element.style.height = "200px";
  element.style.backgroundColor = "blue";
  element.style.border = "1px solid black";

  // Read after all writes are done (single reflow)
  const { width, height } = element.getBoundingClientRect();
}

Correct (batch reads, then writes):

function avoidThrashing(element: HTMLElement) {
  // Read phase - all layout queries first
  let rect1 = element.getBoundingClientRect();
  let offsetWidth = element.offsetWidth;
  let offsetHeight = element.offsetHeight;

  // Write phase - all style changes after
  element.style.width = "100px";
  element.style.height = "200px";
}

Better: use CSS classes

.highlighted-box {
  width: 100px;
  height: 200px;
  background-color: blue;
  border: 1px solid black;
}

function updateElementStyles(element: HTMLElement) {
  element.classList.add("highlighted-box");

  const { width, height } = element.getBoundingClientRect();
}

React example:

// Incorrect: interleaving style changes with layout queries
function Box({ isHighlighted }: { isHighlighted: boolean }) {
  let ref = useRef<HTMLDivElement>(null);

  useEffect(() => {
    if (ref.current && isHighlighted) {
      ref.current.style.width = "100px";
      let width = ref.current.offsetWidth; // Forces layout
      ref.current.style.height = "200px";
    }
  }, [isHighlighted]);

  return <div ref={ref}>Content</div>;
}

// Correct: toggle class
function Box({ isHighlighted }: { isHighlighted: boolean }) {
  return <div className={isHighlighted ? "highlighted-box" : ""}>Content</div>;
}

Prefer CSS classes over inline styles when possible. CSS files are cached by the browser, and classes provide better separation of concerns and are easier to maintain.

See this gist and CSS Triggers for more information on layout-forcing operations.