batch-dom-css-changes

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name: Batch DOM CSS Changes impact: MEDIUM description: reduces reflows/repaints tags: javascript, dom, css, performance, reflow

Batch DOM CSS Changes

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.

Incorrect (interleaved reads and writes force reflows):

Copyfunction updateElementStyles(element: HTMLElement) {
  element.style.width = '100px'
  const width = element.offsetWidth  // Forces reflow
  element.style.height = '200px'
  const height = element.offsetHeight  // Forces another reflow
}

Correct (batch writes, then read once):

Copyfunction 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()
}

Better: use CSS classes

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

Copyfunction updateElementStyles(element: HTMLElement) {
  element.classList.add('highlighted-box')

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

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.