Minimize costly DOM read/write operations

Every time you read a layout property after making a DOM change, the browser must synchronously recalculate layout (reflow) before returning the value. Doing this in a loop — even with just 10 elements — can freeze the page for hundreds of milliseconds. This is called layout thrashing and is one of the most common causes of janky animations and slow renders.

Quick Reference

• Never interleave DOM reads (offsetHeight, getBoundingClientRect) with DOM writes (style changes)
• Batch all reads first, then all writes
• Use requestAnimationFrame for visual updates
• Use DocumentFragment or innerHTML for bulk DOM insertion
• Avoid per-frame React state updates across many visible components when a simpler CSS or reduced-frequency approach would work
• Use passive listeners for scroll, touch, and wheel when you do not call preventDefault

Check

Identify any layout thrashing in this code — places where DOM reads and writes are interleaved in loops. Also flag scroll, touch, and wheel listeners that should be passive to avoid blocking the main thread.

Fix

Refactor this code to batch all DOM reads before writes, eliminating forced synchronous layouts. Mark relevant listeners as passive when they do not need to cancel scrolling.

Explain

Explain layout thrashing, what causes it, and how batching reads and writes prevents it.

Code Review

Review scripts, client components, and browser execution paths related to Minimize costly DOM read/write operations. Flag exact imports, event handlers, runtime side effects, or blocking operations that violate the rule, and state how the change should be verified in the browser.

---

For full implementation details, code examples, and framework-specific guidance, see references/rule.md.

Rule page: https://frontendchecklist.io/en/rules/javascript/dom-performance