Why is DOM manipulation slow and how can it be optimized?
Jul 01, 2025 am 01:28 AM
The main reasons for slowing operation of DOM are the high cost of rearrangement and low access efficiency. Optimization methods include: 1. Reduce the number of accesses and cache read values; 2. Batch read and write operations; 3. Merge modifications, use document fragments or hidden elements; 4. Avoid layout jitter, and centrally handle read and write; 5. Use framework or requestAnimationFrame to update asynchronously.
Directly operating the DOM is indeed easy to slow down, especially when frequent updates or processing large numbers of nodes. There are two main reasons: one is that the DOM itself is an independent data structure and is managed separately from the JavaScript engine; the other is that each modification may trigger the browser's reflow (repaint) and repaint (repaint) and these operations are very expensive.
To optimize DOM operations, the key is to reduce the number of visits, merge modification actions, and avoid forced synchronous layouts.
Reduce the number of DOM visits
DOM operations are much slower than ordinary JS variable operations, especially when reading attributes such as offsetWidth or getBoundingClientRect() . This type of "read" operation forces the browser to calculate the style immediately, resulting in forced synchronous layout.
Suggested practices:
- Caches the values ??that need to be read multiple times and only read once.
- Try to save state with variables in JS, rather than repeatedly obtaining it from the DOM.
- Batch reading and writing: first read all the data, then perform logical processing, and finally write to the DOM uniformly.
for example:
const elements = document.querySelectorAll('.item');
let totalHeight = 0;
elements.forEach(el => {
totalHeight = el.offsetHeight; // Read only once});Merge DOM modifications to avoid frequent re-arrangement
Each time the DOM is modified, a reorder may trigger. If it is modified five times in a row, the browser may perform five reorders. But in reality, we only need the last result.
Optimization method:
- Use a document fragment (DocumentFragment) to batch insert nodes.
- Hide the element and then operate it, and display it after completion.
- Use virtual DOM or frameworks (such as React) to automatically make differential updates.
For example:
const fragment = document.createDocumentFragment();
for (let i = 0; i < 100; i ) {
const div = document.createElement('div');
div.textContent = 'Item ' i;
fragment.appendChild(div);
}
document.body.appendChild(fragment); // Reordering is triggered only onceAvoid layout jitter (Layout Thrashing)
The so-called layout jitter is to continuously read and write DOM properties in a loop, causing the browser to re-arrange repeatedly.
Common error writing methods:
for (let i = 0; i < items.length; i ) {
items[i].style.width = container.offsetWidth 'px'; // Forced reordering every read}Optimization method:
- Read offsetWidth first and cache it.
- All write operations are processed centrally.
Use modern tools and asynchronous update strategies
Most front-end frameworks now have built-in performance optimization mechanisms, such as React's reconciliation and Vue's responsive systems, which will batch update the DOM at the right time instead of taking effect immediately.
If you implement similar logic yourself, you can use requestAnimationFrame to control the update rhythm, or use anti-shake throttling to control the DOM operation frequency in high-frequency events (such as resize, scroll).
In general, slow DOM operation is mainly due to the high cost of rearrangement and redrawing and low access efficiency. As long as you achieve "less read and write, merge operations, and delay update", you can effectively improve performance. These techniques are not necessarily complicated, but they are very practical.
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