What Forces Layout / Reflow in Browsers and Why It Matters
When optimizing a website’s performance, understanding what causes layout reflow is essential. Reflow, also known as layout, occurs when the browser recalculates the position and size of elements on the page. This process can be costly in terms of performance, especially when triggered repeatedly.
In this blog post, we’ll dive into specific JavaScript methods that force reflow, explain why they occur, and provide actionable steps to reduce reflows for smoother user experiences.
What Causes Reflow?
Reflows can be triggered by various DOM changes, CSS modifications, or JavaScript methods that require the browser to re-evaluate the layout. Let’s break down some of the most common culprits:
- Layout Properties
Anytime you access properties like:
- offsetWidth
- offsetHeight
- clientWidth
- scrollWidth
These properties force the browser to calculate the latest layout, which can lead to performance issues if accessed too often, especially inside loops.
2. getComputedStyle()
This method retrieves the computed styles of an element, such as its dimensions or applied styles. The act of calling this function makes the browser resolve any pending layout recalculations.
3. Scroll Properties and Methods
Accessing or modifying scroll-related properties and methods, such as:
- scrollTop
- scrollLeft
- scrollTo()
These force layout recalculations to ensure the scrolling position is accurate.
4. DOM Manipulation
Adding, removing, or changing DOM elements through methods like:
- appendChild()
- removeChild()
- innerHTML
These actions require the browser to adjust the layout to accommodate the new content, often causing reflow.
5. Class Manipulations
Using methods like:
- classList.add()
- classList.remove()
When class changes affect the element’s size or position, reflow is triggered.
6. Focus Management
Calling methods such as focus() or blur() on elements can also cause reflow, especially if focusing causes the browser to scroll the element into view.
7. SVG Element Updates
Manipulating attributes like width, height, or position in SVG elements causes reflow. Since SVG graphics are part of the document layout, any change here affects the overall page rendering.
Why Reflow Is Bad for Performance
Reflows are expensive because when the browser recalculates the layout, it often has to re-render parts or the entire page. If multiple reflows occur in rapid succession — such as when a loop repeatedly queries layout properties and changes the DOM — it can cause significant lag and a poor user experience.
Layout Thrashing
One common problem is layout thrashing — this happens when scripts make layout changes and then immediately read layout properties again. This forces the browser to perform reflow repeatedly, resulting in inefficient page rendering and degraded performance.
How to Minimize Reflows
To prevent reflows from becoming a performance bottleneck, here are some strategies to follow:
1. Batch DOM Updates: Instead of making individual changes one at a time, group multiple DOM manipulations together. For example, modify the DOM in a document fragment before attaching it back to the main document.
2. Avoid Accessing Layout Properties in Loops: Avoid reading layout properties like `offsetWidth` or `scrollTop` inside loops. Instead, store values in variables and reuse them.
3. Use CSS Transforms for Animations: Animating with CSS properties like `transform` and `opacity` avoids triggering reflow since they only affect the element’s visual rendering and not the layout.
4. Virtual DOM: Frameworks like React utilize a virtual DOM that minimizes actual DOM changes, significantly reducing the number of reflows in dynamic web applications.
Conclusion
Reflow is an inevitable part of rendering a web page, but by understanding what causes it and how to avoid it, developers can make their sites more efficient. Reducing unnecessary reflows leads to smoother user experiences, better performance, and faster load times.
By following best practices like batching DOM updates, avoiding layout thrashing, and using CSS transforms, you can ensure your web pages run efficiently and provide a seamless experience for users.
For more in-depth information and examples of JavaScript methods that force reflow, you can check out Paul Irish’s [comprehensive guide](https://gist.github.com/paulirish/5d52fb081b3570c81e3a).
