Forced Reflow While Executing JavaScript

In this article, we’ll delve deep into forced reflow While Executing JavaScript, its causes, implications, and how to mitigate its impact effectively.

What is forced reflow while executing JavaScript?

Forced reflow while executing JavaScript is a technical term that refers to the browser’s process of recalculating the layout of a web page when certain JavaScript operations are performed.

This can lead to a delay in rendering, negatively impacting the user experience. Let’s break down the key aspects of this phenomenon.

Understanding Reflow

Reflow, also known as layout or re-layout, is the browser’s process of determining the position and size of elements on a web page.

When a reflow occurs, the browser recalculates the geometry of each element affected by the change, which can be triggered by various actions, including:

  • Adding or removing elements from the DOM (Document Object Model).
  • Changing CSS styles, such as dimensions, margins, or padding.
  • Modifying the content of form elements, like input fields or textareas.

Understanding the Causes

Forced reflow can be caused by various factors, including:

  • DOM Manipulation: When JavaScript modifies the Document Object Model (DOM), the browser may need to recalculate the layout.
  • Style Changes: Altering CSS properties like dimensions or positions can trigger a reflow.
  • Requesting Dimensions: Accessing properties like offsetWidth or offsetHeight can force the browser to reflow.

How forced reflow while executing JavaScript Occur?

Forced reflow, also known as layout thrashing, occurs in web development when JavaScript code triggers the browser to recalculate the layout of elements on a web page.

This can be a costly operation in terms of performance because it forces the browser to pause and recalculate the position and size of elements on the page, which can lead to slow and janky user experiences.

There are several common scenarios where forced reflow can occur:

  1. Reading Layout Properties: When JavaScript code reads layout properties like offsetWidth, offsetHeight, clientWidth, clientHeight, or getComputedStyle, it can trigger a reflow because the browser needs to provide accurate values for these properties.
   const element = document.getElementById('myElement');
   const width = element.offsetWidth; // Can trigger a reflow

  1. Changing Style Properties: Modifying CSS properties of an element using JavaScript can also trigger reflows, especially if multiple properties are changed in quick succession.
   const element = document.getElementById('myElement'); = '100px'; // Can trigger a reflow = '100px'; // Can trigger another reflow

  1. Batching Changes: Making multiple style changes in a loop without batching them can result in multiple reflows.
   for (let i = 0; i < 100; i++) { = i + 'px'; // Causes multiple reflows

To minimize forced reflows and improve performance, consider the following strategies:

  1. Batch DOM Updates: Whenever possible, batch DOM updates together to minimize the number of reflows. Use techniques like requestAnimationFrame to schedule updates for the next frame.
  2. Avoid Frequent Layout Reads/Writes: Minimize the use of layout-reading properties in loops or repeatedly. Cache the values when possible.
  3. Use CSS Transitions and Animations: Instead of directly modifying element styles with JavaScript, use CSS transitions and animations for smoother animations, which are often more optimized by the browser.
  4. Use the transform Property: For animations and transformations, use the transform property as it typically doesn’t trigger a reflow.
  5. Defer Non-Critical Operations: Defer non-critical JavaScript operations that could trigger reflows until after the page has finished rendering. This can be achieved using setTimeout or requestIdleCallback for less critical tasks.

By following these best practices, you can reduce the occurrence of forced reflows in your JavaScript code and improve the overall performance and responsiveness of your web applications.

I think we already covered everything we need to know about this article trying to convey.

Nevertheless, you can also check these articles to enhance your JavaScript manipulation skills.


In conclusion, forced reflow while executing JavaScript is a critical consideration in web development that can significantly impact the performance and user experience of a website. This phenomenon occurs when JavaScript operations trigger the browser to recalculate the layout of elements on a web page, leading to delays in rendering and potentially causing a jarring user experience.

Leave a Comment