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A Beginner's Guide to useInsertionEffect Hook: Understanding the Basics

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Rakesh Purohit

ReactJS Developer Advocate
September 11, 2023

Rakesh Purohit

September 11, 2023

Welcome to our blog post on the intriguing topic of useInsertionEffect. As we continue on our journey through the fascinating world of React, we keep discovering new gems like useInsertionEffect. As with any discovery in coding, the most important thing is understanding how and where to utilize this new feature. In this blog post, we will unfold this mystery and bring clarity to what useInsertionEffect really is, and how it can enhance our React projects. Let's get started!

What is useInsertionEffect?

useInsertionEffect is a part of the React.js library that deals primarily with CSS-in-JS styles. It is a hook that is specifically used to insert styles into your DOM in a React project.

Although it might sound similar to other hooks such as useEffect or useLayoutEffect, useInsertionEffect has its own unique behaviour. The fundamental difference lies in the timing of execution. While useEffect runs after layout and paint, useInsertionEffect runs synchronously before paint, which allows for styles insertion prior to the first paint.

How useInsertionEffect Works in a React Project.

In a typical React project, useInsertionEffect works by inserting CSS rules into a style tag in your DOM. It's important to understand that the hook's main purpose is handling CSS styles in your components.

Let's take a quick overview of how useInsertionEffect can be utilized in a React project. Here's a simplified example of the hook's usage:

In the above code, the useInsertionEffect hook is used to dynamically add a style to the head of your document, giving you the freedom to manage the presentation of your components.

Understanding the Basics of useInsertionEffect Hook.

As useInsertionEffect is a hook, it should be used in the functional component of your React project. Similar to useEffect, it runs its callback function when the component renders for the first time. This is known as the initial render.

It's crucial to note that unlike other hooks, useInsertionEffect runs synchronously, which means it doesn't wait for browser paint to complete. In other words, your styles get inserted into the DOM before anything gets painted on the screen. This behavior ensures there is no visible transition from default styles to new styles.

Moreover, you can use dependencies to control when your useInsertionEffect runs, allowing you to target your styles more specifically. Furthermore, useInsertionEffect also accepts a clean-up function which we will discuss in the next section.

A Clear Explanation About Clean Up Function.

Clean-up functions are an essential part of using React hooks, including useInsertionEffect. A clean-up function is what it sounds like: a function that cleans up after a side effect runs.

For useInsertionEffect, the clean-up function runs before the component is removed from the UI to prevent memory leaks. It's used to remove any styles associated with the component that were inserted into the DOM. Here's an example:

In the above code, the clean-up function is used to remove the appended cssRules from the parent node, preventing any potential overflow of styles in your document's head. Thus, the clean up function ensures that your code stays clean, and only the necessary components receive the styles.

Exploring useInsertionEffect

Having breezed through the introduction and basic understanding of useInsertionEffect hook in Part One, let's descend a bit further. In this second part of our blog post, we'll discuss in detail the differences between useEffect and useInsertionEffect, the initial render, the styles and CSS rules insertion, and more.

Diving Deep: Differences Between useEffect and useInsertionEffect.

While useEffect and useInsertionEffect might seem similar due to their comparable syntax and application, they differ greatly in their underlying behavior. The key difference lies in their timing of execution.

useEffect, a crucial tool in handling side-effects in React, runs its callback asynchronously after the browser paints the screen - it operates after the render phase. On the other hand, useInsertionEffect runs synchronously before the browser paint phase. This aspect gives useInsertionEffect a unique ability to insert or manipulate styles before they are reflected on the screen.

This nuanced difference empowers useInsertionEffect with a new set of possibilities, especially when dealing with CSS-in-JS libraries and ensuring a flicker-less style rendering.

Role of Initial Render in useInsertionEffect.

The initial render plays a pivotal role in React apps, as it's the moment when a component is rendered for the first time. The useInsertionEffect hook utilizes this opportunity to perform its function.

Parallel to useEffect, useInsertionEffect executes its callback function during the initial render. However, the significant difference is that unlike useEffect which waits till after the rendering phase, useInsertionEffect acts before paint - applying styles just in time and eliminating unwanted layouts shift or visible transitions from default to new styles.

Insertion of Styles and CSS Rules.

In a React project, styles and CSS rules are instrumental in defining the appearance of components. With useInsertionEffect, these can be inserted into the DOM in a way that directly impacts the layout rending, creating impressive effects while avoiding the flickering or janky transitions associated with rendering styles post-paint.

The hook leverages JavaScript to assign the rules to particular DOM nodes. The defined style rules are then appended to the document head, allowing your components to reflect the styles as desired.

Detailed Account of Reading Layout, CSS in JS Libraries, and Layout Effects.

A major use-case, as well as a key aspect to understand when dealing with useInsertionEffect, involves the process of reading layout, the application of CSS in JS Libraries, and executing layout effects.

Reading layouts involves obtaining measurements or computations about the DOM node layout, such as elements' offsets or dimensions. Generally, reading layout following a write operation can force layout recalculations or reflows, causing a hit to the performance. This principle, known as layout thrashing, is avoided by useInsertionEffect as it executes synchronously before browser paint.

In the context of CSS-in-JS libraries, where styles are created or manipulated using JavaScript, useInsertionEffect can offer significant improvements. The hook allows for the styles to be injected into the DOM just before the first paint, circumventing issues that could arise from style computations or changes reflected after the initial render.

Finally, layout effects refer to the changes that you may want your styles to bring about on the DOM nodes. With useInsertionEffect, you can control these effects more granularly, ensuring your components render as intended without style-related anomalies.

In the next section, we'll dive deeper into advanced understanding and applications of useInsertionEffect. Stay tuned!

Advanced Understanding

Warm welcome to the third part of our blog post about useInsertionEffect. This section covers complex subjects like concurrent rendering and how it impacts useInsertionEffect. We will also discuss server rendering, synchronous re-rendering, and selective hydration. Finally, we will explain the utilization of useInsertionEffect in DOM mutations and response to state updates. So, fasten your seat belts and let's dive right into it!

Concept of Concurrent Rendering and its Impact on useInsertionEffect.

Concurrent rendering -- a highly anticipated feature of React – deals with the ability to work on several tasks simultaneously without blocking the main thread. This means that expensive rendering tasks can be split over several frames instead of blocking the browser. In essence, it allows React to be more responsive under heavy computational tasks and render more smoothly.

However, when concurrent rendering is in place, useInsertionEffect may behave slightly differently. It can run more than once before the screen gets updated due to the interruption and continuation of rendering. It is crucial then for any code within useInsertionEffect to be idempotent, meaning running this code multiple times will yield the same effects on the DOM.

In-depth Discussion on Server Rendering, Synchronously Re-Render, and Selective Hydration.

Server Rendering & useInsertionEffect

Server rendering refers to generating HTML directly on the server for a specific request. This HTML is then sent to the client. With server rendering, useInsertionEffect may not execute as there is no DOM available at the time of running this hook. This is important to keep in mind when you are planning to use it in a server-rendered application.

Synchronous Re-Render

A synchronous re-render takes place when your state or the props of your React components change. This causes an immediate re-render of your components. useInsertionEffect runs before these updates are painted on the screen, keeping the styles and layout consistent and preventing unnecessary reflows.

Selective Hydration

Selective hydration is a strategy in which parts of an application are left dehydrated (or uninteractive) while others are being hydrated. useInsertionEffect could be used to enhance this concept by applying CSS rules before an interactive component is rendered for a better user experience.

Utilizing useInsertionEffect in DOM Mutations and Reacting to State Updates.

useInsertionEffect can be utilized effectively with DOM mutations and state updates. Since useInsertionEffect runs dynamically before browser paint, it ensures that any style changes are reflected before visually rendering the changes on the screen.

When state updates occur in a component, React re-renders the component to accurately visualize the state data on the interface. With useInsertionEffect, you can be sure that any style changes related to these state updates are handled before the browser paints the updated components. This way, you maintain the user interface's visual integrity and performance, even during frequent updates or mutations.

That wraps up our discussion on the advanced understanding of useInsertionEffect. As we move into the conclusion in our next section, we’ll offer some real-world applications and tips for library authors. Stay tuned!


We have reached the concluding part of our in-depth exploration of useInsertionEffect in React. We've discussed what it is, how it works, and how it differs from useEffect. We've also dived deep into concurrent rendering, server rendering, synchronous re-rendering, and how useInsertionEffect can be effectively utilized with DOM mutations and state updates. Now, let's wrap up with some examples, real-world applications, tips for library authors, and some final thoughts on useInsertionEffect.

Examples and Real World Applications of useInsertionEffect.

Let's visualize the usage of useInsertionEffect with a simple example. Consider a Scenario where we need to mutate the DOM by applying a CSS rule to a particular DOM node dynamically.

Disclaimer: The following code snippets are simplified for understanding. In a real-world scenario, your CSS-in-JS solution will likely have a more sophisticated way of inserting dynamic styles.

Adding a style rule:

In many real-world applications, especially when dealing with CSS-in-JS libraries, useInsertionEffect brings a lot of value. It eliminates flickers or unwanted transition effects between default and the new styles, offering seamless UI updates.

Important Notes for Library Authors.

Library authors who deal with CSS in JS libraries can benefit greatly from useInsertionEffect. It can be a valuable tool when applying dynamic styles in React components. However, while useInsertionEffect offers benefits, there are some aspects to be cautious about.

Firstly, it's essential to remember that useInsertionEffect runs synchronously before the browser paint. Therefore, it should not include heavy computations that might delay the painting of the components, leading to sluggish UI.

Secondly, with concurrent rendering, useInsertionEffect can run several times before the browser paint happens. Therefore, it's critical to ensure that the code within the hook is idempotent, able to yield the same result even when executed multiple times.

Lastly, in a server-rendered scenario, useInsertionEffect may not execute due to the absence of the DOM. Hence, it's advisable to consider an alternative or fallback for server-rendered scenarios.

Final Thoughts on useInsertionEffect in React.

useInsertionEffect introduces new possibilities in handling dynamic styles in React components. Its ability to synchronously apply style changes before paint avoids layout shifts or flickers, contributing to better, smoother user interfaces.

The understanding of useInsertionEffect and when to use it appropriately can make a huge difference in crafting high-performance React applications. With it, we are equipped with a tool to make components behave more fluently, rendering styles as we intend.

Remember, each tool or hook has its own strengths and trade-offs. It's crucial to understand the underlying concepts well before applying them.

And thus, with useInsertionEffect, you're more empowered to innovate and impel the limit of what's possible with React. Happy coding!

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