Migrating from React 16 to React 18: Challenges and SolutionsMaster the transition to React 18 with insights into common migration issues, performance optimization, and practical solutions for modern React applications.

Understanding the Shift: React 16 vs. React 18

React, one of the most popular JavaScript libraries for building user interfaces, has evolved significantly from version 16 to version 18. The release of React 18 introduced major architectural changes and performance improvements, including concurrent rendering, automatic batching, and updates to hooks like useEffect. While these features enhance React’s capabilities, migrating an existing application from React 16 to React 18 can be challenging, particularly for large or complex codebases.

This blog post aims to guide developers through the common issues faced during migration from React 16 to React 18, with practical examples and solutions to ensure a smooth transition. Whether you’re a frontend developer maintaining legacy code or upgrading to leverage new features, this guide will prepare you for the challenges ahead.

Key Features of React 18

Before diving into migration issues, it’s essential to understand the core features of React 18 and how they differ from React 16. React 18 introduces a new rendering model designed to improve performance and responsiveness.

Concurrent Rendering

React 18’s concurrent rendering enables the library to work on multiple tasks simultaneously. This means React can pause and resume rendering work, prioritizing more urgent tasks like user interactions. While this change improves app responsiveness, it can also expose timing issues or race conditions in components that rely heavily on synchronous rendering patterns.

Automatic Batching

In React 16, state updates triggered by different events are processed individually. React 18 introduces automatic batching, which combines multiple state updates into a single render, reducing unnecessary renders and improving performance. However, this change can alter how components behave, especially if your app relies on side effects triggered by intermediate renders.

Updated Hooks Behavior

React 18 modifies the behavior of hooks like useEffect. In React 16, useEffect runs asynchronously, typically after the browser paints the screen. React 18 changes this by running useEffect synchronously for discrete user inputs, ensuring the effect is applied immediately. This can impact components that perform expensive computations or side effects in useEffect.

Common Migration Issues and Solutions

1. Delayed Large Contentful Paint (LCP)

One of the most reported issues during migration is increased LCP, a core web vital that measures the time it takes to render the largest visible content on the page. This happens because synchronous useEffect in React 18 can block the main thread if it contains heavy computations.

Solution:

Optimize expensive computations inside useEffect. Move tasks like parsing cookies or fetching data to asynchronous methods using requestIdleCallback or Web Workers. Here’s an example:

import { useEffect } from "react";

const useOptimizedEffect = (callback, deps) => {
  useEffect(() => {
    const id = requestIdleCallback(() => callback());
    return () => cancelIdleCallback(id);
  }, deps);
};

useOptimizedEffect(() => {
  parseHeavyData();
}, [data]);

2. Inconsistent Side Effects

React 18’s updated useEffect behavior can cause side effects to run sooner than expected, leading to race conditions or unintended behavior in components that rely on timing.

Solution:

Audit your useEffect calls to ensure they are idempotent and handle potential race conditions. Use useRef to store intermediate states or to cancel outdated effects.

useEffect(() => {
  let isCancelled = false;

  fetchData().then((data) => {
    if (!isCancelled) {
      setData(data);
    }
  });

  return () => {
    isCancelled = true;
  };
}, [dependency]);

3. Automatic Batching and State Update Order

In React 16, state updates are processed individually, while React 18 batches them automatically. This can alter the order in which updates occur, leading to unexpected results.

Solution:

Review state updates and ensure they don’t rely on specific intermediate states. Use functional updates to manage state transitions more predictably.

setCount((prevCount) => prevCount + 1);
setValue((prevValue) => computeNewValue(prevValue));

4. Deprecated API Usage

React 18 deprecates some legacy APIs and lifecycle methods, like componentWillMount and componentWillReceiveProps, which were already flagged in React 16 but are now more strictly enforced.

Solution:

Refactor your class components to use modern lifecycle methods or convert them to functional components with hooks.

// Before
class MyComponent extends React.Component {
  componentWillReceiveProps(nextProps) {
    if (nextProps.value !== this.props.value) {
      this.setState({ value: nextProps.value });
    }
  }
}

// After
function MyComponent({ value }) {
  const [state, setState] = useState(value);

  useEffect(() => {
    setState(value);
  }, [value]);

  return <div>{state}</div>;
}

5. Testing and Debugging Failures

React 18’s StrictMode runs components twice during development to help identify side effects and rendering issues. This can cause tests or debugging sessions to behave unpredictably.

Solution:

Update your test suite to handle double-rendering scenarios, and refactor effects to ensure they are idempotent. Use libraries like React Testing Library for more robust testing.

Async Behavior in React 16 vs. Sync Behavior in React 18

One of the most notable differences between React 16 and React 18 lies in how useEffect is handled. In React 16, useEffect runs asynchronously after the browser has painted the updated UI. This asynchronous nature allows effects to be scheduled as macrotasks, ensuring that the user interface remains responsive while performing operations like data fetching, event subscriptions, or DOM manipulations.

React 18, however, introduces synchronous behavior for certain useEffect executions. When triggered by discrete input events, useEffect in React 18 executes synchronously, ensuring the effect is processed before the next event is handled. This change aims to reduce latency and improve consistency by ensuring state updates and their side effects occur in a predictable order.

While this synchronous behavior can be advantageous, it introduces the potential for blocking the main thread. For example, if a useEffect contains a computationally expensive operation—such as parsing large cookies or processing complex data—React 18 will prioritize executing that effect synchronously. This prioritization can delay subsequent rendering and input handling, leading to degraded performance and increased Largest Contentful Paint (LCP).

To avoid blocking the main thread, developers should: - Offload intensive computations to web workers or debounce effects where possible. - Avoid placing heavy logic directly within useEffect. Instead, move such logic into utility functions that can be executed asynchronously. - Monitor performance using tools like React Profiler or browser devtools to identify and optimize expensive operations.

By understanding these behavioral differences and adapting strategies accordingly, developers can ensure their applications remain performant while taking advantage of React 18’s new capabilities.

Microtask vs. Macrotask: Understanding the Analogy in React's useEffect Behavior

The differences in how useEffect works in React 16 and React 18 can be explained using the analogy of microtasks and macrotasks from JavaScript's event loop. While useEffect itself is not directly tied to the JavaScript event loop, its scheduling and prioritization in React’s lifecycle closely resemble these concepts.

Macrotasks and React 16

In React 16, useEffect callbacks are deferred to run asynchronously after the browser has completed its render and paint cycles. This behavior is similar to how macrotasks are handled in JavaScript:

• Macrotask Characteristics:
  • Executed after the current rendering and painting are completed.
  • Typically used for operations that don’t need to block the user experience (e.g., setTimeout, setInterval, and I/O operations).
  • Ensures that UI updates are visible to the user before any additional computations.

React 16 defers the execution of useEffect in a similar manner. After updating the DOM, the browser paints the changes, ensuring that the user sees the updated UI immediately. Then, React processes the useEffect callbacks in the next idle cycle. This helps maintain a smooth user experience by keeping heavy computations or side effects out of the critical rendering path.

Microtasks and React 18

In React 18, the introduction of concurrent rendering and task prioritization modifies how useEffect is scheduled, especially in response to discrete user inputs (e.g., a button click). Here, useEffect callbacks are executed more synchronously in React's lifecycle, akin to the behavior of microtasks in JavaScript:

• Microtask Characteristics:
  • Executed immediately after the current synchronous task completes but before the next macrotask.
  • Often used for high-priority, fine-grained tasks (e.g., Promise.then, MutationObserver).
  • Ensures that critical updates or operations happen as soon as possible.

React 18 mimics this behavior by prioritizing useEffect for discrete user events. After React processes a state update caused by a user interaction, it runs the corresponding useEffect synchronously to ensure the effect's side effects (like event listener attachments or derived computations) are ready before the next input is processed. However, this synchronous execution can block the main thread, delaying the browser's ability to render or paint new content.

Key Differences Between React 16 and React 18

Why This Matters

Understanding this analogy helps developers recognize the potential performance implications of using useEffect in React 18. While the synchronous scheduling ensures correctness for state-dependent side effects, it can lead to increased blocking of the main thread if the useEffect callback involves heavy computations. In contrast, React 16’s deferred approach prioritizes rendering and painting, which might feel more responsive to users but could delay critical side effects.

By treating React 18’s useEffect like a microtask, developers can better plan for scenarios where synchronous execution impacts performance and optimize their applications accordingly.

Migrating from React.js to React 18 and tackling the challenges

Migrating from React 16 to React 18 introduces changes in how useEffect behaves, particularly its synchronous execution in some scenarios. To mitigate potential performance issues, you can strategically use useDeferredValue, useLayoutEffect, and a custom useIdleEffect. Here's how you can use these hooks effectively:

1. Using useDeferredValue

useDeferredValue allows you to prioritize rendering while deferring less critical computations that depend on state. This is particularly useful when expensive state-based computations cause performance bottlenecks.

When to Use:

• When your useEffect logic depends on a rapidly changing state that doesn't need to update immediately.
• To prioritize rendering and delay computation until the browser is idle.

Example:

import { useState, useDeferredValue, useEffect } from "react";

const ExpensiveComponent = ({ input }: { input: string }) => {
  const deferredInput = useDeferredValue(input);

  useEffect(() => {
    // Expensive computation based on deferredInput
    console.log("Processing deferred input:", deferredInput);
  }, [deferredInput]);

  return <div>Input: {deferredInput}</div>;
};

const App = () => {
  const [input, setInput] = useState("");

  return (
    <div>
      <input
        type="text"
        value={input}
        onChange={(e) => setInput(e.target.value)}
        placeholder="Type something..."
      />
      <ExpensiveComponent input={input} />
    </div>
  );
};

How It Helps:

• The expensive computation in the effect runs only when the browser has time to process the deferred value, improving responsiveness.

2. Using useLayoutEffect

useLayoutEffect runs synchronously after DOM mutations but before the browser paints. This makes it suitable for layout-critical updates. However, it's essential to use it sparingly because it blocks rendering until it completes.

When to Use:

• When the logic directly impacts the layout (e.g., measuring DOM elements).
• When you need precise timing to avoid flickering or layout inconsistencies.

Example:

import { useLayoutEffect, useRef } from "react";

const LayoutSensitiveComponent = () => {
  const divRef = useRef<HTMLDivElement | null>(null);

  useLayoutEffect(() => {
    if (divRef.current) {
      const rect = divRef.current.getBoundingClientRect();
      console.log("Div dimensions:", rect);
    }
  }, []);

  return (
    <div
      ref={divRef}
      style={{ width: "200px", height: "100px", backgroundColor: "lightblue" }}
    >
      Measure Me!
    </div>
  );
};

How It Helps:

• Ensures layout-critical updates happen synchronously, avoiding visual inconsistencies caused by delayed effects.

3. Using a Custom useIdleEffect

A custom useIdleEffect leverages requestIdleCallback to defer non-critical logic until the browser is idle. This approach helps offload heavy computations from the main thread.

When to Use:

• For non-urgent computations like analytics, cookie parsing, or preloading data.
• To ensure smooth user interactions by deferring heavy tasks.

Example:

import { useEffect } from "react";

const useIdleEffect = (callback: () => void, deps: React.DependencyList) => {
  useEffect(() => {
    const id = requestIdleCallback(callback);
    return () => cancelIdleCallback(id);
  }, deps);
};

const IdleComputationComponent = () => {
  useIdleEffect(() => {
    console.log("Idle computation started...");
    // Simulate heavy computation
    for (let i = 0; i < 1e6; i++) {
      Math.sqrt(i);
    }
    console.log("Idle computation finished.");
  }, []);

  return <div>Check the console for idle computation logs.</div>;
};

How It Helps:

• Defers non-critical tasks until the browser has spare time, preventing main thread blocking and improving metrics like LCP.

Using These Hooks Together

You can combine these hooks to handle different scenarios in your application:

Scenario: Input-dependent heavy computations

1. Use useDeferredValue to prioritize rendering.
2. Use useIdleEffect to defer the computation to idle time.

Scenario: Layout-sensitive updates

1. Use useLayoutEffect for precise DOM measurements.
2. Use useIdleEffect for non-urgent tasks triggered by layout changes.

Example:

import { useState, useDeferredValue, useLayoutEffect } from "react";

const CombinedHooksComponent = () => {
  const [input, setInput] = useState("");
  const deferredInput = useDeferredValue(input);

  useLayoutEffect(() => {
    console.log("Synchronous layout update for:", deferredInput);
  }, [deferredInput]);

  useIdleEffect(() => {
    console.log("Deferred computation for:", deferredInput);
    // Simulate heavy computation
    for (let i = 0; i < 1e6; i++) {
      Math.sqrt(i);
    }
  }, [deferredInput]);

  return (
    <div>
      <input
        type="text"
        value={input}
        onChange={(e) => setInput(e.target.value)}
        placeholder="Type something..."
      />
      <div>Deferred Input: {deferredInput}</div>
    </div>
  );
};

Key Takeaways

1. useDeferredValue: Prioritize rendering over non-critical updates for responsive UI.
2. useLayoutEffect: Use for layout-sensitive tasks but avoid heavy computations.
3. Custom useIdleEffect: Defer non-critical logic to idle periods to improve performance metrics like LCP.

By applying these hooks appropriately, you can mitigate issues caused by React 18’s synchronous behavior and optimize your app for both responsiveness and performance.

Conclusion

Migrating from React 16 to React 18 is a significant step that brings new capabilities but also new challenges. By understanding the architectural changes and addressing common issues proactively, developers can take full advantage of React 18’s performance improvements and modern features. Optimizing useEffect calls, handling side effects carefully, and refactoring legacy code are key strategies for a successful migration.

While the transition might seem daunting, the benefits of React 18’s concurrent rendering and automatic batching outweigh the initial effort. With careful planning and execution, you can ensure a seamless upgrade that improves both the developer experience and end-user performance.

Reads & Resources

• medium.com - The Foundational Update to Core Rendering model React 18
• stackoverflow - Contrast between useEffect callback execution between v16 & v18 of react, what is the intended behavior?