React & Next.js

Why React Compiler Can’t Completely Fix Memory Leaks

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React developers are likely familiar with the nuisance of memory leak issues, especially when using closures and memoization hooks like `useCallback` or `useEffect`. This article explores how the new React compiler can mitigate these problems and the areas where it still falls short.


The Limitations of the React Compiler: It Can’t Prevent All Memory Leaks

If you were expecting the new React compiler to completely solve memory leak issues, you might be slightly disappointed. [While the React compiler can prevent memory leaks caused by closures under certain conditions, it doesn’t apply universally.] The React team explains that the compiler can cache certain memoized values to prevent closure-related memory leaks, but leaks can still occur depending on the values referenced by closures.

For example, in the code below, every time the state is changed in the memoized `useCallback` function, a new instance of `BigObject` is created, which isn’t collected by the garbage collector.

import { useState, useCallback } from 'react';

class BigObject {
  public readonly data = new Uint8Array(1024 * 1024 * 10);
}

export const App = () => {
  const [countA, setCountA] = useState(0);
  const [countB, setCountB] = useState(0);
  const bigData = new BigObject();

  const handleClickA = useCallback(() => {
    setCountA(countA + 1);
  }, [countA]);

  const handleClickB = useCallback(() => {
    setCountB(countB + 1);
  }, [countB]);

  const handleClickBoth = () => {
    handleClickA();
    handleClickB();
    console.log(bigData.data.length);
  };

  return (
    <div>
      <button onClick={handleClickA}>Increment A</button>
      <button onClick={handleClickB}>Increment B</button>
      <button onClick={handleClickBoth}>Increment Both</button>
      <p>A: {countA}, B: {countB}</p>
    </div>
  );
};

When this code runs in the React compiler, you’ll notice in the memory snapshot that instances of `BigObject` keep being allocated. This occurs because new instances are created with each state change, leading to a memory leak.

Approaches to Resolving the Issue

One way to address this issue is to bypass closures entirely. For instance, by using the `bind` method to pass values directly to a function, you avoid reliance on the shared context object across closures. This approach helps prevent memory leaks.

Here is an example using `bind`:

import { useState } from 'react';

class BigObject {
  constructor(public state: string) {}
  public readonly data = new Uint8Array(1024 * 1024 * 10);
}

function bindNull<U extends unknown[]>(f: (args: U) => void, x: U): () => void {
  return f.bind(null, x);
}

export const App = () => {
  const [countA, setCountA] = useState(0);
  const [countB, setCountB] = useState(0);
  const bigData = new BigObject(`${countA}/${countB}`);

  const handleClickA = bindNull(([count, setCount]) => {
    setCount(count + 1);
  }, [countA, setCountA] as const);

  const handleClickB = bindNull(([count, setCount]) => {
    setCount(count + 1);
  }, [countB, setCountB] as const);

  const handleClickBoth = bindNull(([countA, setCountA, countB, setCountB]) => {
    setCountA(countA + 1);
    setCountB(countB + 1);
    console.log(bigData.data.length);
  }, [countA, setCountA, countB, setCountB] as const);

  return (
    <div>
      <button onClick={handleClickA}>Increment A</button>
      <button onClick={handleClickB}>Increment B</button>
      <button onClick={handleClickBoth}>Increment Both</button>
      <p>A: {countA}, B: {countB}</p>
    </div>
  );
};

This code operates as expected in the React compiler without causing memory leaks.

Conclusion

While the React compiler can enhance performance through code optimization, it can’t prevent all memory leaks. [When working with React, be cautious with closures and memoization, and follow best practices to prevent memory leaks.] Writing small, pure components and using memory profilers to identify issues are crucial steps.

Understanding and addressing this issue will significantly improve your React development skills. Take a moment today to inspect your code for potential memory leaks!

Reference: Schiener.io, “Sneaky React Memory Leaks: How the React compiler won’t save you”

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