Avoiding Pitfalls: Why You Shouldn’t Put Refs in a Dependency Array in React

In the context of React, useEffect is a built-in hook that allows you to perform side effects in functional components. Side effects can include actions like fetching data from an API, subscribing to events, or manipulating the DOM. The useEffect hook replaces the lifecycle methods in class components, such as componentDidMount, componentDidUpdate, and componentWillUnmount.

useEffect(() => {
  // Side effect code here
  // This code will run after the component is rendered
  // You can perform any necessary operations here

  // Optional cleanup function
  return () => {
    // Cleanup code here
    // This code will run before the component is unmounted
    // You can perform any necessary cleanup operations here
  };
}, [dependencyArray]);

The second argument of useEffect is an array called the "dependency array" or "dependency list." It is an optional parameter that specifies which values the effect depends on. The effect will only run if any of the values in the dependency array change between renders. If the dependency array is empty, the effect will run only once, after the initial render.

By including dependencies in the array, you can control when the effect should run. If any of the dependencies change, the effect will be re-evaluated. If no dependencies are provided, the effect will run every time the component re-renders. Specifying the dependencies correctly is important for preventing unnecessary re-renders and managing resource usage efficiently.

In React, when using the `useEffect` hook, it’s generally not recommended to include mutable references (such as objects or arrays) directly in the dependency array. The dependency array is used to determine when the effect should be re-executed. Adding mutable references to the dependency array can lead to unexpected or undesired behavior. Let me explain with an example.

Consider the following code snippet:

import React, { useEffect, useState } from 'react';

const MyComponent = () => {
  const [count, setCount] = useState(0);
  const mutableRef = { value: 0 };
  useEffect(() => {
    console.log('Effect triggered');
  }, [mutableRef]);
  const handleClick = () => {
    mutableRef.value += 1;
    setCount(count + 1);
  };
  return (
    <div>
      <button onClick={handleClick}>Increment</button>
      <p>Count: {count}</p>
    </div>
  );
};
export default MyComponent;

In this example, we have a component with a button that increments a count state variable and updates a mutable reference mutableRef. The effect is supposed to trigger whenever mutableRef changes.

The problem with including `mutableRef` in the dependency array is that React performs a shallow comparison to check if any dependency has changed. Since `mutableRef` is an object and its reference remains the same, React will not detect changes and will not re-execute the effect. This can lead to unexpected bugs and issues.

Instead, if you have a need to trigger an effect when a specific value within a mutable reference changes, you can use the `useEffect` hook with a function as the dependency and perform a manual comparison.

Here’s an updated version of the code to achieve this:

import React, { useEffect, useState, useRef } from 'react';

const MyComponent = () => {
  const [count, setCount] = useState(0);
  const mutableRef = useRef({ value: 0 });
  useEffect(() => {
    console.log('Effect triggered');
  }, [mutableRef.current.value]);
  const handleClick = () => {
    mutableRef.current.value += 1;
    setCount(count + 1);
  };
  return (
    <div>
      <button onClick={handleClick}>Increment</button>
      <p>Count: {count}</p>
    </div>
  );
};
export default MyComponent;

In this updated code, we use the useRef hook to create a mutable reference mutableRef. By accessing the value using mutableRef.current, we can now include mutableRef.current.value in the dependency array. React will then detect changes in the specific value and trigger the effect accordingly.

By following this approach, you can avoid potential issues caused by including mutable references directly in the dependency array of the `useEffect` hook.