Why useeffect is running twice in react
Tags: react, javascript, webdev, discuss
Published: 2023-02-11 11:48:47

The `useEffect` hook allows you to perform side effects in your React components. It takes a callback function as its first argument, which is called whenever the component is updated. The second argument is an array of dependencies, which is a list of values that the effect depends on. When any of these dependencies change, the effect is re-run. If the array is omitted or is empty, the effect will run on every render.

In React, the `useEffect` hook is called multiple times because it is designed to re-run whenever certain values (referred to as "dependencies") change.

Here are a few reasons why an `useEffect` might be called twice:

- **Missing dependencies:** If you forget to include all of the values that the effect depends on, it will run twice. This is because it will re-run on every render, not just the ones where the data actually changed.
- **Changing state inside the effect:** If you change the state inside an effect, it will cause a re-render, which will then cause the effect to re-run. To avoid this, make sure to wrap state updates inside a `useState` or `useReducer` call.
- **Async updates:** If you're updating state asynchronously (for example, in a `setTimeout`), the state may change after the effect has already run, causing the effect to re-run on the next render.

If you want to prevent your `useEffect` from running twice, make sure to list all of its dependencies correctly and avoid changing state inside the effect.

Here's an example of an `useEffect` that causes unnecessary re-renders because of missing dependencies:

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

function Example() {
  const [count, setCount] = useState(0);
  const [name, setName] = useState('John');

  useEffect(() => {
    console.log(`The count is ${count}`);
  });

  return (
    <div>
      <p>{`The count is ${count}`}</p>
      <button onClick={() => setCount(count + 1)}>Increase count</button>
      <button onClick={() => setName('Jane')}>Change name</button>
    </div>
  );
}
```

In this example, the `useEffect` will run every time the component re-renders, even though the `count` is the only thing that actually changed. To fix this, we can list `count` as a dependency of the effect:

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

function Example() {
  const [count, setCount] = useState(0);
  const [name, setName] = useState('John');

  useEffect(() => {
    console.log(`The count is ${count}`);
  }, [count]); // <-- Add the dependency here

  return (
    <div>
      <p>{`The count is ${count}`}</p>
      <button onClick={() => setCount(count + 1)}>Increase count</button>
      <button onClick={() => setName('Jane')}>Change name</button>
    </div>
  );
}
```

With this change, the `useEffect` will only run when the `count` changes, and not every time the component re-renders.


When using the `useEffect` hook with an array of dependencies, you can use the `useRef` hook to avoid unnecessary re-renders in some cases.

For example, if you have an effect that depends on some values that don't change during the lifetime of your component, you can use a ref to store these values and exclude them from the list of dependencies. This way, the effect will only re-run when the values that actually matter change.

Here's an example:

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

function Example() {
  const [count, setCount] = useState(0);
  const [text, setText] = useState('Hello');
  const countRef = useRef(count);

  useEffect(() => {
    console.log(`The count is ${countRef.current}`);
    document.title = `Count: ${countRef.current}`;
  }, [countRef]);

  return (
    <div>
      <p>{`The count is ${count}`}</p>
      <p>{`The text is ${text}`}</p>
      <button onClick={() => setCount(count + 1)}>Increase count</button>
      <button onClick={() => setText('Hello again')}>Change text</button>
    </div>
  );
}
```

In this example, we're using a ref to store the value of `count` so that we can use it in the `useEffect` hook. Since the ref is not a state value, updating it will not trigger a re-render, and the effect will only re-run when the value of `count` actually changes.

It's important to note that while `useRef` can help you avoid unnecessary re-renders, it should be used carefully. Overusing `useRef` can lead to complex and hard-to-debug code, so it's usually best to use state variables when possible.