Why does my React useEffect hook run multiple times even with an empty dependency array?

The Question
While working on a React project, I encountered an issue with the useEffect hook. My goal was to fetch data from an API only once when the component mounts. However, the useEffect kept running multiple times, even though I provided an empty dependency array.

Here’s the code snippet:

import React, { useEffect, useState } from "react";
import axios from "axios";

const MyComponent = () => {
  const [data, setData] = useState([]);

  useEffect(() => {
    console.log("Fetching data...");
    axios.get("https://jsonplaceholder.typicode.com/posts")
      .then(response => setData(response.data))
      .catch(error => console.error(error));
  }, []);
  return (
    <div>
      <h1>Data</h1>
      <ul>
        {data.map(item => (
          <li key={item.id}>{item.name}</li>
        ))}
      </ul>
    </div>
  );
};
export default MyComponent;

Despite the empty dependency array ([]), the useEffect was executed multiple times. I tried restarting the development server, but the issue persisted. After some research and troubleshooting, I identified the root cause and resolved it.
The Answer

Why This Happens

Strict Mode in Development:

If your app is running in React's development mode with StrictMode enabled, React intentionally mounts and unmounts components multiple times. This is a development-only behavior meant to detect side effects that may cause issues.

Re-renders or Hot Module Replacement (HMR):

During development, changes in the code may trigger Hot Module Replacement, causing the component to re-render and useEffect to execute again.

How to Fix or Handle This Behavior

Identify Strict Mode:

If you're using StrictMode, understand that this behavior happens only in development and won’t affect the production build. You can temporarily disable it by removing

import React from "react";
import ReactDOM from "react-dom";
import App from "./App";
ReactDOM.render(<App />, document.getElementById("root"));  

However,it’s better to leave it enabled and adapt your code to handle potential side effects gracefully.

Prevent Duplicate API Calls:

Use a flag to ensure that the API call happens only once during the component’s lifecycle, even

import React, { useEffect, useState, useRef } from "react";
import axios from "axios";

const MyComponent = () => {
  const [data, setData] = useState([]);
  const isFetched = useRef(false);

  useEffect(() => {
    if (isFetched.current) return;

    console.log("Fetching data...");
    axios.get("https://api.example.com/data")
      .then(response => setData(response.data))
      .catch(error => console.error(error));

    isFetched.current = true;
  }, []);

  return (
    <div>
      <h1>Data</h1>
      <ul>
        {data.map(item => (
          <li key={item.id}>{item.name}</li>
        ))}
      </ul>
    </div>
  );
};
export default MyComponent;

Using a useRef ensures that the API call happens only once, regardless of the additional renders caused by StrictMode.

Key Takeaways
. React’s Strict Mode in development is intentional and safe to leave on.
. Production builds won’t have this issue. . Use useRef or other techniques to manage side effects when necessary.
Use useRef or other techniques to manage side effects when necessary.
Production builds won’t have this issue.
Use useRef or other techniques to manage side effects when necessary.