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How React Strict Mode Works For Great Development

React Strict Mode helps improve a React project’s development process by identifying and addressing common pitfalls and potential issues.

If you want to understand how React strict mode works, you’ve come to the right place.

In this article, we will cover all you need to know about React Strict Mode, its benefits, and best practices to ensure your project goes well. Let’s dive in!

Brief Overview of React

React is a JavaScript library for building user interfaces (UIs). It encourages a modular, component-based approach to UI development.

React’s main features include a virtual DOM for efficient updates, JSX for declarative user interface creation, state management, lifecycle methods, and React Hooks. 

It is widely used in front-end development due to its flexibility and active community support.

Understanding Strict Mode in React

React introduced the Strict Mode to help developers catch and fix potential problems in their React applications. 

Strict Mode is not intended for production use; instead, it is a development-only feature. When enabled, it performs additional checks and warnings during the rendering process. This helps developers identify common mistakes and potential issues early in the development phase.

React’s Commitment to Backward Compatibility

React is known for its commitment to backward compatibility, ensuring that new versions of the library do not break existing applications. 

This commitment allows developers to update their applications to newer versions of React without fear of disruptions. 

Strict Mode aligns with this commitment by providing a non-intrusive way for developers to improve their code and catch potential issues without affecting the production environment.

What is React Strict Mode?

React Strict Mode is a development feature that helps to identify potential problems in React applications. It does this by performing additional checks and providing warnings during the development build. 

The React Strict Mode contributes to writing safer and more reliable code by conducting additional checks and providing warnings during the app development build.

It identifies potential issues, such as deprecated lifecycles, legacy string refs, and unintended side effects. It also alerts developers to areas that may lead to bugs or unpredictable behavior.

The Scope of Strict Mode

The scope of React Strict Mode is limited to the development build of a React application and does not impact the production phase. 

Wrapping your components or app with the <StrictMode> tag only activates additional checks, warnings, and debugging assistance during the development phase. These checks help you catch potential issues early and improve code quality. 

However, in the production phase, Strict Mode has no impact. It doesn’t affect the behavior of your app for end users. Remember, it’s a powerful tool for development, but it won’t affect your live application!

Benefits of Using Strict Mode

Strict Mode helps to catch components with deprecated or unsafe lifecycles. This gives developers a heads-up to make necessary adjustments for future-proofing their codebase.

Using Strict Mode allows developers to receive warnings about outdated practices, such as using the legacy string ref API. It encourages them to adopt modern approaches for better maintainability.

Strict Mode helps to detect unintended side effects that can be tricky to identify. It does this by pinpointing these side effects during development, reducing the chances of subtle bugs slipping through the cracks.

To keep up with React’s advancements, Strict Mode ensures that developers are aware of any legacy context API usage, promoting the adoption of newer, more efficient alternatives.

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Implementing React Strict Mode

Now that we understand the importance of Strict Mode, let’s explore how to implement it effectively in your application.

In just a few steps, using an example, we will give you a detailed tour of how you can wrap your application or components in strict mode.

Navigate to your project directory and open it in your code editor. If you don’t have a React app setup for your project, please see the video below for steps to help you get started.

In your src folder, open your index.js file (or the entry point of your application). After that, you’ll need to import StrictMode from React and wrap your entire application or the specific component you want to include in Strict Mode. 

Here, we’ll wrap the entire application in StrictMode:

// src/index.js
import React, { StrictMode } from "react"; // Import React and StrictMode
import ReactDOM from "react-dom";
import App from "./App"; // Your root component

const rootElement = document.getElementById("root");

    <App />

The code above sets up a React application by importing necessary libraries, including the root component (App), and uses React Strict Mode for enhanced development checks. 

The ReactDOM.render function renders the App component, wraps it with the StrictMode tag, and mounts it to the specified HTML element with the id “root”.

Implementing Strict Mode on a Specific Component

Next, we will create a simple component called MyComponent.jsx in the src folder. In the MyComponent.js file insert the code below:

// MyComponent.jsx
import React from "react";

const MyComponent = () => {
  // Some component logic
  return <div>This is MyComponent</div>;

export default MyComponent;

Within the src folder, navigate to the App.js file and open it. If you only want to apply Strict Mode to a specific component, you can wrap it directly. See the code below for more reference.

import React, { StrictMode } from "react"; // Import React and StrictMode
import MyComponent from "./MyComponent";

const App = () => {
  return (
      <h1>Hello React!</h1>
        <MyComponent />

export default App;

In this example, the MyComponent is imported and rendered within the App component. We then wrap the new component with StrictMode.

Running Your Dev Server

Save your files and run your React application. To start your application, run the npm start or yarn start command in your command prompt.

Your React app should open in your browser automatically but if it doesn’t, you can access it at ‘http://localhost:3000/’.

Since the Strict Mode only affects the development build and not the production build, open your browser’s developer tools, navigate to the Console tab, and observe any warnings related to Strict Mode. These might include information about deprecated lifecycles, string refs, or unexpected side effects.

Address these warnings by updating your code. For example, if there are warnings about deprecated lifecycles, update your component’s lifecycle hooks to use the latest methods.

Using Strict Mode in Class-based Components

In the example above, we showed you how to implement Strict Mode on function-based components. Now, let’s see how to implement it in a Class-based component.

Here are some practical examples of class-based components using React Strict Mode:

Class Component: Create a new file in your src folder. For example, ClassComponent.jsx and initialize your class component. Here is an example implementation.

import React, { Component } from "react";

class MyClassComponent extends Component {
  constructor(props) {
    // Your component logic here

  render() {
    return (
        <h1>Class Component with Strict Mode</h1>
         <p>This is a class component with some content.</p>

export default MyClassComponent;

Now that we have successfully created the class component, the next step is to import it into the App.js file and wrap it with the Strict Mode tag.

import React, { StrictMode } from "react";
import MyClassComponent from "./ClassComponent";

function App() {
  return (
        <MyClassComponent />

export default App;

Running the Dev Server

Save all the files, return to the terminal window, and run the npm start command to start the development server.

Your application will look like this in the production and developement phase.

With this, you can see that Strict mode does not cause any changes in the behavior of your components in the Production build.

It is important to note that the primary purpose of the React Strict Mode is to help identify and address potential issues during the development phase. 

Strict Mode introduces additional checks and warnings to the console that can catch common mistakes, unsafe actions, and deprecated features. However, these checks do not alter the behavior of your components in a production environment.

Accessing Brower’s Dev Tools

To check for errors in the above example, open your browser’s developer tools and navigate to the “Console” tab. If there are any issues or warnings related to strict mode, they will be displayed there.

Note: To access your developer menu, Right-click on any portion of your Project page and left-click on Inspect. You will see a list of developer tools. Then click on Console to open it.

Class-based Components vs Function-based Component

Below is a table differentiating function-based components from class-based components

FeaturesFunction-Based ComponentsClass-Based Components
SyntaxDefined as JavaScript functions.Defined as ES6 classes.
State ManagementUses Hooks (e.g., useState, useEffect).Uses this.state and this.setState.
Lifecycle MethodsUse Hooks (e.g., useEffect).Use lifecycle methods (e.g., componentDidMount, componentDidUpdate).
Use of Lifecycle MethodsNo traditional lifecycle methodsCan use traditional lifecycle methods
Access to PropsPassed as function arguments.Accessed via this.props.
ReadabilityShorter and more concise syntaxLonger syntax with class definition
Compatibility with HooksFully compatibleNot fully compatible, needs additional setup
Code StructureEasier to read and understandCan be more complex and verbose
Code ReusabilitySupports custom hooks for reusabilityExtends other components for reusability
Context APIYou can use the useContext hook.Can use contextType or static contextType.

Remember that function-based components are now preferred in React projects due to their simplicity, readability, and better performance. However, class-based components are still widely used, especially in legacy codebases. Choose the one that best fits your project and team preferences.

Best Practices

Below are some best practices for Using Strict Mode in a React Application.

When and Where To Use Strict Mode in Your Application

Development Environment: We advise developers to always use strict mode in the development environment. Doing so helps them catch common mistakes and potential issues early in their app development process.

Integrated Testing: Utilize Strict Mode alongside unit and integration tests. Running tests with strict mode enabled can help identify issues in a controlled testing environment.

Adopt Early and Stay Updated: React Strict Mode ensures you stay informed about React updates and deprecations. Adopt strict mode and address warnings early to ensure a smoother transition when upgrading React versions.

Enable at the Top Level: Wrap your entire application or the root component with <React.StrictMode> to apply the Strict Mode checks throughout the component tree.

Consistent Development Setup: We encourage you to use React Strict Mode across the development team. Ensure that all developers use strict mode consistently to maintain a standardized development setup.

Integrating Strict Mode in a Legacy Application

Integrating Strict Mode into a legacy React application involves a gradual and systematic approach. You’ll first need to access the codebase and selectively enable Strict Mode in well-maintained sections. 

Thorough testing, regular console checks for warnings, and prompt issue resolution contribute to maintaining a clean and robust codebase.

Effective documentation and communication within the development team are crucial as developers modernize the code by addressing deprecated features and refactoring components. 

Utilizing version control, updating dependencies, and following best practices contribute to successful integration, aligning the legacy React code with modern React standards.

Tips for Addressing and Fixing Issues Highlighted by Strict Mode

Here are some tips for addressing and fixing issues highlighted by the React Strict Mode.

Check Console Warnings

Each Strict Mode warning points to a specific potential problem. Read the message carefully to understand its meaning and why it’s flagged. Don’t just dismiss it – it’s there to help you write better code.

Research and Analyze

Dive deeper! Google the warning message, refer to React documentation, or seek help from online communities. Understanding the root cause is key to a lasting solution.

Fix the Code

Once you understand the issue, it’s time to take action. The solution might involve:

  • Refactoring code: Rewrite code sections to comply with modern React practices and eliminate the warning.

  • Updating libraries: Check if outdated libraries are causing the issue and update them if possible. 

Test Thoroughly

After any fix, test your component thoroughly to ensure the warning is gone and the behavior remains correct. You can consider writing unit tests specifically targeting the fixed area.

Consider Scope

If a warning only affects a specific component, we recommend you wrap that component selectively with <StrictMode> instead of applying it globally. This helps you target the issue and reduce unnecessary performance impact.

Utilize Hooks

You should consider refactoring class components to functional components with hooks. Using hooks provides a more modern and concise way of managing state and side effects.

Continuous Learning

Remember, Strict Mode is a learning tool. Use the warnings as opportunities to grow your understanding of React best practices and write cleaner, more maintainable code in the future.

If you follow these tips and approach Strict Mode with a curious and proactive mindset, you’ll transform those warnings into stepping stones for building more robust React applications.

Frequently Asked Questions

These are answers to some of the questions concerning React Strict Mode:

What Issues Does React Strict Mode Help Identify?

Strict Mode helps identify components with unsafe lifecycles, warns about using the legacy string ref API, detects unexpected side effects, and pinpoints legacy context API usage.

Does React Strict Mode Affect Performance?

No, Strict Mode does not affect the performance of your application in the production environment. 

Is React Strict Mode on by default?

No, React Strict Mode is not on by default. It is an opt-in feature that developers need to explicitly enable in their applications by wrapping their components or the entire application with <React.StrictMode>. React does not enable Strict Mode automatically because it is a development tool designed to highlight potential issues in an application, such as side effects in the render phase, use of deprecated APIs, and other common problems that could lead to unstable UIs or performance issues.

Should I Use Strict Mode in Production?

No, the React Strict Mode is designed for development use only. It should be disabled in production to avoid unnecessary overhead.

How Do I Resolve Warnings Thrown by Strict Mode?

To resolve warnings thrown by the Strict Mode, you will need to review each warning carefully, refer to React’s documentation for guidance, and make the necessary code adjustments to address the issues raised.

Can I disable React Strict Mode?

Yes, you can disable Strict Mode. React Strict Mode is an optional tool for highlighting potential problems in an application, and it’s wrapped around your components during development to help identify issues. If you find that Strict Mode is causing confusion or you prefer to not use it for any reason, you can simply remove the <React.StrictMode> wrapper from your application’s entry point.

Is React Strict Mode Compatible With All Versions of React?

Yes, the React Strict Mode is compatible with all versions of React, ensuring developers can leverage its benefits regardless of their project’s setup.

Why Does useEffect Run Twice in Strict Mode?

When using React’s useEffect hook in strict mode, you might observe that it runs twice instead of once as expected. This behavior is intentional and is part of React’s strategy to help developers catch side effects that are not properly managed within their components.

Final Thoughts on Using React Strict Mode

The answer is a resounding YES! Using React Strict Mode in your development workflow ensures a proactive approach to identifying and addressing potential bugs. This, in turn, leads to a more robust and reliable application.

React Strict Mode is a valuable tool for every React developer. It contributes to creating high-quality, bug-free applications by providing early warnings and checks. 

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For Further Reading:

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David Ozokoye

David Ozokoye

Software Engineer and Technical Writer