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What is StrictMode in React? How to Use it?

Last updated:
7th Jun, 2023
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What is StrictMode in React? How to Use it?

What Is StrictMode? 

React StrictMode is a JavaScript feature that allows a program to run in a “safe” mode where potential problems are detected and accordingly prevented. This includes making specific variables or objects read-only. It also prevents particular JavaScript functions such as eval() and arguments from being used for programming.

It is primarily used for developing data-driven applications and automating tasks, including databases. StrictMode streamlines installation and configuration making it easier to carry out tasks immediately.

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Read on to learn more about the uses of StrictMode.

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Who Can Use StrictMode?

StrictMode can be used by any user or developer who wants to improve the performance of an application by reducing issues such as network access, disk reads and writes and enforcing various rules related to threading.

Developers can use StrictMode in Android code to enforce rules such as limiting network access to the main thread and preventing disk reads and writes involving the main thread.

As it was recently introduced in JavaScript, the StrictMode isn’t an expression but a statement to be put forward in the program. The “use strict” command is used for directly putting out a statement instructing the program.

While using StrictMode, you cannot use undeclared variables. The command is given out to instruct the system to execute actions in StrictMode.

Note: StrictMode is used in almost all browsers except for Internet Explorer 9 and similar versions.

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How Does React’s Strict Mode Work?

React’s strict mode consists of tools and functions that detect and identify potential errors. It detects unsafe use of setState(), apart from identifying performance issues related to third-party libraries and tools. StrictMode implements the same with development-specific checks and warnings.

Here are the extensive features of StrictMode:

  • React’s Strict mode helps developers identify and fix issues in their code.
  • Strict mode runs in development mode and can be enabled by adding a component at the beginning of the application.
  • It checks for potential problems in the code, such as possible memory leaks, and warns the user about their presence.

Why You Should Use StrictMode

StrictMode is extremely easy to implement, making it ideal for projects of varied sizes and capacities. Here are the other reasons it is a perfect fit for React developments:

  • StrictMode catches potential bugs and minute code errors that would otherwise be missed.
  • StrictMode warns the developer of any unsafe code or non-strict practices.
  • It offers security from potential resource and memory leaks.
  • It can make developers aware of the state of their applications by mainly focusing on their performance.

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How To Use StrictMode in React?

StrictMode is essentially a React.js component that is used for detecting and fixing problems in any given code for deploying a flawless application. 

To use StrictMode in React, users can use the following code:

import React, { StrictMode } from 'react'; 
function App() { 
  return ( 
    <StrictMode> // components in here will follow strict mode rules 
        App component
export default App;

To wrap an entire programme using the StrictMode syntax, follow this:

import React from 'react';
import ReactDOM from 'react-dom';
const rootElement = document.getElementById('root');
    <MyApp />

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Component Stack Warnings 

React StrictMode detects and deploys warnings for different stacks. When an issue is detected, the StrictMode-enabled application prints out warnings in the development console.

On the counterpart, if a developer wishes to opt out of warnings triggered by the use of StrictMode, they can configure it using the following commands:


The warning trigger represents the type of warnings that are currently issued. Similarly, the fallback command defines the actions to be taken when the warning is triggered.

For instance, if users want to access performance-based warnings only, they can set the warningTrigger to “performance”. On the other hand, if they wish to forgo warnings, they can turn it off by setting the warningTrigger to “off”.

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Deprecation Warnings 

The depreciation warning represents an older and deprecated feature in Android and other currently unused applications. Upon receiving a deprecation warning, users can compare their code with the existing version and replace any deprecated elements.

Before looking at the depreciated codes, access the source code first by following these instructions:

// Set StrictMode options
StrictMode.ThreadPolicy threadPolicy = new StrictMode.ThreadPolicy.Builder().detectAll().penaltyLog().build();
StrictMode.VmPolicy vmPolicy = new StrictMode.VmPolicy.Builder().detectAll().penaltyLog().build(); 

After the developer has accessed the source code, they can identify the deprecated methods.

public void myOldMethod()
    myMethod();   // May generate a warning
// Non-deprecated example code
// Won't generate a warning
public void myNewMethod()
    myNewerMethod();   // Won't generate a warning

Avoiding the Test Cliffs 

When accessing test cliffs in StrictMode, it is important to ensure that all tests are scored and updated according to the latest usage. This can be accomplished by running tests routinely and running them in multiple environments.

A developer can use the following tips to avoid test cliffs in an application:

1) Pre-validate memory accesses: Validate memory access operations before running them. This is done by enabling security checks provided by validation libraries or tools.

2) Use appropriate compiler flags: Enable compiler flags such as ‘-Wall‘ and ‘-Werror‘ to generate compilation warnings. Fix the errors when found wherever possible.

3) Understand pointer arithmetic: Understand the implications of pointer arithmetic before checking the code.

4) Avoid implicit type conversions: Check argument types when noting pointer arguments. Additionally, think twice before performing operations with uninitialised pointers.

5) Optimise user input as much as possible: Consider user input and validate input parameters. These parameters help find buffer overflows, type errors, and data aborts.

6) Look for compiler warnings: Many compiler warnings can point out potential problems in a code that can help users avoid testing cliffs. This makes it even more important to pay attention to compiler warnings.

Detecting Unexpected Side-Effects

You can use Strict in React to instantly detect an application’s memory leaks, performance issues, and misused components. It also displays and triggers warnings for deprecated methods and intentional API mismatches between different environments.

Here are some examples of detecting potential errors:

  1. Logging errors for using deprecated props
const MyComponent = (props) => { 
  return React.StrictMode(
      <h1>My Component</h1>
      <MyChildComponent {...props.useDeprecatedProp} />
    onSetError: (error) => console.log(`Deprecated prop usage warning: ${error}`)
  1. Logging warnings for nested arrays 
const MyComponent = () => { 
  return React.StrictMode(
      <h1>My Component</h1>
    onSetWarning: (warning) => console.log(`Nested array warning: ${warning}`)

When detecting potential side effects in React, StrictMode does a brilliant job. 

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Detecting Legacy String Ref API Usage

React StrictMode logs a warning when it detects legacy string ref API usage. This warning is logged to the console when a component with a string ref is detected while a component is newly installed. 

A code snippet of a legacy string ref API is shown below:

const myRef = React.createRef();
// legacy string ref
const myStringRef = 'my-string-ref';
class MyComponent extends React.Component {
  render() {
    return <div ref={myStringRef}>My Component</div>;

If a user were to run this code with StrictMode enabled, React would issue a warning admitting that the legacy string ref API is being used. It would recommend that the user switch to a callback ref API instead.


StrictMode React is a handy feature that helps users and developers identify potential problems with their applications. It highlights potential risks in the code that can cause UI issues, unexpected crashes, and other related problems like memory and resource leaks. 

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Frequently Asked Questions



Pavan Vadapalli

Blog Author
Director of Engineering @ upGrad. Motivated to leverage technology to solve problems. Seasoned leader for startups and fast moving orgs. Working on solving problems of scale and long term technology strategy.

Frequently Asked Questions (FAQs)

1What are the limitations of using StrictMode in React?

The biggest limitation of StrictMode is that it is only applied in development and does not affect the application's behaviour in production. Also, certain issues may not be detected or displayed by StrictMode. This may cause the developer to use other methods, such as performance testing and code reviews, to identify undetected issues.

2What is React StrictMode, and is it optional in React?

StrictMode is a streamlining command that is highly efficient for use in React programs. It is an optional feature in React because it provides additional checks and warnings to identify potential errors. However, it is not necessarily required for React applications.

3What types of warnings are missed by StrictMode?

As efficient as the StrictMode command may be, it does not detect all possible problems within a React system. For example, StrictMode does not detect logic errors or memory leaks within a React application.