- ## **React 19 Deep Dive: Features, Migration Guide, and Performance Optimization Tips**

React has long been the backbone of modern web development, powering everything from small projects to enterprise-scale applications. With the release of React 19, developers gain access to a suite of new tools, improved performance, and simplified workflows that make building interactive UIs faster and more intuitive. Let’s dive into the highlights.

### 🚀 What’s New in React 19

React 19 introduces several developer-friendly features designed to streamline state management, server-side operations, and user interactions:
<div class="list-disc">

- **New Hooks**

    <span class="highlight-small">**useActionState →**</span> Simplifies handling state changes triggered by user actions.

    <span class="highlight-small">**useFormStatus →**</span> Provides real-time feedback on form submissions, making UX smoother.

    <span class="highlight-small"> **useOptimistic →**</span> Enables optimistic UI updates, so users see instant changes while data is still processing.


- **Server Components & Server Actions**

These allow developers to offload heavy logic to the server, reducing client-side complexity and improving performance.


- **React Compiler (Experimental)**

Automatically optimizes React code into efficient JavaScript, eliminating the need for manual memoization in many cases.


- **Improved Suspense & Hydration**

Faster pre-warming for suspended trees and better hydration strategies mean smoother page loads and fewer flickers.


- **Static DOM APIs**

New APIs for manipulating DOM elements more efficiently, especially useful for large-scale apps.
</div> 

Upgrade React and React DOM to version 19 via npm or yarn.

### 🔄 Migration Tips: Moving from React 18 to 19


Migrating to React 19 is straightforward but requires attention to detail. Here’s a step-by-step guide:

<div class="list-disc">

- Update Dependencies

Upgrade React and React DOM to version 19 via npm or yarn.

```javascript

npm install react@19 react-dom@19

```

- Set Up Server Actions & Components

Replace client-heavy state logic with server actions where possible. This reduces bundle size and improves scalability.

- Update Routing & Suspense

Ensure your app’s routing integrates with the improved Suspense features for better hydration.

- Refactor State Management

Use the new hooks <span class="highlight-small">(useActionState, useFormStatus, useOptimistic) </span> to simplify form handling and user interactions.

- Test for Breaking Changes

Review the official upgrade guide for deprecated APIs or altered behaviors.

</div>

### ⚡ Performance Tricks in React 19

React 19 isn’t just about new features—it’s about speed. Here are some performance optimization strategies:
<div class="list-disc">

- Leverage the React Compiler

Stop overusing React.memo. The compiler automatically optimizes components, reducing unnecessary re-renders.

- Use Server Components Wisely

Offload expensive computations to the server. This keeps client bundles lean and improves load times.

- Optimistic UI Updates

With useOptimistic, users see immediate feedback, reducing perceived latency.
- Smarter Suspense

Pre-warming suspended trees ensures smoother transitions between routes and components.

- Hydration Improvements
</div>
Faster hydration means better performance for apps with heavy server-side rendering.

### 📈 Why Upgrade to React 19?
<div class="list-disc">

- Better Developer Experience → Simplified hooks and APIs reduce boilerplate.
- Improved User Experience → Faster hydration and optimistic updates make apps feel snappier.
- Future-Proofing → Server Components and the React Compiler are paving the way for the next generation of React apps.
</div>

### 🎯 Conclusion

React 19 is more than just an upgrade—it’s a leap forward in how developers build modern web applications. With new hooks, server-side capabilities, and performance optimizations, migrating from React 18 is worth the effort. Whether you’re running a small project or scaling enterprise apps, React 19 ensures your UI is faster, smarter, and future-ready.
Upgrade now, experiment with the new features, and let React 19 power your next big idea.


Discover React 19’s new features, migration tips from React 18, and performance tricks to build faster, smarter web apps. Learn how to optimize your projects with the latest React release.

React 19 Deep Dive: Features, Migration Guide, and Performance Optimization Tips


Whenever creating any kind of application, you must think about its performance. Nowadays, performance is the biggest factor in applications. Consider if you have a competitive application, then if you lose the performance the users will go for the alternative application which leads you to the biggest loss.

When you start to create an application at the time you have to start to think about the performance as well. If you think later, then it will be a mess for you, because the performance factor will come to place when you are adding more facility to the application. When we add the features to the application gradually, the bundle size will also increase, and the browser needs to download the whole required files from the bundles.

We know the deployment bundle has all the files, so if you try to download all the files, that might face performance problems. So, here we can have one concept, that's called <span class="highlight-small">Lazy loader</span> or <span class="highlight-small">Code-splitting</span>.

<div class="highlight">

## What is ReactJS Lazy loading?

A lazy function lets you defer the loading of a component's code until it is rendered for the first time. Before, it will remain in the bundle. So that we can reduce the load of the application.

</div>

What is the syntax for lazy loading?

```javascript
import { lazy } from "react";

const componentName = lazy(load);
```

The function `lazy` is derived from react package and the lazy function has the only parameter that is our component. Once passed as a parameter to the function, then it will return as a new component that will be stored in the variable and we can use it accordingly.

E.g:-

```javascript
import { lazy } from "react";

const About = lazy(() => import("./Aboutus"));
```

The above example is doing the <span class="highlight-small">dynamic import()</span> of a component using lazy function.

Along with the lazy function we can use another component, i.e.; <span class="highlight-small">`React Suspense`</span>. This function will specify while the component is loading, look at the below picture to clear the confusion.

![How to do lazy loading in ReactJS](/assets/blog/helpers/lazyloading-gif.gif)

The above picture shows loading text while moving to the next item, but it is shown only once, because the component is downloaded from the bundle and does not require further loading of the same component. The loading text is coming from `Suspense` component.

```javascript
import React, { Suspense, lazy } from "react";

import { BrowserRouter as Router, Routes, Route, Link } from "react-router-dom";

const About = lazy(() => import("./Aboutus"));

const HomeComp = lazy(() => import("./Home"));

const Contact = lazy(() => import("./contactus"));

function App() {
  return (
    <>
      <Router>
        <Suspense fallback={<div className="loading">Loading...</div>}>
          <Routes>
            <Route path="/" element={<Layout />}>
              <Route path="/" element={<HomeComp />} />

              <Route path="/about" element={<About />} />

              <Route path="/Contact" element={<Contact />} />
            </Route>
          </Routes>
        </Suspense>
      </Router>
    </>
  );
}
```

The above code is the abstract of the react lazy loader, please find the GitHub link for your references.

<a rel="nofollow" href="https://github.com/ullas1000/react-lazy-loader">React Lazy loader</a>

Thank you. <span>&#128526; <span>


A lazy function lets you defer the loading of a components code until it is rendered for the first time. Before, it will remain in the bundle. So that we can reduce the load of the application. 

How to do lazy loading in ReactJS

- ## **Writing test cases for modal popup in Jest**
Writing the test cases and asserting are very easy right after mounting the component. But some cases we will not recieve any result after mounting the component. that means mounted component will return null.

this only the reason I thought to share some steps to resolve the problem if anybody facing this already.

E.g.:- Modal popups, bootstrap modal and modal window

The modal winow or modal popup will rendered the top layer of the entire view or out of the body. So the Modal popup won't be captured at the time of mount operation. To capture the Modal popup we required additional settings before mounting the Modal component.

settings are nothing but mock function we have to write it.

I have demostrated the below jest mock function for the modal popup

```javascript
import React from 'react'
import { mount } from 'enzymme'
import { MemoryRouter } from 'react-router'
import SampleComponent from 'samplecomponent' // contains Modal component

describe('Test SampleComponent', () => {
    it('should load the Component', () => {
        const elemment = mount(<SampleComponent />)
        expect(element.html()).not.toBeNull()
    })
})
```

in the above code the variable *element* will not recieve the *Modal* component, eventually the test case will fail.

**How to fix the error then ?**

To fix the test case we have to add the mock function for Modal to the component. Given example in the below code snippet.

```javascript
import React from 'react'
import { mount } from 'enzymme'
import { MemoryRouter } from 'react-router'
import SampleComponent from 'samplecomponent' // contains Modal component

jest.mock('component library were the Modal is imported', ()=>{
    function Modal({children}) {
       return <div>{children}</div>
    }
    Modal.Header = 'Modal.Header'
    Modal.Body = 'Modal.Body'
    Modal.Footer = 'Modal.Footer'

    return Modal
})

describe('Test SampleComponent', () => {
    it('should load the Component', () => {
        const elemment = mount(<SampleComponent />)
        expect(element.html()).not.toBeNull()
    })
})
```

Hope this solution helped for your test case of bootstrap modal or other modal window pluggins


Writing test cases for modal popup in jest

Writing test cases for modal popup in Jest

- ## **Uncaught TypeError: Cannot read property 'location' of undefined**
It is a common error when you executing the test cases for useLocation in Jest. It is not like any other hooks since it is working with the router.(```'react-router-dom'```) it requres some extra input before running the test cases.

# **what is useLocation ?**

This isbone react hook, this hook will returns the current location object. This can be useful if you'd like to perform some side effect whenever the current location changes.

```javascript
import { useLocation } from 'react-router-dom'
```
Look at to the below sample test case snippet.

let consider `useLocation` has been used in the component `SampleComponent` and the below test case is for testing the component using mount function of enzyme

```javascript
import React from 'react'
import { mount } from 'enzymme'
import SampleComponent from 'samplecomponent'

describe('Test SampleComponent', () => {
    it('should load the Component', () => {
        const elemment = mount(<SampleComponent />)
        expect(element.html()).not.toBeNull()
    })
})
```
the result of the above code will be failed and says **Uncaught TypeError: Cannot read property 'location' of undefined**

because the program have used useLocation and when writing test case for that component also need to provide the initial value

lets solve the issue by a simple step, that nothing but wrap the compoent with `MemoryRouter`.
```javascript
import { MemoryRouter } from 'react-router'
```

then the program would be like this.

```javascript
import React from 'react'
import { mount } from 'enzymme'
import { MemoryRouter } from 'react-router'
import SampleComponent from 'samplecomponent' // the component contains useLocation hook

describe('Test SampleComponent', () => {
    it('should load the Component', () => {
        const elemment = mount(
            <MemoryRouter initialEntries={['/']}>
                <SampleComponent />
            </MemoryRouter>
        )
        expect(element.html()).not.toBeNull()
    })
})
```
MemoryRouter is a wrapper function which will help us to provide the correct input to the useLocation hook of the component.





Cannot read property location of undefined, this is an common test cases error in react jest while using useLocation hook in your react component

[Solved] Uncaught TypeError: Cannot read property 'location' of undefined


## **What is Markdown?**

Markdown is a lightweight markup language that you can use to add formatting elements to plaintext text documents. Created by John Gruber in 2004, Markdown is now one of the world’s most popular markup languages. ".MD" is the file extension.

**What are the advantages of Markdown file.**

- **1.** Markdown is easy to read and write.</br>
- **2.** Writers and publishers love writing with Markdown as it reduces distractions.</br>
- **3.** It's intentionally simple.</br>
- **4.** Lets you focus on important things and increases productivity.</br>
- **5.** Converts plain text into HTML.</br>
- **6.** Multi-platform.</br>
- **7.** It doesn't break.
- **8.** Online markdown editors also available so easy to create it.

There are lot of markdown editors are available, if you are an expert then notepad is enough to write the contents on markdown file.

## **How to read/load the markdown aka .md files in ReactJS**

We going to use the NPM package to read the markdown or .md file. Currently many NPM pacakges are available to read the .md file, among that "marked" package we imported.

**Step 1.**

Initialize the project

```javascript
npx create-react-app markdownsample
```

**Step 2.**

Add the package

```javascript
npm i marked
```

**Step 3.**

create a .md file and name it readme.md or anything as your wish under the src folder.

**Step 4.**

Edit the App.js, were we added the logics

```javascript
import React, { useEffect, useState } from "react";
import readmePath from "./README.md";
import { marked } from "marked";
function App() {
  const [data, setData] = useState("");
  useEffect(() => {
    fetch(readmePath)
      .then((response) => {
        return response.text();
      })
      .then((text) => {
        setData(marked(text));
      });
  }, []);
  return (
    <React.Fragment>
      <article dangerouslySetInnerHTML={{ __html: data }} />
    </React.Fragment>
  );
}

export default App;
```

Here we imported the readme.md file and read using fetch function.
fetch function read the file into text. Once it is responded as text, it is passed as an argument to the "marked" function and directly set the result to state variable.

Finally state variable rendered to the article html element.

then the result would be like this in the below screenshot

![markdown file result](/assets/blog/helpers/markdown-result.JPG)

**Conclusion**

The above program snippet is helping to read and render the markdown or .md file.
Also you can fork the project from the given [Github link](https://github.com/ullas1000/markdownsample).


 Markdown is a lightweight markup language that you can use to add formatting elements to plaintext text documents. 

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