Understanding JSX Syntax

Anton Ioffe - August 31st 2023 - 16 minutes read

Introduction to JSX

JSX, short for Javascript eXtensible Markup Language, is a syntax extension integrated within a Javascript file. It has the ability to code a document in a format which is easily understandable by humans and can be interpreted by machines. A valuable characteristic of JSX is that it facilitates developers to build HTML structures within Javascript code, enhancing comprehension of the code.

JSX was initially developed as part of a library and rapidly gained popularity due to its clarity and effective UI generation ability. With JSX, developers can conveniently create and define elements in their codes. Here is a simple example that demonstrates the use of JSX in constructing HTML structures:

function createDirectory(blogPost){
  return <li>{blogPost.title}</li>;

The use of curling braces {} in the above example is a prominent JSX-specific feature. It allows the inclusion of Javascript expressions within the jsx, which eases the understanding of the code.

A common mistake while working with JSX is not enclosing multi-line JSX expressions within parentheses. Let's understand this with an example:

// Incorrect
function createDirectory(blogPost){

// Correct
function createDirectory(blogPost){
  return (

In the incorrect version, as the return statement is on a new line, the function returns undefined. This issue can be tackled by enclosing multi-line JSX expressions within parentheses.

In conclusion, JSX plays a critical role in making code more accessible and manageable, however, its effective use also necessitates awareness of the common syntax errors.

Understanding JSX syntax

JSX, or JavaScript XML, is an extension to the JavaScript language syntax. It closely resembles HTML or XML syntax and is used within React to declare elements and components in an intuitive and visually clear way. Let's dive deeper into its structure and common uses.

Component parts

Every JSX expression is defined by its tags, either an opening and closing pair <>...</> for non-self-closing tags or a self-closing tag such as <MyComponent /> which does not require a closing tag, similar to img or br tags in HTML.

The content within these tags is known as 'children' and can consist of more JSX, strings or numbers, and can be dynamically structured using {} to denote JavaScript expressions.

Practical Explanation

Consider the following example:

const myElement = (
    <h1>Hello, world!</h1>
    {'I love ' + 'JSX.'}

function renderElement() {
  ReactDOM.render(myElement, document.getElementById('root'));

This is a standard JSX syntax where myElement is a JSX variable that contains a div element with child elements h1 and a string concatenated using JavaScript within {}. ReactDOM.render is used to render this JSX onto an HTML with id 'root'.

Common Syntax mistakes

One of the common mistakes in JSX syntax is not closing tags properly, believing that it follows the JavaScript object notation. For instance:

// Incorrect JSX syntax
const myElement = <div className='myDiv'; // Missing closing tag and incorrect attribute assignment

// Correct JSX syntax
const myElement = <div className='myDiv'>; // Correct JSX syntax with closing tag and attribute assignment


Compared to vanilla JavaScript, JSX simplifies creating complex HTML structures with JavaScript. Regular JS, for instance, would require nested document.createElement and element.appendChild calls. This can make the code harder to read and maintain, which is not the case with JSX as it resembles the familiar HTML syntax.

To summarize, JSX is an essential part of the React ecosystem, allowing developers to write declarative and visually clear component structures. Making full use of its ability to embed JavaScript in {}, one can create highly dynamic components. However, it's crucial to be aware of its syntax rules and common pitfalls to ensure an error-free development experience. Be aware of correctly closing tags and embedding JavaScript expressions within {}. With practice, JSX will undoubtedly become a powerful tool in your React development arsenal.

Distinction between JSX and Other Technologies

We find JSX at the convergence of JS, HTML, and React components. The amalgamation of the benefits of these three core technologies makes JSX a powerful tool in any developers' toolbox, especially when creating applications with React. Let's discuss the distinctions between JSX and the aforementioned technologies.


When comparing JSX to JavaScript (JS), the most notable difference lies in syntax. JS follows a more traditional programming syntax, whereas JSX appears closer to HTML - let's take a look at some examples:

// JS Syntax
var divElement = document.createElement('div');
divElement.textContent = 'Hello World!';

// JSX Syntax
const element = <div>Hello World!</div>;
ReactDOM.render(element, document.getElementById('app'));

While these snippets essentially do the same thing (create a div with text 'Hello World!'), the JSX version is significantly cleaner and easier to read. A common mistake is to assume that JSX is simply JavaScript. Therefore, idiomatic JavaScript constructs, such as if-else statements or for-loops, do not work within JSX.


While JSX and HTML look strikingly similar at first glance, it's important to note they are not the same. Key distinctions include the use of className in JSX instead of class in HTML, due to class being a reserved keyword in JavaScript - this is a common trip-up for developers shifting from HTML to JSX.

Furthermore, JSX employs camelCase property naming conventions, such as onClick or tabIndex, unlike the lowercase naming convention of HTML attributes like onclick or tabindex.

Let's compare a simple element creation:

<!-- HTML Syntax -->
<div class="greet">Hello, world!</div>

// JSX Syntax
<div className="greet">Hello, world!</div>

JSX vs React Components

JSX forms the backbone of React's component structure, enabling users to create dynamic user interfaces with ease. However, JSX and React components should not be misconstrued as identical.

React components are JavaScript classes or functions, encapsulating logic and rendering UI using JSX. JSX is merely a syntax extension, allowing for the composition of these components.

Consider this simple component creation:

// JSX in a React Component
function Greeting() {
  return <h1>Hello, world!</h1>;

ReactDOM.render(<Greeting />, document.getElementById('app'));

In this scenario, Greeting is a functional React component returning JSX that is rendered to the DOM.

Understanding the demarcation between these technologies is crucial, since it's easy to blur the lines between JSX vs JS or HTML, or treat JSX as interchangeable with React components.

Knowing the fundamental distinctions not only enhances your development skills but also boosts your problem solving capabilities when working with libraries like React. It's equally imperative to avoid these common pitfalls to stay productive as a developer.

The working process of JSX

JSX (JavaScript XML) is a syntax extension for JavaScript. It enables us to compose HTML-like syntax that later transforms into regular JavaScript. However, browsers do not inherently comprehend this.

Babel, a JavaScript compiler, is where we find our solution. Babel transpiles your JSX code into ES5, a version of JavaScript that all browsers can process and understand.

To illustrate the pivotal role of Babel, let's look at a simple theoretical example:

Consider this straightforward JSX code:

const element = <h1>'Hello, world!'</h1>;

The Babel transpiler turns this JSX code into:

const element = React.createElement(
  'Hello, world!'

This transformation from Babel enables your JSX code to be recognized and executed by browsers. Without Babel, such a task would not be possible, leading us to a common coding mistake.

Common Mistakes in Using JSX

An issue commonly encountered when working with React is the attempt to execute JSX code without first transpiling it with Babel. This results in errors like "Uncaught SyntaxError: Unexpected token <" as JSX code is incomprehensible to browsers when interpreted directly. Always remember, it's paramount to employ a transpiler, like Babel, to guarantee successful interpretation and execution of your JSX code.

You'll see the effectiveness of the transpilation process directly when inspecting the transformed code. Does the transformed code look like it will function as desired? This level of review post-transpilation is a habit every developer should cultivate.

Knowing how Babel interprets your JSX code allows you to anticipate potential issues and write more efficient, effective code. Always remember that while JSX improves code readability and DOM manipulation, it can't be interpreted by browsers without Babel's input. Previewing and understanding the transformed code post-transpilation is a key part of the coding process.

Practical Usage of JSX

JSX, the Javascript XML, is a syntax extension for JavaScript. It's like a chisel that has been added to the carpentry workshop of JavaScript, allowing developers to structure component rendering using HTML-like syntax, which is familiar to many. In this section, we will dive deep into the practical uses of JSX, from looping and mapping over components, rendering JavaScript variables, running JavaScript functions and scheduling tasks, and even optimizing performance by assigning appropriate 'keys' to JSX elements.

Let's jump right into the core of it and begin with the first practical use case of JSX — looping over elements.

Looping Through Elements in JSX

One of the most common and useful ways to use JSX is for mapping an array to a new array of JSX elements with JavaScript's map() function. Simply iterate over each element in an array and return a new array with JSX syntax. Let's have a look at it:

import React from 'react';
import ReactDOM from 'react-dom';

let itemList = ['One', 'Two', 'Three', 'Four', 'Five'];

let listItem = itemList.map((item, index) => (
  <li key={index}>{item}</li>


Using Variables in JSX

We're in JavaScript land. So, naturally, we can use JavaScript variables within our JSX! All it takes is wrapping them in {} and we're good to go. Let's give it a try:

import React from 'react';
import ReactDOM from 'react-dom';

let name = 'John Doe';

  <div>Hello, {name}</div>,

This ReactDOM.render() call will render a <div> element onto the 'root' div element with the text 'Hello, John Doe'.

Rendering Functions in JSX

Managing code modularity and reusability can be crucial in building effective applications. JSX also allows us to run and render the output of functions directly in our JSX syntax. But be aware of common mistakes, such as passing a function call instead of the function itself. Let's see what's right and what's not:

import React from 'react';
import ReactDOM from 'react-dom';

const greeting = (name) => (
  <h1>Hello, {name}!</h1>

  greeting('John Doe'),

// Below is a common mistake that you should avoid.
// Here, greeting() will be called immediately, NOT when we expect!

Classes and JSX

We all love classes! But since class is a reserved keyword in JavaScript, JSX makes use of the className attribute to add CSS classes to our elements. We can set it up like so:

import React from 'react';
import ReactDOM from 'react-dom';

let element = <div className='myClass'>Hello world!</div>;


Common Pitfalls

It's easy to stumble over some common pitfalls when working with JSX. Let's take a look at some of them:

  1. Forgetting to provide the key prop when working with loops can lead to potential performance issues and warnings on the console.
import React from 'react';
import ReactDOM from 'react-dom';

let itemList = ['One', 'Two', 'Three', 'Four', 'Five'];

// We are missing the key prop in the map function
// Watch out and avoid doing this!
let listItem = itemList.map((item) => (

  1. When using variables in JSX, remember to put them in curl braces {}, not in quotes. Otherwise, JSX will interpret the variable name as a string.
import React from 'react';
import ReactDOM from 'react-dom';

let name = 'John Doe';

// This will NOT render "The user's name is {name}"
// It will render "The user's name is John Doe"
let userMessage = <div>The user's name is {name}</div>;  


Stay tuned, and practice, and you'll avoid these and other pitfalls when writing JSX code.

Advanced Topics in JSX

JSX with TypeScript

Working with JSX in TypeScript is as intuitive as it is essential. TypeScript provides a type-safe experience when working with JSX, which is particularly important when dealing with complex applications. Here is an example using TypeScript with JSX:

// This is a functional component, using a TypeScript 'interface' for prop types
interface MyComponentProps {
    name: string;
    age: number;
    isActive: boolean;

// The react component using the props
const MyComponent: React.FC<MyComponentProps> = ({ name, age, isActive }) => {
    return (
            <h1>{`Hello, ${name}`}</h1>
            <p>{`Age: ${age}, Is Active: ${isActive}`}</p>

Note how we leverage TypeScript's ‘interfaces’ in defining and enforcing the type of our component's props. This eliminates a common mistake of passing the wrong type of data to a prop.

Another common pitfall is not properly typing event handlers. Event handlers in JSX accept synthetic events that wrap over native events. Since every DOM event type has its associated properties, we should ensure that we correctly annotate these event types in our JSX events handlers. Consider the following example:

const clickHandler = (event) => {

const changeHandler = (event: React.ChangeEvent<HTMLInputElement>) => {

In the incorrect usage above, TypeScript doesn't know what an "event" is, therefore it won't know the properties on this object. In the correct version, we're telling TypeScript exactly what type the event is going to be, and TypeScript can correctly infer the properties on this object.

JSX Types

The JSX in TypeScript can take on several types, two of which are Element and Element[] (an array of Elements). Here's a clear distinction between these two types:

// Example 1 uses single 'Element'
const singleElement: JSX.Element = <div>Hello World</div>;

// Example 2 uses 'Element[]'
const multipleElements: JSX.Element[] = [<div>Hello</div>, <div>World</div>];

In the first instance, we assign JSX that represents a single div element to a JSX.Element variable. The second example is a common mistake where multiple JSX elements are bracketed together. This should only be assigned to a JSX.Element[] variable as seen in the corrected example.

Storing JSX

Being able to store JSX in variables or JSON can greatly improve code readability and modularity, especially in complex JSX structures. For example:

// Storing JSX in a variable
let welcomeMessage: JSX.Element = <h1>Hello World</h1>;

// Example of JSX Stored as a value in JSON
const messages: { [key: string]: JSX.Element } = {
    welcome: <h1>Welcome back!</h1>,
    goodbye: <h1>Goodbye, see you soon!</h1>

In the first instance, we store JSX in a variable. This can simplify more complex JSX structures. In the second example, we're storing JSX as values in a JSON object. This can be useful when you have dynamic content to display based on various states or properties of your application.

Keep in mind that, while storing JSX in variables or JSON can simplify complex JSX structures, overdoing it can also lead to bloated and complex code. Always strive for balance and remember to organize your code in a way that makes it easy to understand and maintain.

On the journey to mastering JSX, remember that errors are merely stepping stones leading towards better understanding. Continue experimenting and learning advanced concepts in JSX to better your skills as a developer.

Criticisms and Limitations of JSX

JSX is widely used, primarily in conjunction with React. It allows for HTML-like syntax to be used in your JavaScript code, which can enhance readability and make it easier for developers from an HTML/CSS background to understand the code. However, as with every technology, there are criticisms and limitations that need to be noted.


A steadfast criticism of JSX is the combination of concerns that it promotes. Traditionally, HTML, CSS, and JavaScript have been kept separate, with clear boundaries in place. While some argue that this is an outdated practice, many developers find comfort in the separation of concerns and believe it leads to cleaner, more manageable code.

Another criticism is the steep learning curve for developers unfamiliar with either React or the JSX syntax. For example, className is used instead of class to prevent the clash with the reserved keyword in JavaScript. This slight deviation can lead developers to confusion and possible frustration as they adapt. Moreover, the use of JSX can lead to a slippery slope for less disciplined developers who might begin to include too much logic in their components, resulting in complex dependencies and challenges in debugging. These factors ultimately increase the difficulty for beginners in understanding JSX.

Common Mistake:

Ignoring the limitations and criticisms of JSX can be a common mistake made by developers. Before opting for JSX, it's essential to understand what it brings to the table and whether the limitations outweigh the benefits for your specific use case. Blindly following trends or using technology without understanding its full implications can potentially create future technical debt.


One of the main limitations is that JSX is primarily used and supported by React. This narrow focus reduces the flexibility developers have if they decide to move away from React or would like to use JSX in another context. While there are other libraries that use or support JSX, they are relatively few and as such, JSX has not been widely adopted outside the React ecosystem.

JSX is also not standard JavaScript. This means that your code will need to be transcompiled using a tool like Babel for it to run in a browser. This additional step can seem unnecessary, especially when considering that Vanilla JavaScript does not have this requirement.

Another limitation lies in the additional mental overhead of differentiating between JavaScript and JSX syntax. For example, using curly braces {} has different connotations in JSX than in standard JavaScript, which can lead to confusion for less experienced developers.

JSX Beyond React:

While JSX is primarily used with React, it's not exclusively tied to it. Libraries such as Preact, Inferno, or even implementing your custom render method can make use of JSX. However, given the limitations mentioned above, its adoption beyond React hasn't been extensive. JSX's future beyond React may largely depend on how these limitations are addressed, and whether the broader JavaScript community sees benefits in adopting JSX outside the React ecosystem.

A note on potential deprecation - like any technology, JSX could face deprecation. As the JavaScript ecosystem evolves and new technologies emerge, developers should stay informed about these changes.

In conclusion, while JSX offers certain advantages, especially in tandem with React, these benefits need to be weighed against its limitations and the criticisms it attracts. It's also crucial to remember that ignoring these limitations and criticisms can lead to potential pitfalls in the development process.


In the world of web development, developers have shown a definitive preference for the use of JSX over HTML. This inclination is not without reason. Rather, it is heavily rooted in the advantages that JSX offers in comparison to HTML and the development boost it provides in constructing React applications.

For starters, JSX provides a more straightforward and more efficient environment for developers to code in. With JSX, developers can write JavaScript logic directly inside what seems to be an HTML file, enabling them to hone in their focus on coding without needing to constantly switch between JavaScript and HTML files. This simply makes the whole process of coding more enjoyable and less strenuous.

Comparatively, although HTML could be used with regular JavaScript, the frequent to and fro between JavaScript and HTML files during development tends to become tiresome and even confusing at times. JSX provides a way through this maze by merging all the necessary elements in a neat and organized way. This factor alone makes JSX a more palatable option for developers.

Furthermore, the syntax of JSX is not just more intuitive, it also improves code readability and maintainability. By essentially combining JavaScript and HTML, JSX paves the way for cleaner and more comprehensible code. This enhances developers' productivity and streamlines the coding workflow significantly. HTML, although a potent language in its own right, simply lacks this feature integration which JSX has capitalized on.

However, JSX's edge over HTML isn't solely based on its syntax or its cohesive coupling with JavaScript. What truly sets JSX apart is its compilation process. JSX converts its syntax into JavaScript function calls rather than regular HTML. This expedites the execution of the code and makes the process considerably faster than traditional HTML.

Fundamentally, choosing between JSX and HTML boils down to a developer’s preference, and the project requirements at hand. However, the advantages of using JSX have proven to be considerable in many scenarios, hence its growing adoption in the developer community. The utmost flexibility, efficiency, and performance gain provided by JSX allows developers to focus more on creating robust applications, consequently propelling it forward as an instrumental tool in modern web development.

In conclusion, developers prefer JSX over HTML due to its enhanced capabilities, streamlined integration, and more efficient execution. JSX has not only provided an innovative alternative to HTML but also has set new benchmarks in terms of productivity, flexibility, and performance in web development. Therefore, it comes as no surprise that developers today are increasingly gravitating towards the use of JSX in their applications.

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