# Writing Markup with JSX *JSX* is a syntax extension for JavaScript that lets you write HTML-like markup inside a JavaScript file. Although there are other ways to write components, most React developers prefer the conciseness of JSX, and most codebases use it. ### JSX: Putting markup into JavaScript The Web has been built on HTML, CSS, and JavaScript. For many years, web developers kept content in HTML, design in CSS, and logic in JavaScript—often in separate files! Content was marked up inside HTML while the page’s logic lived separately in JavaScript:
HTML markup with purple background and a div with two child tags: p and form.
HTML
Three JavaScript handlers with yellow background: onSubmit, onLogin, and onClick.
JavaScript But as the Web became more interactive, logic increasingly determined content. JavaScript was in charge of the HTML! This is why **in React, rendering logic and markup live together in the same place—components.**
React component with HTML and JavaScript from previous examples mixed. Function name is Sidebar which calls the function isLoggedIn, highlighted in yellow. Nested inside the function highlighted in purple is the p tag from before, and a Form tag referencing the component shown in the next diagram.
`Sidebar.js` React component
React component with HTML and JavaScript from previous examples mixed. Function name is Form containing two handlers onClick and onSubmit highlighted in yellow. Following the handlers is HTML highlighted in purple. The HTML contains a form element with a nested input element, each with an onClick prop.
`Form.js` React component Keeping a button’s rendering logic and markup together ensures that they stay in sync with each other on every edit. Conversely, details that are unrelated, such as the button’s markup and a sidebar’s markup, are isolated from each other, making it safer to change either of them on their own. Each React component is a JavaScript function that may contain some markup that React renders into the browser. React components use a syntax extension called JSX to represent that markup. JSX looks a lot like HTML, but it is a bit stricter and can display dynamic information. The best way to understand this is to convert some HTML markup to JSX markup. #### Note JSX and React are two separate things. They’re often used together, but you *can* [use them independently](https://reactjs.org/blog/2020/09/22/introducing-the-new-jsx-transform.html#whats-a-jsx-transform) of each other. JSX is a syntax extension, while React is a JavaScript library. ### Converting HTML to JSX Suppose that you have some (perfectly valid) HTML: ```html

Hedy Lamarr's Todos

Hedy Lamarr ``` And you want to put it into your component: ```jsx export default function TodoList() { return ( // ??? ) } ``` If you copy and paste it as is, it will not work: ```jsx export default function TodoList() { return ( // This doesn't quite work!

Hedy Lamarr's Todos

Hedy Lamarr ); } ``` ### Error ```jsx /App.js: Adjacent JSX elements must be wrapped in an enclosing tag. Did you want a JSX fragment <>...? (5:4) 3 | // This doesn't quite work! 4 |

Hedy Lamarr's Todos

> 5 | Hedy Lamarr #### 1. Return a single root element To return multiple elements from a component, **wrap them with a single parent tag.** For example, you can use a `
`: ```html

Hedy Lamarr's Todos

Hedy Lamarr
``` If you don’t want to add an extra `
` to your markup, you can write `<>` and `` instead: ```jsx <>

Hedy Lamarr's Todos

Hedy Lamarr ``` This empty tag is called a [*Fragment.*](https://react.dev/reference/react/Fragment) Fragments let you group things without leaving any trace in the browser HTML tree. #### 2. Close all the tags JSX requires tags to be explicitly closed: self-closing tags like `` must become ``, and wrapping tags like `
  • oranges` must be written as `
  • oranges
    • `. This is how Hedy Lamarr’s image and list items look closed: ```jsx <> Hedy Lamarr ``` #### 3. camelCase ~~all~~ most of the things! JSX turns into JavaScript and attributes written in JSX become keys of JavaScript objects. In your own components, you will often want to read those attributes into variables. But JavaScript has limitations on variable names. For example, their names can’t contain dashes or be reserved words like `class`. This is why, in React, many HTML and SVG attributes are written in camelCase. For example, instead of `stroke-width` you use `strokeWidth`. Since `class` is a reserved word, in React you write `className` instead, named after the [corresponding DOM property](https://developer.mozilla.org/en-US/docs/Web/API/Element/className): ```jsx Hedy Lamarr ``` You can [find all these attributes in the list of DOM component props.](https://react.dev/reference/react-dom/components/common) If you get one wrong, don’t worry—React will print a message with a possible correction to the [browser console.](https://developer.mozilla.org/docs/Tools/Browser_Console) #### Pitfall For historical reasons, [`aria-*`](https://developer.mozilla.org/docs/Web/Accessibility/ARIA) and [`data-*`](https://developer.mozilla.org/docs/Learn/HTML/Howto/Use_data_attributes) attributes are written as in HTML with dashes. #### Pro-tip: Use a JSX Converter Converting all these attributes in existing markup can be tedious! We recommend using a [converter](https://transform.tools/html-to-jsx) to translate your existing HTML and SVG to JSX. Converters are very useful in practice, but it’s still worth understanding what is going on so that you can comfortably write JSX on your own. Here is your final result: ```jsx export default function TodoList() { return ( <>

    Hedy Lamarr's Todos

    Hedy Lamarr ); } ``` ### Recap Now you know why JSX exists and how to use it in components: * React components group rendering logic together with markup because they are related. * JSX is similar to HTML, with a few differences. You can use a [converter](https://transform.tools/html-to-jsx) if you need to. * Error messages will often point you in the right direction to fixing your markup. --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://edrus.gitbook.io/mt-it/2nd-month/week-6/react-js/components-basics/writing-markup-with-jsx.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.