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---
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id: jsx-in-depth
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title: JSX in Depth
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layout: docs
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permalink: jsx-in-depth.html
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prev: displaying-data.html
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next: jsx-gotchas.html
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---
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JSX is a JavaScript XML syntax transform recommended for use
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with React.
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## Why JSX?
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React works out of the box without JSX. Simply construct your markup using the
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functions on `React.DOM`. For example, here's how to construct a simple link:
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```javascript
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var link = React.DOM.a({href: 'http://facebook.github.io/react'}, 'React');
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```
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We recommend using JSX for many reasons:
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* It's easier to visualize the structure of the DOM.
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* Designers are more comfortable making changes.
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* It's familiar for those who have used MXML or XAML.
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## The Transform
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JSX transforms from an XML-like syntax into native JavaScript. XML elements and
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attributes are transformed into function calls and objects, respectively.
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```javascript
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var Nav;
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// Input (JSX):
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var app = <Nav color="blue" />;
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// Output (JS):
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var app = Nav({color:"blue"});
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```
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Notice that in order to use `<Nav />`, the `Nav` variable must be in scope.
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JSX also allows specifying children using XML syntax:
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```javascript
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var Nav, Profile;
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// Input (JSX):
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var app = <Nav color="blue"><Profile>click</Profile></Nav>;
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// Output (JS):
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var app = Nav({color:"blue"}, Profile(null, "click"));
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```
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Use the [JSX Compiler](/react/jsx-compiler.html) to try out JSX and see how it
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desugars into native JavaScript.
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If you want to use JSX, the [Getting Started](getting-started.html) guide shows
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how to setup compilation.
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> Note:
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>
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> Details about the code transform are given here to increase understanding, but
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> your code should not rely on these implementation details.
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## React and JSX
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React and JSX are independent technologies, but JSX was primarily built with
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React in mind. The two valid uses of JSX are:
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* To construct instances of React DOM components (`React.DOM.*`).
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* To construct instances of composite components created with
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`React.createClass()`.
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### React DOM Components
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To construct a `<div>` is to create a variable that refers to `React.DOM.div`.
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```javascript
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var div = React.DOM.div;
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var app = <div className="appClass">Hello, React!</div>;
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```
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### React Composite Components
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To construct an instance of a composite component, create a variable that
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references the class.
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```javascript
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var MyComponent = React.createClass({/*...*/});
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var app = <MyComponent someProperty={true} />;
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```
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See [Multiple Components](multiple-components.html) to learn more about using composite components.
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> Note:
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>
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> Since JSX is JavaScript, identifiers such as `class` and `for` are discouraged
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> as XML attribute names. Instead, React DOM components expect attributes like
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> `className` and `htmlFor`, respectively.
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## DOM Convenience
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Having to define variables for every type of DOM element can get tedious
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(e.g. `var div, span, h1, h2, ...`). JSX provides a convenience to address this
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problem by allowing you to specify a variable in an `@jsx` docblock field. JSX
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will use that field to find DOM components.
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```javascript
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/**
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* @jsx React.DOM
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*/
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var Nav;
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// Input (JSX):
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var tree = <Nav><span /></Nav>;
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// Output (JS):
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var tree = Nav(null, React.DOM.span(null));
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```
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> Remember:
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>
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> JSX simply transforms elements into function calls and has no notion of the
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> DOM. The docblock parameter is only a convenience to resolve the most commonly
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> used elements. In general, JSX has no notion of the DOM.
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## JavaScript Expressions
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### Attribute Expressions
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To use a JavaScript expression as an attribute value, wrap the expression in a
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pair of curly braces (`{}`) instead of quotes (`""`).
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```javascript
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// Input (JSX):
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var person = <Person name={window.isLoggedIn ? window.name : ''} />;
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// Output (JS):
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var person = Person({name: window.isLoggedIn ? window.name : ''});
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```
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### Child Expressions
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Likewise, JavaScript expressions may be used to express children:
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```javascript
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// Input (JSX):
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var content = <Container>{window.isLoggedIn ? <Nav /> : <Login />}</Container>;
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// Output (JS):
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var content = Container(null, window.isLoggedIn ? Nav(null) : Login(null));
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```
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### Comments
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It's easy to add comments within your JSX; they're just JS expressions:
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```javascript
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var content = <Container>{/* this is a comment */}<Nav /></Container>;
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```
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## Tooling
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Beyond the compilation step, JSX does not require any special tools.
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* Many editors already include reasonable support for JSX (Vim, Emacs js2-mode).
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* JSX syntax highlighting is available for Sublime Text and other editors
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that support `*.tmLanguage` using the third-party
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[`JavaScript (JSX).tmLanguage`][1].
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* Linting provides accurate line numbers after compiling without sourcemaps.
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* Elements use standard scoping so linters can find usage of out-of-scope
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components.
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[1]: https://github.com/yungsters/sublime/blob/master/tmLanguage/JavaScript%20(JSX).tmLanguage
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## Prior Work
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JSX is similar to several other JavaScript embedded XML language
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proposals/projects. Some of the features of JSX that distinguish it from similar
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efforts include:
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* JSX is a simple syntactic transform.
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* JSX neither provides nor requires a runtime library.
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* JSX does not alter or add to the semantics of JavaScript.
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JSX is similar to HTML, but not exactly the same. See [JSX gotchas](./jsx-gotchas.html) for some key differences.
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