For the remainder of this tutorial, we'll be writing our JavaScript code in this script tag.
## Your first component
React is all about modular, composable components. For our comment box example, we'll have the following component structure:
```
- CommentBox
- CommentList
- Comment
- CommentForm
```
Let's build the `CommentBox` component, which is just a simple `<div>`:
```javascript
// tutorial1.js
var CommentBox = React.createClass({
render: function() {
return (
<divclass="commentBox">
Hello, world! I am a CommentBox.
</div>
);
}
});
React.renderComponent(
<CommentBox/>,
document.getElementById('content')
);
```
### JSX Syntax
The first thing you'll notice is the XML-ish syntax in your JavaScript. We have a simple precompiler that translates the syntactic sugar to this plain JavaScript:
Its use is optional but we've found JSX syntax easier to use than plain JavaScript. Read more on the [JSX Syntax article](syntax.html).
### What's going on
We pass some methods in a JavaScript object to `React.createClass()` to create a new React component. The most important of these methods is called `render` which returns a tree of React components that will eventually render to HTML.
The `<div>` tags are not actual DOM nodes; they are instantiations of React `div` components. You can think of these as markers or pieces of data that React knows how to handle. React is **safe**. We are not generating HTML strings so XSS protection is the default.
You do not have to return basic HTML. You can return a tree of components that you (or someone else built). This is what makes React **composable**: a key tenet of maintainable frontends.
`React.renderComponent()` instantiates the root component, starts the framework, and injects the markup into a raw DOM element, provided as the second argument.
# Composing components
Let's build skeletons for `CommentList` and `CommentForm` which will, again, be simple `<div>`s:
```javascript
// tutorial2.js
var CommentList = React.createClass({
render: function() {
return (
<divclass="commentList">
Hello, world! I am a CommentList.
</div>
);
}
});
var CommentForm = React.createClass({
render: function() {
return (
<divclass="commentForm">
Hello, world! I am a CommentForm.
</div>
);
}
});
```
Next, update the `CommentBox` component to use its new friends:
Notice how we're mixing HTML tags and components we've built. HTML components are regular React components, just like the ones you define, with one difference. The JSX compiler will automatically rewrite HTML tags to "React.DOM.tagName" expressions and leave everything else alone. This is to prevent the pollution of the global namespace.
Let's create our third component, `Comment`. We will want to pass it the author name and comment text so we can reuse the same code for each unique comment. First let's add some comments to the `CommentList`:
```javascript{6-7}
// tutorial4.js
var CommentList = React.createClass({
render: function() {
return (
<divclass="commentList">
<Commentauthor="Pete Hunt">This is one comment</Comment>
<Commentauthor="Jordan Walke">This is *another* comment</Comment>
</div>
);
}
});
```
Note that we have passed some data from the parent `CommentList` component to the child `Comment` component as both XML-like children and attributes. Data passed from parent to child is called **props**, short for properties.
## Using props
Let's create the Comment component. It will read the data passed to it from the CommentList and render some markup:
```javascript
// tutorial5.js
var Comment = React.createClass({
render: function() {
return (
<divclass="comment">
<h2class="commentAuthor">
{this.props.author}
</h2>
{this.props.children}
</div>
);
}
});
```
By surrounding a JavaScript expression in braces inside JSX (as either an attribute or child), you can drop text or React components into the tree. We access named attributes passed to the component as keys on `this.props` and any nested elements as `this.props.children`.
## Adding Markdown
Markdown is a simple way to format your text inline. For example, surrounding text with asterisks will make it emphasized.
First, add the third-party **Showdown** library to your application. This is a JavaScript library which takes Markdown text and converts it to raw HTML. This requires a script tag in your head (which we have already included in the React playground).
Next, let's convert the comment text to Markdown and output it:
All we're doing here is calling the Showdown library. We need to convert `this.props.children` from React's wrapped text to a raw string that Showdown will understand so we explicitly call `toString()`.
But there's a problem! Our rendered comments look like this in the browser: "`<p>`This is `<em>`another`</em>` comment`</p>`". We want those tags to actually render as HTML.
That's React protecting you from an XSS attack. There's a way to get around it but the framework warns you not to use it:
```javascript{5,11}
// tutorial7.js
var converter = new Showdown.converter();
var Comment = React.createClass({
render: function() {
var rawMarkup = converter.makeHtml(this.props.children.toString());
This is a special API that intentionally makes it difficult to insert raw HTML, but for Showdown we'll take advantage of this backdoor.
**Remember:** by using this feature you're relying on Showdown to be secure.
## Hook up the data model
So far we've been inserting the comments directly in the source code. Instead, let's render a blob of JSON data into the comment list. Eventually this will come from the server, but for now, write it in your source:
```javascript
// tutorial8.js
var data = [
{author: 'Pete Hunt', text: 'This is one comment'},
{author: 'Jordan Walke', text: 'This is *another* comment'}
];
```
We need to get this data into `CommentList` in a modular way. Modify `CommentBox` and the `renderComponent()` call to pass this data into the `CommentList` via props:
```javascript{7,15}
// tutorial9.js
var CommentBox = React.createClass({
render: function() {
return (
<divclass="commentBox">
<h1>Comments</h1>
<CommentListdata={this.props.data}/>
<CommentForm/>
</div>
);
}
});
React.renderComponent(
<CommentBoxdata={data}/>,
document.getElementById('content')
);
```
Now that the data is available in the `CommentList`, let's render the comments dynamically:
```javascript{4-6}
// tutorial10.js
var CommentList = React.createClass({
render: function() {
var commentNodes = this.props.data.map(function (comment) {
Let's replace the hard-coded data with some dynamic data from the server. We will remove the data prop and replace it with a URL to fetch:
```javascript{2}
// tutorial11.js
React.renderComponent(
<CommentBoxurl="comments.json"/>,
document.getElementById('example')
);
```
This component is different from the prior components because it will have to re-render itself. The component won't have any data until the request from the server comes back, at which point the component may need to render some new comments.
## Reactive state
So far, each component has rendered itself once based on its props. `props` are immutable: they are passed from the parent and are "owned" by the parent. To implement interactions, we introduce mutable **state** to the component. `this.state` is private to the component and can be changed by calling `this.setState()`. When the state is updated, the component re-renders itself.
`render()` methods are written declaratively as functions of `this.props` and `this.state`. The framework guarantees the UI is always consistent with the inputs.
When the server fetches data, we will be changing the comment data we have. Let's add an array of comment data to the `CommentBox` component as its state:
`getInitialState()` executes exactly once during the lifecycle of the component and sets up the initial state of the component.
### Updating state
When the component is first created, we want to GET some JSON from the server and update the state to reflect the latest data. In a real application this would be a dynamic endpoint, but for this example, we will use a static JSON file to keep things simple:
```javascript
// tutorial13.json
[
{'author': 'Pete Hunt', 'text': 'This is one comment'},
{'author': 'Jordan Walke', 'text': 'This is *another* comment'}
]
```
We will use jQuery 1.5 to help make an asynchronous request to the server.
Note: because this is becoming an AJAX application you'll need to develop your app using a web server rather than as a file sitting on your file system. The easiest way to do this is to run `python -m SimpleHTTPServer` in your application's directory.
```javascript{4-11}
// tutorial13.js
var CommentBox = React.createClass({
getInitialState: function() {
$.ajax({
url: 'comments.json',
dataType: 'json',
mimeType: 'textPlain',
success: function(data) {
this.setState({data: data});
}.bind(this)
});
return {data: []};
},
render: function() {
return (
<divclass="commentBox">
<h1>Comments</h1>
<CommentListdata={this.state.data}/>
<CommentForm/>
</div>
);
}
});
```
The key is the call to `this.setState()`. We replace the old array of comments with the new one from the server and the UI automatically updates itself. Because of this reactivity, it is trivial to add live updates. We will use simple polling here but you could easily use WebSockets or other technologies.
All we have done here is move the AJAX call to a separate method and call it when the component is first loaded and every 5 seconds after that. Try running this in your browser and changing the `comments.json` file; within 5 seconds, the changes will show!
Now it's time to build the form. Our `CommentForm` component should ask the user for their name and comment text and send a request to the server to save the comment.
Let's make the form interactive. When the user submits the form, we should clear it, submit a request to the server, and refresh the list of comments. To start, let's listen for the form's submit event and clear it.
React attaches event handlers to components using a camelCase naming convention. We attach an `onSubmit` handler to the form that clears the form fields when the form is submitted with valid input.
We always return `false` from the event handler to prevent the browser's default action of submitting the form. (If you prefer, you can instead take the event as an argument and call `preventDefault()` on it – read more about [event handling](event-handling.html).)
`React.autoBind()` is a simple way to ensure that a method is always bound to its component. Inside the method, `this` will be bound to the component instance.
#### Refs
We use the `ref` attribute to assign a name to a child component and `this.refs` to reference the component. We can call `getDOMNode()` on a component to get the native browser DOM element.
#### Callbacks as props
When a user submits a comment, we will need to refresh the list of comments to include the new one. It makes sense to do all of this logic in `CommentBox` since `CommentBox` owns the state that represents the list of comments.
We need to pass data from the child component to its parent. We do this by passing a `callback` in props from parent to child:
Our application is now feature complete but it feels slow to have to wait for the request to complete before your comment appears in the list. We can optimistically add this comment to the list to make the app feel faster.
