This article represents code sample for ReactJS UI controlled components along with related details. Please feel free to comment/suggest if I missed to mention one or more important points. Also, sorry for the typos.
The code sample represents an input field consisting of “Calvin”. Once you change the text, the “calvin” written with “Hello” would also change. Following are the key points described later in this article:

• What are controlled components?
• Code Example – Controlled Component
What are Controlled Component?

As per the ReactJS Form page, An <input> with “value” set is a controlled component. In a controlled <input>, the value of the rendered element will always reflect the value prop.

In the example below, the textfield consists of “value” attribute which is assigned to “this.state.value”. Following code represents the controlled component:

<input type="text" value={this.state.value} onChange={this.handleChange}/>

Code Example – Controlled Component

The code below represents following:

• The controlled UI component, namely, <input> field with value set.
• “onChange” event handler with a callback method “handleChange” which is invoked as the text in input field changes
• “setState” function which is invoked from handleChange method, thereby setting the value of “value” field to changed value
• “getInitialState” function setting the initial value of “value”

Changing text inside the text field will change the text ahead of “Hello”. Following screenshot represents the UI.

Following is the code sample:

<!DOCTYPE html>
<html>
<title>Hello ReactJS!</title>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/0.13.3/react.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/0.13.3/JSXTransformer.js"></script>
<body class="container">
<div id="content"></div>
<script type="text/jsx">
var HelloUI = React.createClass({
getInitialState: function(){
return {value: 'Calvin'};
},
handleChange: function(event) {
this.setState({value: event.target.value});
},
render: function(){
return(
<div>
<h1>Hello, {this.state.value}</h1>
<hr/>
<input type="text" value={this.state.value} onChange={this.handleChange}/>
</div>
);
}
});
React.render(<HelloUI />, document.getElementById("content"))
</script>
</body>
</html>

Hope the article was helpful in presenting concepts and code samples related with UI Controlled Component in ReactJS.