Getting error Cannot read property 'setState' of undefined - reactjs

Question

I'm new with Reactjs. I'm trying to do something very simple: update a div inside a render function when the user change the text inside text area. Any suggestions?
class HTMLEditor extends React.Component {
constructor(props) {
super(props);
this.state = {value: 'Put here HTML'};
}
handleChange(e) {
this.setState({value: e.currentTarget.value});
}
render() {
return (
<div>
<textarea defaultValue={this.state.value} onChange={ this.handleChange } />
<div>{this.state.value}</div>
</div>
);
}
}
ReactDOM.render(
<HTMLEditor />,
document.getElementById('container')
);

Answer 1

You should bind the handleChange function. You are getting this error because, in your handleChange function this keywork doesn't refer to the context of the React Class and hence you need to bind the function.
See this answer on why do you need to bind event handlers in React
class HTMLEditor extends React.Component {
constructor(props) {
super(props);
this.state = {value: 'Put here HTML'};
}
handleChange = (e) =>{
this.setState({value: e.currentTarget.value});
}
render() {
return (
<div>
<textarea defaultValue={this.state.value} onChange={ this.handleChange } />
<div>{this.state.value}</div>
</div>
);
}
}
ReactDOM.render(
<HTMLEditor />,
document.getElementById('container')
);
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react-dom.min.js"></script>
<div id="container"></div>