What happens when using this.setState multiple times in React component?

Question

I wanted to check what happens when you use this.setState multiple times (2 times for the sake of the discussion). I thought that the component will be rendered twice but apparently it's rendered only once. Another expectation I had was that maybe the second call for setState will run over the first one, but you guessed it - worked fine.

Link to a JSfiddle

var Hello = React.createClass({
  render: function() {
    return (
      <div>
        <div>Hello {this.props.name}</div>
        <CheckBox />
      </div>
    );
  }
});

var CheckBox = React.createClass({
  getInitialState: function() {
    return {
      alex: 0
    };
  },

  handleChange: function(event) {
    this.setState({
      value: event.target.value
    });
    this.setState({
      alex: 5
    });
  },

  render: function() {
    alert('render');
    return (
      <div>
        <label htmlFor="alex">Alex</label>
        <input type="checkbox" onChange={this.handleChange} name="alex" />
        <div>{this.state.alex}</div>
      </div>
    );
  }
});

ReactDOM.render(
  <Hello name="World" />,
  document.getElementById('container')
);

As you'll see, an alert that says 'render' pops up on every render.

Do you have an explanation for why it worked properly?

Answer 1

React batches state updates that occur in event handlers and lifecycle methods. Thus, if you update state multiple times in a <div onClick /> handler, React will wait for event handling to finish before re-rendering.

To be clear, this only works in React-controlled synthetic event handlers and lifecycle methods. State updates are not batched in AJAX and setTimeout event handlers, for example.