Why calling react setState method doesn't mutate the state immediately?

Question

I'm reading Forms section of reactjs documentation and just tried this code to demonstrate onChange usage (JSBIN).

var React= require('react');

var ControlledForm= React.createClass({
    getInitialState: function() {
        return {
            value: "initial value"
        };
    },

    handleChange: function(event) {
        console.log(this.state.value);
        this.setState({value: event.target.value});
        console.log(this.state.value);

    },

    render: function() {
        return (
            <input type="text" value={this.state.value} onChange={this.handleChange}/>
        );
    }
});

React.render(
    <ControlledForm/>,
  document.getElementById('mount')
);

When I update the <input/> value in the browser, the second console.log inside the handleChange callback prints the same value as the first console.log, Why I can't see the result of this.setState({value: event.target.value}) in the scope of handleChange callback?

Answer 1

From React's documentation:

setState() does not immediately mutate this.state but creates a pending state transition. Accessing this.state after calling this method can potentially return the existing value. There is no guarantee of synchronous operation of calls to setState and calls may be batched for performance gains.

If you want a function to be executed after the state change occurs, pass it in as a callback.

this.setState({value: event.target.value}, function () {
    console.log(this.state.value);
});