setStateAsync.js

class SomeForm extends React.Component {

  handleFirstNameChange = (event) => {
    this.setState({firstName: event.currentTarget.value});
    console.log(this.state.firstName)
  }

  render () {
    return (
      <div>
        <input onChange={this.handleFirstnameChange} value={this.state.firstName}/>
      </div>
    );
  }
}