react-form-management.js

class SomeForm extends Component {
  state = {
    email: '',
    password: '',
  };

  // - using arrow functions will auto-bind to 'this', so no need 
  //   for doing this.updateFormField = this.updateFormField.bind(this)
  //   in the constructor (although there are some caveats to this
  //   approach when writing unit tests)
  updateFormField = (e) => {
    const { name, value } = e.target;
    this.setState({ [name]: value });
  };

  submit = (e) => {
    const { email, password } = this.state;
    // submit the form...
  };
  
  render() {
    const { email, password } = this.state;
    
    return (
      <form onSubmit={this.submit}>
        <input value={email} name='email' onChange={this.updateFormField} />
        <input value={password} name='password' onChange={this.updateFormField} />
      </form>
    );
  }
}