ControlledStatefulReactComponent.js

class SignInForm extends React.Component {
  constructor() {
    super();
    this.state = {
      username: "",
      password: ""
    };
  }

  onChange(key, value) {
    this.setState({ [key]: value});
  }
  
  onSubmit(ev) {
    ev.preventDefault();
    const username = this.state.username;
    const password = this.state.password;
    alert (`You submitted username=${username} password=${password}`);
  }

  render() {
    const onUsernameChange = ev => this.onChange("username", ev.target.value);
    const onPasswordChange = ev => this.onChange("password", ev.target.value);
    return (
      <form onSubmit={ev => this.onSubmit(ev)}>
        <p>Username: <input name="username" type="text" value={this.state.username} onChange={onUsernameChange} /></p>
        <p>Password: <input name="password" type="password" value={this.state.password} onChange={onPasswordChange} /></p>
        <button type="submit">Sign In</button>
      </form>
    );
  }
}

UncontrolledStatefulReactComponent.js

class SignInForm extends React.Component {
  onSubmit(ev) {
    ev.preventDefault();
    const username = this.refs.username.value;
    const password = this.refs.password.value;
    alert (`You submitted username=${username} password=${password}`);    
  }

  render() {
    return (
      <form onSubmit={ev => this.onSubmit(ev)}>
        <p>Username: <input name="username" type="text" ref="username" /></p>
        <p>Password: <input name="password" type="password" ref="password" /></p>
        <button type="submit">Sign In</button>
      </form>
    );
  }
}

UncontrolledStatelessReactComponent.js

const onSubmit = ev => {
  ev.preventDefault();
  const username = ev.target.elements.username.value;
  const password = ev.target.elements.password.value;
  alert (`You submitted username=${username} password=${password}`);    
};

const SignInForm = () => (
  <form onSubmit={onSubmit}>
    <p>Username: <input name="username" type="text" /></p>
    <p>Password: <input name="password" type="password" /></p>
    <button type="submit">Sign In</button>
  </form>
);