Basic stateless React.js form

login-form.js

/*
This is obviously a very basic example. It takes props to render the inputs with their values, and 
to render any error state. It also takes props to update the state, or submit the form.
*/

const LoginForm = props => (
  <form onSubmit={props.onSubmit}>
    <input type="text" name="username" onChange={props.onChangeUsername} value={props.username}/>
    { props.userNameError && <span class="error">{ props.usernameError }</span> }
    <input type="password" name="password" onChange={props.onChangePassword} value={props.password}/>
    { props.passwordError && <span class="error">{ props.passwordError }</span> }
    <input type="submit" value="Log in"/>
  </form>
);

LoginForm.propTypes = {
  username: React.PropTypes.string.isRequired,
  usernameError: React.PropTypes.string.isRequired,
  password: React.PropTypes.string.isRequired,
  passwordError: React.PropTypes.string.isRequired,
  onChangeUsername: React.PropTypes.func.isRequired,
  onChangePassword: React.PropTypes.func.isRequired,
  onSubmit: React.PropTypes.func.isRequired
};

export default LoginForm;

/*
This is just one approach - it may very well be overkill to notify a parent / dispatch an action
on every keypress. Instead, you could write a stateful component where the inputs' onChange events
change the local, intermediate state, and then onBlur (and probably also onSubmit) is what causes 
that state to leave the component and be persisted somewhere else.

Both of those approaches are assuming that the state is ultimately stored outside the form component.
I like to put it in a redux store, because a) you can put business logic like validation either in
a reducer, or a reselect selector, and b) when submitting the form you can dispatch a thunk action
which reads the store using getState, and then submits it.
*/