How to submit a form using Enter key in react.js?

Change <button type="button" to <button type="submit". Remove the onClick. Instead do <form className="commentForm" onSubmit={this.onFormSubmit}>. This should catch clicking the button and pressing the return key.

const onFormSubmit = e => {
  e.preventDefault();
  // send state to server with e.g. `window.fetch`
}

...

<form onSubmit={onFormSubmit}>
  ...
  <button type="submit">Submit</button>
</form>