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>

It's been quite a few years since this question was last answered. React introduced "Hooks" back in 2017, and "keyCode" has been deprecated.

Now we can write this:

  useEffect(() => {
    const listener = event => {
      if (event.code === "Enter" || event.code === "NumpadEnter") {
        console.log("Enter key was pressed. Run your function.");
        event.preventDefault();
        // callMyFunction();
      }
    };
    document.addEventListener("keydown", listener);
    return () => {
      document.removeEventListener("keydown", listener);
    };
  }, []);

This registers a listener on the keydown event, when the component is loaded for the first time. It removes the event listener when the component is destroyed.