Determine which element triggered a form submit event

You can use document.activeElement This will work for clicks and when the user tabs to a button and hits the enter button.

Listen for the submit event on the form, then inside of the callback, reference document.activeElement, it will be the button that the user clicked or tabbed to.

Example: http://codepen.io/anon/pen/KVoJrJ

The submit event of a <form> element actually contains a reference to the submitter:
https://developer.mozilla.org/en-US/docs/Web/API/SubmitEvent/submitter

form.onsubmit = (event) => {
    const formElement = event.target;
    const formSubmitter = event.submitter;
    console.log("The form element itself", formElement);
    console.log("The submitter of the form (usually an HtmlInputElement)", formSubmitter);

    const formData = new FormData(formElement);
    if(formSubmitter) formData.set(formSubmitter.name, formSubmitter.value);

    fetch(formElement.action, {
        method: formElement.method,
        body: formData,
    })

    event.preventDefault();
}

Note: check the compatibility of this feature.