How to do something before on submit with JavaScript?

To do something before on submit with JavaScript, we call preventDefault in the submit handler.

For instance, we write

<form id="formId" action="/" method="POST" onsubmit="prepareForm(event)">
  <input type="text" value="" />
  <input type="submit" value="Submit" />
</form>

to add a form.

We set the onsubmit attribute to call the prepareForm function.

Then we write

function prepareForm(event) {
  event.preventDefault();
  // ...
  document.getElementById("formId").requestSubmit();
}

to define the prepareForm function.

In it, we call preventDefault to stop the default submit behavior.

And then we select the form with getElementByid and call requestSubmit to submit the form.