When working with web forms, you often need to check user input before the data is sent to the server. The best moment to perform this check is when the user tries to submit the form. In JavaScript, you can listen for the submit event on a form element. This event fires whenever the form is submitted, either by clicking a submit button or pressing Enter in a field. By handling this event, you can intercept the submission, examine the form data, and decide whether it should be sent or if the user needs to fix something first. This makes the submit event the ideal place to run your validation logic.

Calling event.preventDefault() inside your submit event handler is crucial for custom validation. By default, submitting a form causes the browser to send form data and reload the page. If you want to check the input and show error messages before sending anything, you must stop this default behavior. Preventing the default submission gives you full control: you can validate the data, display helpful messages, and only allow the form to submit if everything is correct.