Handling forms in JavaScript

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Forms are an extremely important part of HTML and the Web Platform. They allow users can interact with the page and

• search something on the site
• trigger filters to trim result pages
• send information

and much much more.

By default, forms submit their content to a server-side endpoint, which by default is the page URL itself:

<form>
  ...
  <input type="submit">
</form>

We can override this behavior by setting the action attribute of the form element, using the HTML method defined by the method attribute, which defaults to GET:

<form action="/contact" method="POST">
  ...
  <input type="submit">
</form>

Upon clicking the submit input element, the browser makes a POST request to the /contact URL on the same origin (protocol, domain and port).

Using JavaScript we can intercept this event, submit the form asynchronously (with XHR and Fetch), and we can also react to events happening on individual form elements.

Intercepting a form submit event

I just described the default behavior of forms, without JavaScript.

In order to start working with forms with JavaScript you need to intercept the submit event on the form element:

const form = document.querySelector('form')
form.addEventListener('submit', event => {
  // submit event detected
})

Now inside the submit event handler function we call the event.preventDefault() method to prevent the default behavior and avoid a form submit to reload the page:

const form = document.querySelector('form')
form.addEventListener('submit', event => {
  // submit event detected
  event.preventDefault()
})

At this point clicking the submit event button in the form will not do anything, except giving us the control.

Working with input element events

We have a number of events we can listen for in form elements

• input fired on form elements when the element value is changed
• change fired on form elements when the element value is changed. In the case of text input elements and textarea, it’s fired only once when the element loses focus (not for every single character typed)
• cut fired when the user cuts text from the form element
• copy fired when the user copies text from the form element
• paste fired when the user pastes text into the form element
• focus fired when the form element gains focus
• blur fired when the form element loses focus

Here’s a sample form demo on Codepen:

See the Pen Form events demo by Flavio Copes (@flaviocopes) on CodePen.

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