Change events are a crucial part of building dynamic and responsive web applications with Javascript. The change event allows developers to execute code in response to changes to form inputs, enabling features like form validation, live updates, and data binding. In this comprehensive article, we’ll cover everything you need to know about the Javascript change event – from what it is under the hood to practical use cases and examples. Let’s dive in!

What is the Javascript Change Event?

The change event is fired when the value of an element has been changed. This event only applies to <input>, <select>, and <textarea> HTML elements.

Some key things to know about the change event:

• It fires after the value changes and the element loses focus. This differs from the input event which fires instantly as the value changes.
• It bubbles up the DOM tree like other events. This means change events fired on child elements will trigger handlers on parent elements too.
• The event does not fire if the value is changed programmatically with Javascript, only from user interaction.
• Unlike input which fires for every keystroke, change only fires when focus leaves the element after a change. This makes it ideal for validation and final form submission.

Let’s look at a quick example to see the change event in action:

<input type="text" id="fname">

<script>
const input = document.getElementById('fname');

input.addEventListener('change', () => {
  console.log('Input value changed!')  
});
</script>

Here we attach a change event listener to an <input> element. It will log a message when the user types into the input box then changes focus by tabbing or clicking away.

Now that we’ve covered the basics, let’s look at some common use cases and examples of using the change event.

Common Use Cases

There are a few main ways the change event is commonly used:

1. Form Validation

One of the most common uses of change is to validate form inputs. For example:

const emailInput = document.getElementById('email');

emailInput.addEventListener('change', () => {
  if (!validateEmail(emailInput.value)) {
    // Show error
  } else {
    // Email valid
  }
})

Here we validate the email input every time the value changes and the field loses focus.

You can validate inputs like required fields, email format, numbers within a range, and more. Keyup or input events work for instant validation, but change is better for final validation before form submission.

2. Dynamic Updates

Another common use case is to dynamically update other parts of the UI based on a changed input value:

const qtyInput = document.getElementById('quantity');
const totalElement = document.getElementById('total');

qtyInput.addEventListener('change', () => {
  let total = qtyInput.value * productPrice;

  totalElement.innerText = `Total: $${total}`; 
})

Here we recalculate and update the total price every time the quantity changes.

Examples include updating shopping cart totals, dynamically filtering tables, changing charts or graphics based on inputs, and more.

3. Data Binding

The change event is also useful for binding data between your UI and Javascript model or back-end data storage.

const model = {
  firstName: 'John'
}

const input = document.getElementById('fname');

input.addEventListener('change', () => {
  model.firstName = input.value;

  // Also update backend
  saveToDatabase(model);
})

Two-way data binding keeps your data in sync between the front-end UI and backend data model.

Now let’s look at how we can manually trigger the change event in Javascript.

Manually Firing the Change Event

Sometimes you need to programmatically trigger the change event handler instead of waiting for user input. There are a couple ways to do this:

Using dispatchEvent()

The standard DOM API way is to create a new Event object, set the type to 'change', and call the dispatchEvent() method on the target element:

const input = document.getElementById('fname');

// Create event
const changeEvent = new Event('change');

// Dispatch on input 
input.dispatchEvent(changeEvent);

This will trigger the change handler in the same way as if the user changed the input value.

With jQuery

If you are using jQuery, you can simply call the .trigger() method and pass the event type:

$('#fname').trigger('change'); 

This dispatches the change event on the matched element(s).

That covers the basics of manually triggering the change event. Let’s look at some practical examples and use cases next.

Common Use Cases and Applications

Now that we’ve covered how to listen for and dispatch change events, let’s go over some of the most common applications and use cases.

Form Validation

One of the most common uses for change events is form validation. For example, to validate that a field was filled out before submitting:

const form = document.getElementById('signup-form');
const nameInput = document.getElementById('name');

nameInput.addEventListener('change', () => {
  if(nameInput.value === '') {
    showError('Name is required');
  } else {
    clearError();
  }
})

form.addEventListener('submit', (e) => {
  // Check for errors
  if(hasErrors()) {
    e.preventDefault(); // Stop submission
  }
})

We validate on change and prevent form submission if errors. Other examples are validating email format, numbers within a range, matching password fields, and more.

Dynamic Form Elements

Another example is showing or hiding certain form elements based on a value change:

const jobInput = document.getElementById('job-role');
const otherInput = document.getElementById('other-role');

jobInput.addEventListener('change', () => {
  if(jobInput.value === 'other') {
     show(otherInput); // Show other input
  } else {
     hide(otherInput); // Hide 
  }
})

Here we show a special “other” input if the user selects “other” from a job dropdown. Useful for multi-step forms.

Updating Totals / Calculations

Change events are commonly used to update calculations and totals based on input changes:

const qtyInput = document.getElementById('qty');
const totalElement = document.getElementById('total');

qtyInput.addEventListener('change', () => {
  const total = qtyInput.value * productPrice;

  totalElement.innerText = `Total: $${total}`;
}) 

Anytime quantity changes, we recalculate and update the total. Other examples include shopping carts, dynamic charts/graphs, totaling forms, etc.

Data Binding

We can also use change events for data binding to keep frontend and backend in sync:

const model = {
  firstName: 'John'
};

const input = document.getElementById('fname');

input.addEventListener('change', () => {
  // Update frontend
  model.firstName = input.value;

  // Also update backend
  saveToDatabase(model); 
})

This ensures data is saved whenever the user makes a change.

Auto Saving Forms

Along the same lines, we can use change events to autosave forms and input:

const formInputs = document.querySelectorAll('#form input');

formInputs.forEach(input => {
  input.addEventListener('change', () => {
    saveForm(form); // Save form data
  })
})

This way users won’t lose any entered data if they leave the page.

As you can see there are endless useful applications for change events like these. They are a fundamental component of dynamic Javascript UIs and form workflows.

Change Events on Other Elements

It’s worth noting that change events also work on other HTML form elements like <select> dropdowns and <textarea> multi-line inputs:

Select Dropdowns

const select = document.getElementById('car-make'); 

select.addEventListener('change', () => {
  // Runs when dropdown selection changes
})

Textareas

const textarea = document.getElementById('description');

textarea.addEventListener('change', () => {
  // Runs when textarea loses focus after change  
}) 

This allows change handlers for all kinds of configurable form inputs.

Use With Caution

While change events are useful, be careful not to overuse them. Here are some tips:

• Use input or keyup for instant validation/updates as the user types.
• Don’t attach change handlers to every single input unnecessarily. This can impact performance.
• Disable the handler if making programmatic changes to avoid infinite loops.
• Detach handlers when done to avoid memory leaks.

Follow best practices, and change events will become an invaluable tool in your toolkit!

Conclusion

The change event is a highly useful feature of HTML form elements and the Javascript language. Understanding how to listen for, trigger, and leverage change events enables you to provide a rich, dynamic experience in web applications. Change events help make forms and data input interactive. Mastering their usage will level up your Javascript skills and improve your overall web development ability.So next time you need to respond to user changes in a form, reach for the humble but powerful change event!