First Steps Into Javascript – a Practical Guide 3

Handling DOM events

After we learned the basic of accessing DOM elements and how to modify them, we are ready for the more exciting parts – handling DOM events.

This allows us to make our web way more interactive, and finally we can put our JS knowledge into practice. User input such as keyboard actions or mouse clicks happens one by one and in real time and our web page is expected to respond to it as it happens. This means that web browsers thanks to functions called event handlers actively notify our code when an event is fired.

But first we need to understand in what those DOM events actually consist.

DOM events

Events are signals, that something has happened. In our case, this means that some object in the DOM has generated this kind of signal, but depending on what triggered it, we can group events into different categories. Here is a list o a few:

• Mouse events (click, dblclick, mousewheel, contextmenu, mouseover / mouseout, mousedown / mouseup, mousemove)
• Keyboard events (keypress, keydown, keyup)
• Form element events (submit , focus, blur, change)
• Window element events (scroll, resize, hashchange, load, unload)
• CSS events (transitionend – when a CSS-animation finishes)
• Touch events - triggered on touch devices like smart phones, tablets (touchstart, touchmove, touchend, touchcancel)
• Hint: The names of those properties are case-sensitive, so remember to always use “onclick” and not “ONCLICK”.

Event handlers

Alright, so now we know which events to look for. But how can we react in case an event is triggered? By using event handlers, functions that get executed in case an event is triggered. They are responsible for pairing the DOM element with the event.

There are different ways of assigning those handlers. We will take a look at the the most common and preferred way, which is to use the addEventListener method. This method, in comparison to the other options of how to assign handlers, allows you to add an unlimited number of handlers.

Complementing the addEventListener method, we have the removeEventListener . Which is used to unregister a handler.

Let’s see a simple example of how assigning a handler looks like:

The addEventListene method takes two arguments: the event to listen for and the function to be executed when that event is emitted. Let's start with an example: adding a click listener to a button.

myButton.addEventListener('click', () = {
myButton.classList.add(''text-white');
}
);

This example shows us how to change the style of a button after it was clicked. (The event we listen for, in this case, is ‘click’).But what are these events?

Event objects

Every time a user interacts with his web browser, the browser creates an event object. These objects hold additional information about the event, such as their related data and functionality as properties and methods.

One of those properties is the target event property. It returns the element that triggered the event, this means the element on which the event originally occurred.

Here are some usage examples:

• If we want to know which element triggered a specified event, we can use the event.target property together with the element.tagName property:
• If we want to know which mouse button was pressed, we can look at the event object’s ‘button’ property.
• If we want to know the exact coordinates where a mouse event happened, there is an event object property we can use to obtain them.

OK, that’s enough theory, now let’s practice and use our knew knowledge with validating user input.

Changing input field’s style during user data input

Imagine we have a simple form, where the user is supposed to enter a password of his choosing, but we require that its length be between 5 and 10 characters. As we already know, the input event gets triggered every time data is entered into the input element. So, even during the time that the user is entering his password, character after character, we can set the event listener so that the form dynamically shows the user (using the color of the inputs border) if his password meets the requirements. In other words, we change the style of the input element from red border to green border). How do we proceed?

Our simple and most basic HTML markup could look for example like this;

<form>
<input id="password-input" type="password" />
<form>

And now for the JS part:

First we need to get access to the input element, so we create a constant which hold a reference to the input element by calling the getElementById function.

const password = document.getElementById('password-input');

Now, we want to capture the event object, so we add an event listener to the input field. Then we add a conditional statement that checks the length property of $event.target.value and changes the border using the style property.

password.addEventListener('input', ($event) =>{
  if($event.target.value.length >= 5 && $event.target.value.length <= 10 ){
     password.style.border = "1px solid green"; 
    } else {
      password.style.border = "1px solid red"; 
    }
});

And that’s how we can give the user visual feedback when he enters data.

Changing submit button’s attribute during user data input

The exercise we did so far was simple and easy. But it doesn’t really prevent invalid data to be submitted. How about we add a submit button, with a “disabled” attribute? We can then use a similar approach as we did with the borders to change its state to enabled when our requirements are met.

Now we add a submit button to our HTML part and it looks like something this:

   <form>
      <input type="password" id="password-input">
      <button type="submit" id="submit-button" disabled="true">Submit</button>
</form>

Our little script will grow a bit: similar to how we did before, we save a reference to the DOM elements we want to access in constants. Apart from the input field, we also need to gain access to the form button:

  const password = document.getElementById('password-input');
const button = document.getElementById('submit-button');

Now, in the following lines of code, we, as before, capture the event object which gets triggered upon every data input from the user, and then we access the value of the length using the target property. 

  password.addEventListener('input', ($event) =>{
  if($event.target.value.length >= 5 && $event.target.value.length <= 10 ){
     button.removeAttribute("disabled"); 
    } else {
      button.setAttribute("disabled","true"); 
    }
});

The code in the else statement makes sure the attribute is set to disabled and only when our conditions are met, we remove the disabled attribute from our submit button. Otherwise, It is interesting to observe that instead of setting the attribute to false, we rather use the removeAttribute method.

Changing input field’s style after user data input

To complete our simple user input validation scripts, we can add another interesting yet simple feature. Imagine we need to check if two input fields match, like it happens when we want the user to chose a password and then to repeat the same password to make sure he didn’t make a type-o in the first input field.

This validation needs to happen after both input fields are filled out, so the event which suits our needs most is “blur”, which gets fired when the element is out of focus (the user tabbed or clicked away from the input field).

The HTML part for our purposes can be as simple as this:

    <form>
              <input type="password" id="password-input">
              <input type="password" id="confirm-password">
 </form>

Now to our script:

Similar as we did before, we access our input fields and store the reference:

  const password1 = document.getElementById('password');
const password2 = document.getElementById('confirm-password');

Then we attach a blur event listener to our second password input field and add the condition of the two password being equal.

Here we don’t need to capture the event, because we do not need it to check if our condition is met, we only need to compare the two passwords’ input values.

  password2.addEventListener("blur", ()  =>{
if(password1.value === password2.value){
   password1.style.border = "1px solid green"; 
   password2.style.border = "1px solid green"; 
} else{
   password1.style.border = "1 px solid red"; 
  password2.style.border = "1 px solid red"; 
} 
});

Conclusion

To make our web pages more interactive, we familiarized ourselves with concepts like events and event handlers. An easily understandable and practical way of learning how to use events is by writing simple scripts to validate user input.