mustafa-al-shaheri / fewpjs-event-listening-recoded-sanaa-2019

Interacting with the DOM via JavaScript Event Listeners

Learning Goals

• Demonstrate triggering event listeners on DOM nodes with addEventListener()

Introduction

We know how to manipulate nodes in the DOM by using JavaScript. We also know that events can be handled and can do work inside callback functions when the event is triggered. JavaScript programmers often say we are "listening" for an event in order to "execute" or "call" a callback function.

In order to teach nodes (or, "elements") to "listen" for an event, we use addEventListener(). It allows us to associate "hearing" an event with executing a callback.

Demonstrate Defining Event Listeners on DOM Nodes with addEventListener()

addEventListener() takes two arguments:

1. the name of the event
2. a callback function to "handle" the event

Take a look at index.html in the browser. When you click in the <input> area, nothing happens. Now let's set up some event handling.

Start by adding an event listener for the click event on the input#input element in index.html.

Try out the following in the Chrome DevTools console:

const input = document.getElementById('input');
input.addEventListener('click', function(event) {
  alert('I was clicked!');
});

When you click inside of input#input, now, you will get an alert box.

Take this code and paste it into the index.js file's addingEventListener function.

Let's review the parts of this code:

1. The node that will be doing the listening. We store that node in the input constant
2. The invocation of addEventListener on the node that will be doing the listening
3. The first argument is the event name to listen for
4. The second argument is the callback function. It's work that will be executed when the node "hears" the event

Conclusion

When you have pasted the solution into the addingEventListener function, your tests should pass when you run learn. This means you can run learn submit and move to the next lesson!

Resources

• MDN - Web Events
• StackOverflow - Bubbling and Capturing
• QuirksMode - Event order

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