Events

Events are functions that get executed by the browser when the user does some action.

Objectives

• Add events to elements in the DOM

Context

Now that we know how to select DOM elements, we can attach events to them:

• Common Events:

  • change
  • click
  • mouseover
  • mouseout
  • keydown
  • keyup

• List of Event Types

How Events Work

• User does some action ( or something happens in the browser that a listener is set to )
• Browser runs the function associated with that event
• Just like setTimeout and setInterval, we specify something to be run at a later time.

addEventListener

We have different events, but now we need a way to attach these events to elements. The solution? Use the addEventListener function. This function is called on DOM elements and takes two parameters: an event type and a function. The event type refers to a "click", "mouseover", or other type of event. The function contains the code that runs when the event occurs.

console.log("define the event handler");

var clickHandler = function(event){
  console.log("click happened");
};

console.log("about to set event handler on element");

var button = document.querySelector('#my-button');
button.addEventListener('click', clickHandler);

console.log("done setting handler");

Connecting events to the DOM

How do we refer to an element that was clicked on?

With the tools we have now we can do this:

document.querySelector("#user-input")
        .addEventListener('change',function(){
          // we have a click event

          // re-get the element that was clicked on
          var input = document.querySelector('#user-input');
        });

But we can do better and not repeat ourselves.

Method 1: event parameter

• the browser, when executing your callback function, will pass in the event object parameter.
• A dom event object. Contains everything about the event that occured:
• where it occured from
• pixel coordinates for the event
• etc., depending on the type of event
• the DOM element that originated the event will be set to event.target

Example 1: User Text Input

If you have this HTML:

<input id="user-input"/>

You can write this in your javascript:

document.querySelector("#user-input")
        .addEventListener('change',function(event){

          // event is the DOM context for the event
          console.log(event.target);
        });

event.target parameter to the callback contains the element

Example 2: generic method for dealing with an element

What if we have this HTML:

<div id="motorbike">honda</div>
<ul>
  <li id="tomatoes">tomatoes</li>
  <li id="lettuce">lettuce</li>
  <li id="bananas">bananas</li>
  <li id="pinapples">pineapple</li>
  <li id="durian">durian</li>
</ul>

Inside our callback functions, we want a way to access each individual element value but in a generic way.

Obviously we would treat our motorbike div differently, but what if we have the generic produce element, but they each have a different value inside?

var dealWithProduce = function(){
  // How do we complete the string?
  // console.log(' are yummy!!!');
};

// select a set of elements
var listItems = document.querySelectorAll("li");

// set an event listener on each element
for(var i = 0; i < listItems.length; i++){
    listItems[i].addEventListener("click", function() {
      console.log("hello!");
    });
}

We can use event.target to reference our specific context of the element that was clicked on.

this

this keyword is the current context of the function being executed

var dealWithProduce = function(){
  console.log( this );
};

Only the this keyword can be used to set attributes of the element context.

Note: Events can only be attached to specific elements. Therefore, when you return a collection such as the result of document.querySelectorAll you CANNOT simply do this:

var items = document.querySelectorAll("button")
items.addEventListener("click", function(event) {
    console.log("Click worked");
})

Instead you must loop through all of the elements and attach an event to each item individually.

var items = document.querySelectorAll("button")
for(var i = 0; i < items.length; i++){
    items[i].addEventListener("click", function(event) {
        console.log("Click worked");
    });
}

Pairing Exercise: (15 minutes)

Part 1

Create a new html and javascript file.

Using this function:

var saySomething = function(){
  console.log("YES!");
};

And this HTML:

<h1>Hello</h1>
<input id="search" />

• set the click event to the h1
• set mouseover to that element
• set keyup or keydown to the input
• try setting some different kinds of events from MDN on these elements.
• try writing some other elements in the HTML file and setting events on them.

List of Event Types

Part 2

• Start a new index.html and script.js file
• Copy the above code for this and event.
• Run it and see the value of this and event for each time you click.
• Add new elements
• Add a class to the li elements, change selector to class name, run it again
• Add a non-li element with the same class name, run it again

Extra: Method Chaining

• For DOM methods and events we can use the method chaining syntax so that we don't have to capture return values and call methods on those assigned values.

Without method chaining:

var element = document.querySelector("#myDiv");

element.addEventListener("click", function(event) {
    //Your code here
});

With method chaining

document.querySelector("#myDiv").addEventListener("click", function(event) {
    //Your code here
});

Another syntax

document.querySelector("#myDiv")
        .addEventListener("click", function(event) {
          //Your code here
        });
        // be sure to indent the dot method to the previous one