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JS HTML DOM Methods

Week 1 Keywords and Questions

  • What is the DOM?
  • What kind of events are there?
  • What is an event listener?
  • How do I select something in the DOM using JS?

Prerequisites

Motivation

Javascript is one of the most popular programming languages. Earlier Javascript was only used for designing interactive web pages, but now it is used for Full Stack Development. One of many methods of running Javascript is through the web browser, in which we interact with elements on the page. When a web page is loaded, the browser creates a Document Object Model of the page.

Prominent companies like Netflix, Walmart, and PayPal have built their entire applications around JavaScript.

Objectives

Participants will be able to:

  • Write HTML code and make changes with JavaScript
  • Incorporate methods on the DOM to make web pages interactive
  • Select HTML elements by their element type, class, and ID with JavaScript
  • Learn ways to link JavaScript to HTML
  • Learn to use element properties like .insertAdjacentElement()
  • Implement classes & attributes by using .setAttribute() & .getAttribute()
  • Walk through some methods to traverse and delete nodes

Specific Things to Learn

  • How to incorporate JavaScript into a webpage
  • How to select HTML elements
  • Common and useful JavaScript methods for basic interactivity
  • Dynamically add or remove HTML elements

Lesson

Things to Remember

  • Remember to use . to select classes and # to select ids.
  • There are several JavaScript methods that achieve similar goals but through different means. You should understand how they're different so you can use them appropriately.
    • .appendChild() vs .insertAdjacentText()
  • "Vanilla JavaScript" is just a cute way of saying plain JavaScript without any extra libraries; an example of a library is jQuery. Vanilla JavaScript is not its own special form of JavaScript.

DOM Event Basics

DOM stands for Document Object Model. Each event in DOM has an event handler, aka event listener, where developers can add JavaScript to set up any automatic actions they would like to trigger once the event takes place. An example of an event is a button click. An event listener could be given a function that changes the background color of the webpage when the button is clicked.

Some common examples of events are:

  • Mouse events (MouseEvent): mousedown, mouseup, click, dblclick, mousemove, mouseover, mousewheel, mouseout, contextmenu
  • Window events: scroll, resize, hashchange, load, unload
  • Keyboard events (KeyboardEvent): keydown, keypress, keyup

.addEventListener() Function

The addEventListener() method allows us to add event listeners on the HTML DOM object such as Document, Element, or Window. The first argument in addEventListener() should be a string, then the name of the event it should listen for. The second argument in addEventListener() is either a function to call when the event is fired or an object that implements handleEvent.

Syntax: target.addEventListener(type, listener [, options]);

In the previous example, the first argument would be the 'click' event on the button, and the second argument is a function that changes the page background color when the 'click' event fires.

Here's an example with a function as the listener:

const button = document.querySelector('button');
button.addEventListener('click', function () {
  document.body.style.backgroundColor = 'blue';
});

On button click, the background of the body will be changed to blue.

Here's an example with an object as the listener:

const ColorChangeButton = function (element) {
  this.handleEvent = function (event) {
    switch (event.type) {
      case 'click':
        document.body.style.backgroundColor = 'blue';
        break;
      case 'dblclick':
        document.body.style.backgroundColor = 'orange';
        break;
    }
  };

  // Note that the listeners in this case are |this|, not this.handleEvent
  element.addEventListener('click', this, false);
  element.addEventListener('dblclick', this, false);

  // You can properly remove the listeners this way:
  element.removeEventListener('click', this, false);
  element.removeEventListener('dblclick', this, false);
};
const s = new ColorChangeButton(document.body);

Also note in the above example that if you no longer need a particular event listener, you can remove it using .removeEventListener().

Event Bubbling and Propagation

Event propagation is the flow in which the events propagate to reach its target. It is the process of calling all the listeners for the given event type. We can register several listeners on one node for the same event type. The listeners are called in the order of their registration on reaching the node. The flow of propagation can be to the event target from the window and back, thus making it bi-directional. The stopPropagation() method prevents the further propagation of any other event.

Syntax: event.stopPropagation()

Event bubbling is a way of event propagation. It refers to the order in which event handlers are called when an event is nested inside another event and both elements implement a listener for the same event. When an event takes place on an element, first the handlers are run on the inner-most element and then are successively run on its parent till it reaches the outer-most DOM element, like a bubble in water; this is known as event bubbling.

Event Bubbling

In the above diagram, if we are implementing a listener for 'click' event that runs an alert() function on the button as well as on the div and body, then first the event handler is run on the button, then on the div and then the body, as the button is nested inside the div, which is again nested inside the body. On clicking the button we get three alerts, one for the button, one for the outer div, and one for the body(button -> div -> body). So here the event "bubbles" from the innermost element to its parents.

Guided Practice

Work through the JavaScript Event Propagation on TutorialRepublic

Or, work through the JavaScript and DOM tutorial on Treehouse if you have a SF Public Library card

If you aren't using Treehouse, follow this Vanilla DOM Manipulation - App Academy tutorial instead. Try the 5-question quiz that comes after as well. If you don't know an answer after a minute or two, just move on, and try it again at the end of this outline.

Note: Our goal is to only provide free resources, so if you can't get Treehouse for free, don't use it. We have a lot of instances of Treehouse in the curriculum, but now that we’ve expanded beyond the Bay Area not every participant has free or inexpensive access. If you're in the Bay Area you will be able to get Treehouse through the SF Public Library, but you need a library card.

Activity 1 - Trying Out Some important JavaScript DOM methods

For each of the following links, read the code and play around with its functionality. You can refresh the webpage to reset it.

Here is some code and a sandbox for .getElementById() & .innerHTML showing the difference between method and property.

Here is some code and a sandbox for .getElementsByTagName().

Here is some code and a sandbox for .createElement().

Here is some code and a sandbox for .appendChild().

Here is some code and a sandbox for .removeChild().

Here is some code and a sandbox for .getAttribute().

Here is some code and a sandbox for .setAttribute().

Here is some code and a sandbox for .addEventListener().

Here is some code and a sandbox for event bubbling.

Here is some code and a sandbox for .stopPropagation().

Activity 2 - JavaScript DOM method Example

  1. Create a new directory inside your assignments directory and give it the name js-dom-play.

  2. Navigate into js-dom-play. Create a file called index.html.

  3. Read this example code and talk through it with your pair partner. Once you understand what it does, copy it into index.html using TextEdit or WordPad and save the file.

  4. Open index.html in a new Chrome tab. What do you see? Is this what you expected, based on the code you read?

  5. Create a new directory also inside js-dom-play called static. Navigate to static. Remember that all CSS and JS files go in this static folder.

  6. Create a file in the static directory called script.js.

  7. Read this example code and talk through it with your pair partner. Once you understand what it does, copy it into script.js and save the file.

  8. Refresh the Chrome tab that is currently rendering index.html. What happens? What do you see? What are you now able to do?

Text Editor Note: 'Rich text' is something that is often turned on by default in TextEdit or WordPad, but it does not play nicely with JS. One thing it does, for example, is turn quotes into curly quotes. See if you can dig into settings and make sure it's set to 'plain text' instead, or the rich text may start causing problems. Make sure you re-save your files to use plain text too if you were able to change the setting.

Activity 3 - Customize the JS DOM Method Example

  1. Using the JavaScript documentation or other resources as your guide, add another div with a class containing an image, a paragraph describing the image, and a button with JavaScript functionality.

  2. Using the Javascript documentation or other resources as your guide, add a list in between the two divs using appropriate methods.

Supplementary Materials

Check for Understanding

  1. Where in an HTML document do you link an external JavaScript file and why?
  2. What is the DOM?
  3. Name at least 5 methods for making changes to the DOM you used or saw today.
  4. Where would you look to find more JavaScript DOM methods?
  5. What are different ways of traversing the DOM?