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DOM (DOM Selectors)

DOM (DOM Selectors)

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MUTEEB MASOODI
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Table of contents

Document Object Model

  • When HTML and CSS get requested the browser creates DOM, When the webpage has loaded, the browser creates DOM which stands for Document Object Model.

  • DOM is something a browser creates to allow us to modify the HTML and CSS.

What is DOM

  • It is a tree data structure that represents the HTML document and nodes of the tree are HTML tags or elements in the document.

  • DOM is an abstraction of the HTML of web pages.

  • The HTML DOM is a standard object model and programming interface for HTML

Why do we need DOM

  • The HTML DOM is a standard for how to get, change, add, or delete HTML elements.

  • change all the HTML elements on the page

  • change all the HTML attributes on the page

  • change all the CSS styles on the page

  • remove existing HTML elements and attributes

  • add new HTML elements and attributes

  • JavaScript can react to all existing HTML events on the page

  • JavaScript can create new HTML events on the page

JavaScript Engine and DOM

  • Each browser has a JavaScript engine, Chrome has a V8 engine, the edge has a Chakra core, safari has JavaScriptCore Etc. They are the ones that read the javascript file and execute it line by line.

  • Web browsers allow us to access the DOM through the document object

  • A document is just an object it has a parent object it is called a window if you open it up in the console you will see it has a bunch of properties including a document

  • A web browser has a window object that has a property 'document' that specifies what should get displayed, to decide what to get displayed, DOM reads the HTML and CSS and then JavaScript is read line by line by something like a JavaScript engine. In Google Chrome that's the V-8 engine reads through it. And if it ever needs to change anything JavaScript can speak with the document object and modify the HTML and CSS

DOM selectors

  • getElementsByTagName

  • getElementsByClassName

  • getElementById

  • querySelector

  • querySelectorAll

  • getAttribute

  • setAttribute

  •       <!DOCTYPE html>
      <html lang="en">
      <head>
          <title>Dom</title>
      </head>
      <body>
      <h1>Shopping List</h1>
      <p id ="first">Get it done</p>
      <p class="second">No excuses</p>
      <ul>
      <li random="23">Notebook</li>
      <li>jelly</li>
      <li>spinach</li>
      <li>rice</li>
      <li>birthday cake</li>
      <li>Candles</li>
      </ul>
      </body>
      </html>

getElementsByTagName

  • it accepts a tag name

  •       //lets select the h1 tag
      document.getElementsByTagName("h1");

getElementsByClassName

  • If you want to find all HTML elements with the same class name, use getElementsByClassName().

  • This example returns a list of all elements with class="second".

  •       document.getElementsByClassName("second")
      //to access
      document.getElementsByClassName("second")[0]

getElementById

  • The easiest way to find an HTML element in the DOM is by using the element id.

  • This example finds the element with id="first"

  •       //there is the reason it doesn't have plural "s" because you can select one id
      getElementsById("first")

querySelector

  • I can select anything here but it only selects the first item it finds

  •       document.querySelector("h1");
      document.querySelector("li");
      //it will select the first item it will find

querySelectorAll

  • it will select all the items, it works with any CSS selector

  •       document.querySelectorAll("li");
      document.querySelectorAll("li,h1");

  • use querySelector and querySelectorAll to do your selection

getAttribute

  • To get the attribute you first have to select the element

  • You can check for the values of anything

  •       document.querySelector("li");
      //but to store it
      document.querySelector("li").getAttribute("random");
      //23

setAttribute

  • What values do you want to change to

  •       document.querySelector("li");
      //but to store it
      document.querySelector("li").setAttribute("random","1000"); //this will return undefined
      //but if you will check it will change the value to 1000
      document.querySelector("li");
      //the value will be changed from "23" to "1000"

Changing styles

  • You can use getAttribute and setAttribute but the more common ways are as under:

style.{property}

  • let's say we want to style h1

  • This has an issue called separation of concerns, we ideally don't want to use it since we want separation that HTML focuses on the text and CSS focuses on the syle

  • There are better ways to do it and they are "className and classList"

  •       document.querySelector("h1").style; //here we will see a bunch of styles but now let's select the element we want a style  "h1" and change the color to the yellow
      document.querySelector("h1").style.background="yellow";

className

  •       var h1 = document.querySelector("h1");
      h1.className ="coolTitle";  //here you can see i have added a class called cool title

classList

//lets add some class to our code
<h1>Shopping List</h1>
<p id ="first">Get it done</p>
<p class="second">No excuses</p>
<ul>
<li class="bold red" random="23">Notebook</li>
<li>jelly</li>
<li>spinach</li>
<li>rice</li>
<li>birthday cake</li>
<li>Candles</li>
</ul>

var h1 = document.querySelector("li").classList;
//you can see that you will get the class list

  • This class list gives us some methods

  • classList.add

  • classList.remove

  • classList.toggle

classList Methods

  •       //use the CSS file that I have mentioned in
      var h1 = document.querySelector("li").classList.add("coolTitle");
      document.querySelector("li").classList.remove("coolTitle");
      document.querySelector("li").classList.add("done");
      document.querySelector("li").classList.toggle("done"); //if you want to turn on/off back and forth

Note

InnerHTML

  •       document.querySelector("h1");
      document.querySelector("h1").innerHtml = "<strong>Not a list</strong>";

Parent Element

  •       document.querySelectorAll("li")[1]; //jello
      document.querySelectorAll("li")[1].parentElement;
      //ul list
      document.querySelectorAll("li")[1].parentElement.parentElement;  //body

Children

  •       document.querySelectorAll("li")[1].parentElement.parentElement.children;  //i will get all the children of the body

It is very important to CACHE selectors in variables

  • And every time we're selecting new things whenever we want to use them.

    Well, this is using up memory because the computer has to remember the browser has to remember that we want to select this and then we're selecting it again and then we're selecting again and we're selecting

    the same thing.

  •       document.querySelectorAll("li")[1].parentElement.parentElement.children;
      document.querySelectorAll("li")[1].parentElement.parentElement.children;
      document.querySelectorAll("li")[1].parentElement.parentElement.children;

  • So the Web browser is doing actions over and over when all we had to do was variable h1 equals

  •       var h1= document.querySelectorAll("h1");

  • So now any time I need to use H1 the Web browser doesn't have to go look up the DOM find H1 and then store it in memory.

  • We have H1 living until we refresh the page so that the Web browser's work is done.

  • It already told us where H1 is and now we can use it. And that's called Cache Selectors.

Exercises

Open SourceJavaScript#codenewbies