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HTML DOM Manipulation with JavaScript

Intro

The DOM (Document Object Model) is an interface, where you can access and manipulate content, structure and style of a website.

We will have a look at the following topics:

  • What is the DOM?
  • How can we manipulate it?

What is the DOM?

When we open a HTML file in our browser, the DOM gets created by the browser, so the DOM is the representation of a HTML document.

HTML elements, e.g. <body></body>, <p></p>, become so-called nodes. Nodes have relationships to each other and are structured as a tree.

The root of the DOM is called document. By using JavaScript, the DOM can be accessed and manipulated.

Accessing a Node / HTML element

There are various way to access a single node.

querySelector

querySelector is a method to input a complete query and get the first occurrence back.

// access the body tag
document.querySelector("body");

// access the "button"-class
document.querySelector(".button");

// access the "footer"-id
document.querySelector("#footer");

As we can see, we can access an element by tag(body), by class(.button) or by id(#footer).

querySelectorAll

querySelectorAll is a method to input a complete query and get all occurrences back.

// access the "button"-class
document.querySelectorAll(".button");

getElementById

getElementById is a method to get a single element with a specific id.

document.getElementById("header");

We get the element that has the id header. We don’t need # like when using querySelector.

getElementsByClassName

getElementsByClassName is a method to get all elements with a specific class.

document.getElementsByClassName("button");

We get the elements that has the class button. We don’t need . like when using querySelector.

Like we can see in the method name, Elements is plural, therefore we get a collection of HTML elements back, not a single element, even if it is empty.

getElementsByTagName

getElementsByTagName is a method to get all elements with a specific HTML tag.

document.getElementsByTagName("p");

We get the elements that has the HTML tag p.

Like we can see in the method name, Elements is plural, therefore we get a collection of HTML elements back, not a single element, even if it is empty.

What do I use?

I use querySelector and querySelectorAll, because both can be used with tag, class and id. I don’t want to change multiple lines when I change an id into a class.

There is a lot of legacy and transpiled code, therefore you should know how to use all the other methods, too.

More complex queries

Sometimes you have to find more complex stuff

// a tag with a class
document.querySelector("body.main");
// <body class="main">

// a class as a child in a tag
document.querySelector("body > .text");
// <body><p class="text">...</p></body>

// a class anywhere in a tag
document.querySelector("body .text");
// <body><section><p class="text">...</p></section></body>

// a parent
document.querySelector("body").parentNode;

// all children
document.querySelector("body").childNodes;

// first child
document.querySelector("body").firstChild;

// last child
document.querySelector("body").lastChild;

// previous sibling
document.querySelector("body").previousSibling;

// next sibling
document.querySelector("body").nextSibling;

Manipulating elements in the DOM

// change text content of a node
document.querySelector(".text").textContent = "Hello";

// change HTML content
document.querySelector(".text").innerHTML = "<p>Hello</p>";

// change source of an image
document.querySelector(".logo").src = "lion.jpeg";

Creating new elements in the DOM

// create a new element and store it into a variable
const newParagraph = document.createElement("p");

// add text to the newly created paragraph
newParagraph.textContent = "Hello";

// find container where the new paragraph should live in
const container = document.querySelector("body");

// add new paragraph to container
container.appendChild(newParagraph);

Adding an Event Listener to the DOM

<!-- a simple button -->
<button class="my-cool-button">
  Click Me
</button>
// find the button in the DOM by using the class
const myButton = document.querySelector(".my-cool-button");

// add a listener to the button, that waits for a "click" event,
// then run the following function
myButton.addEventListener("click", function() {
  alert("Hi");
});

Outro

These are just a few of the ways to work with the DOM to give you a small primer.

If you want to dive deeper into the topic, read this free book.