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Learn how to create, update, and modify a web page with the Javascript DOM Interface.

JavaScript and the DOM

The *Document Object Model*, abbreviated DOM, is a powerful tree-like structure that organizes the elements on a web page and allows scripting languages to access them. This lesson will focus on some of the most useful methods and properties of the [DOM interface](https://developer.mozilla.org/en-US/docs/Web/API/Document_Object_Model) in JavaScript. This interface is implemented by every modern browser.

First things first! The `document` object in JavaScript is the door to the DOM structure. The `document` object allows you to access the root node of the DOM tree. Before you can access a specific element in the page, first you must access the document structure itself. The `document` object allows scripts to access children of the DOM as properties.

For example, if you want to access the `<body>` element from your script, you can access it as a property of the `document` object by using `document.body`. This property will return the body element of that DOM. 

Similarly, you could access the `<title>` element with the `.title` property. 
Here is a [comprehensive list](https://developer.mozilla.org/en-US/docs/Web/API/Document) of all `document` properties.

Diagram that presents a sample HTML file and its corresponding layout using the DOM Interface

The Document Keyword

When using the DOM in your script to access an HTML element, whether it's an `<li>` element or the entire `<body>` element, you also have access to all of that element's properties.

This includes the ability to modify the contents of the element as well as its attributes and properties, which can range from modifying the text inside a `<p>` element to assigning a new background color to a `<div>`. For example, the `.innerHTML` property allows you to access and set the contents of an element.

Let's take a look at how we can reassign the contents of the `<body>` element to the text `'The cat loves the dog'`:

```js
document.body.innerHTML = 'The cat loves the dog.';
```

The `.innerHTML` property can also add any valid HTML elements. The following example replaces the contents of the `<body>` element by assigning an `<h2>` element as a child inside the `<body>` element:

```js
document.body.innerHTML = '<h2>This is a heading</h2>'; 
```

Tweak an Element

In this lesson, you manipulated a webpage structure by leveraging the Document Object Model (DOM) interface in JavaScript.

Let's review what we learned:
+ The `document` keyword grants access to the root of the DOM in JavaScript.
+ The DOM Interface allows you to select a specific element with CSS selectors by using the `.querySelector()` method.
+ You can access an element directly by its ID with the `.getElementById()` method which returns a single element.
+ You can access an array of elements with the `.getElementsByClassName()` and `.getElementsByTagName()` methods, then call a single element by referencing its placement in the array.
+ The `.innerHTML` and `.style` properties allow you to modify an element by changing its contents or style respectively.
+ You can create, append, and remove elements by using the `.createElement()`,`.appendChild()` and `.removeChild()` methods respectively.
+ The `.onclick` property can add interactivity to a DOM element based on a click event.
+ The `.children` property returns a list of an element's children and the `.parentNode` property returns the element's closest connected node in the direction towards the root.

Review

Let's recap the parent and children relationship in the DOM hierarchy. A _parent node_ is any node that is a direct ancestor of another node. A _child node_ is a direct descendant of another node, called the parent node.

These relationships follow the nested structure of the HTML code. Elements nested within one HTML element are children of that parent element.

Each element has a `.parentNode` and `.children` property. The [`.parentNode` property](https://developer.mozilla.org/en-US/docs/Web/API/Node/parentNode) returns the parent of the specified element in the DOM hierarchy. Note that the `document` element is the _root node_ so its `.parentNode` property will return `null`. The `.children` property returns an array of the specified element's children. If the element does not have any children, it will return `null`.

```html
<ul id='groceries'>
  <li id='must-have'>Toilet Paper</li>
  <li>Apples</li>
  <li>Chocolate</li>
  <li>Dumplings</li>
</ul>
```

In the HTML code above, we have an `<ul>` element with the ID of `groceries` with four `<li>` elements inside. 

```js
let parentElement = document.getElementById('must-have').parentNode; // returns <ul> element
let childElements = document.getElementById('groceries').children; // returns an array of <li> elements
```

Here, the `parentElement` variable stores the `.parentNode` of the `<li>` element with the ID of `must-have`, which will be the `<ul>` element with the ID of `groceries`. The `childElements` variable is set to the children of the `<ul>` element with the ID of `groceries`, which will be an array of four `<li>` elements. 

Traversing the DOM

You can add interactivity to DOM elements by assigning a function to run based on an [event](https://developer.mozilla.org/en-US/docs/Web/Events). Events can include anything from a click to a user mousing over an element. We will learn more about events in the upcoming [DOM Events with JavaScript](https://www.codecademy.com/courses/build-interactive-websites/lessons/dom-events) lesson. For now, let's take a look at how to modify an element when a click event happens.

The `.onclick` property allows you to assign a function to run on when a click event happens on an element:

```js
let element = document.querySelector('button');

element.onclick = function() { 
  element.style.backgroundColor = 'blue' 
};
```

You can also assign the `.onclick` property to a function by name:

```js
let element = document.querySelector('button');

function turnBlue() {
   element.style.backgroundColor = 'blue';
}

element.onclick = turnBlue;
```

In the above example code, when the `<button>` element detects a click event, the `backgroundColor` will change to `'blue'`.

Add Click Interactivity

In addition to modifying or creating an element from scratch, the DOM also allows for the removal of an element. The `.removeChild()` method removes a specified child from a parent.

```js
let paragraph = document.querySelector('p');
document.body.removeChild(paragraph);
```

In the above example code, the `.querySelector()` method returns the first paragraph in the document. Then, the `paragraph` element is passed as an argument of the `.removeChild()` method chained to the parent of the paragraph&mdash;`document.body`. This removes the first paragraph from the document body.

If you want to hide an element rather than completely deleting it, the [`.hidden` property](https://developer.mozilla.org/en-US/docs/Web/API/HTMLElement/hidden) allows you to hide it by setting the property as `true` or `false`:

```js
document.getElementById('sign').hidden = true;
```

The code above did not remove the element with ID of `'sign'` from the DOM but rather, hid it.

Remove an Element

In the previous exercise, we accessed the `<body>` element with the `document` keyword!

What if we wanted to select a specific element besides the entire `<body>` element? The DOM interface allows us to access a specific element with CSS selectors.

_CSS selectors_ define the elements to which a set of CSS rules apply, but we can also use these same selectors to access DOM elements with JavaScript! Selectors can include a tag name, a class, or an ID.

The `.querySelector()` method allows us to specify a CSS selector as a string and returns the first element that matches that selector. The following code would return the first paragraph in the document.

```js
document.querySelector('p');
``` 

Along with `.querySelector()`, JavaScript has more targeted methods that select elements based on their class, id, or tag name.

For example, if you want to access an element directly by its `id`, you can use the aptly named `.getElementById()` method:

```js
document.getElementById('bio').innerHTML = 'The description';
``` 

In this example, we’ve selected the element with an ID of `'bio'` and set its `.innerHTML` to the text `'The description'`. Notice that the ID is passed as a string, wrapped in quotation marks (`' '`).

There are also the `.getElementsByClassName()` and `.getElementsByTagName()` methods which return an array of elements, instead of just one element. You can use bracket notation to access individual elements of an array:

```js
// Set first element of .student class as 'Not yet registered'
document.getElementsByClassName('student')[0].innerHTML = 'Not yet registered';

// Set second <li> tag as 'Cedric Diggory'
document.getElementsByTagName('li')[1].innerHTML = 'Cedric Diggory`;
```

In the above example code, the first element with the `'student'` class and the second `<li>` element have had their inner HTML changed.

Select and Modify Elements

Another way to modify an element is by changing its CSS style. The `.style` property of a DOM element provides access to the inline style of that HTML tag.

The syntax follows an `element.style.property` format, with the `property` representing a CSS property. For example, the following code selects the first element with a `class` of `blue` and assigns blue as the `background-color`:

```js
let blueElement = document.querySelector('.blue');
blueElement.style.backgroundColor = 'blue';
```

Unlike CSS, the DOM `.style` property does not implement a hyphen such as `background-color`, but rather camel case notation, `backgroundColor`. Check out this [W3 Reference on the HTML DOM style object](https://www.w3schools.com/jsref/dom_obj_style.asp) to see a list of how CSS properties are converted into JavaScript. 

The following _chaining_ syntax would also work:

```js
document.querySelector('.blue').style.fontFamily = 'Roboto';
```

Style an Element

Just as the DOM allows scripts to modify existing elements, it also allows for the creation of new ones.

The [`.createElement()` method](https://developer.mozilla.org/en-US/docs/Web/API/Document/createElement) creates a new element based on the specified tag name passed into it as an argument. However, it does not append it to the document. It creates an empty element with no inner HTML.

```js
let paragraph = document.createElement('p');
```

In the example code above, the `.createElement()` method takes `'p'` as its argument which creates an empty `<p>` element and stores it as the `paragraph` variable.

We can assign values to the properties of the newly created element like how we've done previously with existing elements.

```js
paragraph.id = 'info'; 
paragraph.innerHTML = 'The text inside the paragraph';
```

Above, we use the `.id` property to assign `'info'` as ID and the `.innerHTML` property to set `'The text inside the paragraph'` as the content of the `<p>` element.

In order to create an element and add it to the web page, you must assign it to be the child of an element that already exists on the DOM, referred to as the parent element. We call this process _appending_. The `.appendChild()` method will add a child element as the parent element’s last child node. The following code appends the `<p>` element stored in the `paragraph` variable to the document body.

```js
document.body.appendChild(paragraph);
```

The `.appendChild()` method does not replace the content inside of the parent, in this case, `body`. Rather, it appends the new element as the last child of that parent.