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13. DOM and Layout Trees


Window Object

Window object is the global ojbect when we run Javascript in web browser. Every objects are stored under window object. There are DOM, BOM, and Javascript under window object.

Document Object Model (DOM)

Represents all page content as objects, so that we can access and modify with Javascript. document object is the main entry point of page.

Browser Object Model (BOM)

Represents additional objects provide by browser.

Process of Render Tree

  1. Combine DOM & CSSOM.

  2. Exclude invisible nodes.
    (<script>, <meta>, nodes with display: none;)

  3. Layout; Comput exact position and size of each objects.

  4. Paint; Renders pixels to the screen.


Everything in HTML source code in included in DOM and represented as an node(object).

  • element node : HTML tags

  • text node

  • comment node

Manipulating DOM


Methods connects between nodes and events.

  • querySelector()
    Returns first element that has specific CSS selector passed.

  • querySelectorAll()
    Returns all elements that has specific CSS selector passed.

  • creatElement()
    Creates new element.

  • setAttribute()
    Set new attributes for element.


    Allows us to interact with page.

  • addEventListner()

Trasversing DOM

We can walk through nodes of DOM using specific methods.

General methods

  • childNodes
    Returns nodelist of child nodes of given elements. Nodelist are live list of nodes, so if we add or remove elements, it automatically updates.

  • firstChild
    Returns first child of given element

  • nodeName
    Returns name of element ex) "div"

  • nodeVale
    Specific for text & comment nodes, returns value of given node.

Element specified methods

Following methods only considers element nodes, which can be useful in certain circumstances.

  • children Returns nodelist of child element nodes of given element
  • parentNode
    Returns parent element node of given element. It is read-only property, which cannot be assigned.

  • firstElementChild
    Element node version of firstChild

  • closest
    Returns closest ancestor element node which has given CSS selector


DOM vs HTML source code

1. DOM is modified by client-side Javascript = "green";
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If we write above snippet in console, the background color changes to green. It means that DOM has changed. But if we check the HTML source code, it hasn't changed. If we refresh the page, change disappears.

2. Browser automatically fixes error in source code

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Above snippet is wrong, because it doesn't have head & body tag. However, DOM automatically fixes it like this.

DOM vs Render Tree

As I mentioned above, render tree is combination of DOM & CSSOM. Also, it excludes element visually hidden, while DOM includes it.

DOM vs What in DevTools

These two are quite similar, but minor difference is that DevTools include additional information plus DOM like CSS pseudo-elements.

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