Question

Think back to the syntax trees from Chapter 11 for a moment. Their structures are strikingly similar to the structure of a browser’s document. Each node may refer to other nodes, children, which in turn may have their own children. This shape is typical of nested structures where elements can contain sub-elements that are similar to themselves.

We call a data structure a tree when it has a branching structure, has no cycles (a node may not contain itself, directly or indirectly), and has a single, well-defined “root”. In the case of the DOM, document.documentElement serves as the root.

Trees come up a lot in computer science. In addition to representing recursive structures such as HTML documents or programs, they are often used to maintain sorted sets of data because elements can usually be found or inserted more efficiently in a sorted tree than in a sorted flat array.

A typical tree has different kinds of nodes. The syntax tree for the Egg language had variables, values, and application nodes. Application nodes always have children, whereas variables and values are leaves, or nodes without children.

The same goes for the DOM. Nodes for regular elements, which represent HTML tags, determine the structure of the document. These can have child nodes. An example of such a node is document.body . Some of these children can be leaf nodes, such as pieces of text or comments (comments are written between <!-- and --> in HTML).

Each DOM node object has a nodeType property, which contains a numeric code that identifies the type of node. Regular elements have the value 1, which is also defined as the constant property document.ELEMENT_NODE . Text nodes, representing a section of text in the document, have the value 3 ( document.TEXT_NODE ). Comments have the value 8 ( document.COMMENT_NODE ).

So another way to visualize our document tree is as follows:

The leaves are text nodes, and the arrows indicate parent-child relationships between nodes.

This is a book about getting computers to do what you want them to do. Computers are about as common as screwdrivers today, but they contain a lot more hidden complexity and thus are harder to operate and understand. To many, they remain alien, slightly threatening things.