Question

DOM nodes contain a wealth of links to other nearby nodes. The following diagram illustrates these:

Although the diagram shows only one link of each type, every node has a parentNode property that points to its containing node. Likewise, every element node (node type 1) has a childNodes property that points to an array-like object holding its children.

In theory, you could move anywhere in the tree using just these parent and child links. But JavaScript also gives you access to a number of additional convenience links. The firstChild and lastChild properties point to the first and last child elements or have the value null for nodes without children. Similarly, previousSibling and nextSibling point to adjacent nodes, which are nodes with the same parent that appear immediately before or after the node itself. For a first child, previousSibling will be null, and for a last child, nextSibling will be null.

When dealing with a nested data structure like this one, recursive functions are often useful. The following recursive function scans a document for text nodes containing a given string and returns true when it has found one:

function talksAbout(node, string) {
  if (node.nodeType == document.ELEMENT_NODE) {
    for (var i = 0; i < node.childNodes.length; i++) {
      if (talksAbout(node.childNodes[i], string))
        return true;
    }
    return false;
  } else if (node.nodeType == document.TEXT_NODE) {
    return node.nodeValue.indexOf(string) > -1;
  }
}

console.log(talksAbout(document.body, "book"));
// → true

The nodeValue property of a text node refers to the string of text that it represents.

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.