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WebReference.com - Chapter 17 of JavaScript: The Definitive Guide (4th Ed), from O'Reilly & Associates (15/15)

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JavaScript: The Definitive Guide (4th Ed)

The DOM Range API

The DOM Range API consists of a single interface, Range. A Range object represents a contiguous range[1] of document content, contained between a specified start position and a specified end position. Many applications that display text and documents allow the user to select a portion of the document by dragging with the mouse. Such a selected portion of a document is conceptually equivalent to a range.[2] When a node of a document tree falls within a range, we often say that the node is "selected," even though the Range object may not have anything to do with a selection action initiated by the end user. When the start and end positions of a range are the same, we say that the range is "collapsed." In this case, the Range object represents a single position or insertion point within a document.

The Range object provides methods for defining the start and end positions of a range, copying and deleting the contents of a range, and inserting nodes at the start position of a range. Support for the Range API is optional. At the time of this writing, it is supported by Netscape 6.1. IE 5 supports a proprietary API that is similar to, but not compatible with, the Range API. You can test for Range support with this code:

document.implementation.hasFeature("Range", "2.0");  // True if Range is supported 

Start and end positions

The start and end positions of a range are each specified by two values. The first value is a document node, typically a Document, Element, or Text object. The second value is a number that represents a position within that node. When the node is a document or element, the number represents a position between the children of the document or the element. An offset of 0, for example, represents the position immediately before the first child of the node. An offset of 1 represents the position after the first child and before the second. When the specified node is a Text node (or another text-based node type, such as Comment), the number represents a position between the characters of text. An offset of 0 specifies the position before the first character of text, an offset of 1 specifies the position between the first and second characters, and so on. With start and end positions specified in this way, a range represents all nodes and/or characters between the start and end positions. The real power of the Range interface is that the start and end positions may fall within different nodes of the document, and therefore a range may span multiple (and fractional) Element and Text nodes.

To demonstrate the action of the various range-manipulation methods, I'm going to adopt the notation used in the DOM specification for illustrating the document content represented by a range. Document contents are shown in the form of HTML source code, with the contents of a range in bold. For example, the following line represents a range that begins at position 0 within the <body> node and continues to position 8 within the Text node contained within the <h1> node:

<body><h1>Document Title</h2><body>

To create a Range object, call the createRange( ) method of the Document object:

var r = document.createRange(  ); 

Newly created ranges have both start and end points initialized to position 0 within the Document object. Before you can do anything interesting with a range, you must set the start and end positions to specify the desired document range. There are several ways you can do this. The most general way is to call the setStart( ) and setEnd( ) methods to specify the start and end points. Each is passed a node and a position within the node.

A higher-level technique for setting a start and/or end position is to call setStartBefore( ), setStartAfter( ), setEndBefore( ), or setEndAfter( ). These methods each take a single node as their argument. They set the start or end position of the Range to the position before or after the specified node within the parent of that node.

Finally, if you want to define a Range that represents a single Node or subtree of a document, you can use the selectNode( ) or selectNodeContent( ) method. Both methods take a single node argument. selectNode( ) sets the start and end positions before and after the specified node within its parent, defining a range that includes the node and all of its children. selectNodeContent( ) sets the start of the range to the position before the first child of the node and sets the end of the range to the position after the last child of the node. The resulting range contains all the children of the specified node, but not the node itself.

Manipulating ranges

Once you've defined a range, there are a number of interesting things you can do with it. To delete the document content within a range, simply call the deleteContents( ) method of the Range object. When a range includes partially selected Text nodes, the deletion operation is a little tricky. Consider the following range:

<p>This is <i>only</i> a test 

After a call to deleteContents( ), the affected portion of the document looks like this:

<p>This<i>ly</i> a test 

Even though the <i> element was included (partially) in the Range, that element remains (with modified content) in the document tree after the deletion.

If you want to remove the content of a range from a document but also want to save the extracted content (for reinsertion as part of a paste operation, perhaps), you should use extractContents( ) instead of deleteContents( ). This method removes nodes from the document tree and inserts them into a DocumentFragment (introduced earlier in this chapter), which it returns. When a range includes a partially selected node, that node remains in the document tree and has its content modified as needed. A clone of the node (see Node.cloneNode( )) is made (and modified) to insert into the DocumentFragment. Consider the previous example again. If extractContents( ) is called instead of deleteContents( ), the effect on the document is the same as shown previously, and the returned DocumentFragment contains:

is <i>on</i>

extractContents( ) works when you want to perform the equivalent of a cut operation on the document. If instead you want to do a copy operation and extract content without deleting it from the document, use cloneContents( ) instead of extractContents( ).[3]

In addition to specifying the boundaries of text to be deleted or cloned, the start position of a range can be used to indicate an insertion point within a document. The insertNode( ) method of a range inserts the specified node (and all of its children) into the document at the start position of the range. If the specified node is already part of the document tree, it is moved from its current location and reinserted at the position specified by the range. If the specified node is a DocumentFragment, all the children of the node are inserted instead of the node itself.

Another useful method of the Range object is surroundContents( ). This method reparents the contents of a range to the specified node and inserts that node into the document tree at the position of the range. For example, by passing a newly created <i> node to surroundContents( ), you could transform this range:

This is only a test 

into:

This is <i>only</i> a test 

Note that because opening and closing tags must be properly nested in HTML files, surroundContents( ) cannot be used (and will throw an exception) for ranges that partially select any nodes other than Text nodes. The range used earlier to illustrate the deleteContents( ) method could not be used with surroundContents( ), for example.

The Range object has various other features as well. You can compare the boundaries of two different ranges with compareBoundaryPoints( ), clone a range with cloneRange( ), and extract a plain-text copy of the content of a range (not including any markup) with toString( ). The start and end positions of a range are accessible through the read-only properties startContainer, startOffset, endContainer, and endOffset. The start and end points of all valid ranges share a common ancestor somewhere in the document tree, even if it is the Document object at the root of the tree. You can find out what this common ancestor is with the commonAncestorContainer property of the range.


1. That is, a logically contiguous range. In bidirectional languages such as Arabic and Hebrew, a logically contiguous range of a document may be visually discontiguous when displayed.

2. Although web browsers typically allow the user to select document content, the current DOM Level 2 standard does not make the contents of those ranges available to JavaScript, so there is no standard way to obtain a Range object that corresponds to a user's desired selection.

3. Implementing word processor-style cut, copy, and paste operations is actually more complex than this. Simple range operations on a complex document tree do not always produce the desired cut-and-paste behavior in the linear view of the document.


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Created: November 28, 2001
Revised: November 28, 2001

URL: http://webreference.com/programming/javascript/definitive/chap17/15.html