Введение

В этом разделе представлено краткое концептуальное введение в DOM: что это такое, как он предоставляет структуру для HTML и XML-документов, как вы можете получить к нему доступ, и как этот API представляет справочную информацию и примеры.

Что такое DOM?

Document Object Model (DOM) - это программный интерфейс для HTML и XML-документов. Он представляет страницу, чтобы программы могли изменять структуру, стиль и содержание документа. DOM отображает страницу ввиде узлов и объектов. Таким образом, языки программирования могут взаимодействовать со страницей.

Веб-страница - это документ. Этот документ можно либо отобразить в окне браузера, либо в качестве источника HTML. Но в обоих случаях это один и тот же документ. Объектная модель документа (DOM) представляет этот же документ, таким образом, чтобы им можно было манипулировать. DOM представляет собой объектно-ориентированное представление веб-страницы, которое может быть модифицировано с помощью скриптовых языков, таких как JavaScript.

Стандарты W3C DOM и WHATWG DOM реализованы в большинстве современных браузеров. Многие браузеры расширяют стандарт, поэтому следует проявлять осторожность при использовании их в Интернете, где к документам могут обращаться различные браузеры с различными DOM.

Например, стандарт DOM указывает, что метод getElementsByTagName в коде ниже должен возвращать список всех <p> элементов в документе:

var paragraphs = document.getElementsByTagName("P");
// paragraphs[0] первый <p>-элемент
// paragraphs[1] второй <p>-элемент и т.д.
alert(paragraphs[0].nodeName);

Все свойства, методы и события, доступные для управления и создания веб-страниц, организованы в объекты (например, объект документа, который представляет сам документ (document), объект таблица (table), который реализует специальный интерфейс DOM HTMLTableElement для доступа к таблицам HTML и т. д. ). Эта документация предоставляет по-объектную справку по DOM, реализованную в браузерах на базе Gecko.

DOM и JavaScript

The short example above, like nearly all of the examples in this reference, is JavaScript. That is to say, it's written in JavaScript, but it uses the DOM to access the document and its elements. The DOM is not a programming language, but without it, the JavaScript language wouldn't have any model or notion of web pages, HTML documents, XML documents, and their component parts (e.g. elements). Every element in a document—the document as a whole, the head, tables within the document, table headers, text within the table cells—is part of the document object model for that document, so they can all be accessed and manipulated using the DOM and a scripting language like JavaScript.

In the beginning, JavaScript and the DOM were tightly intertwined, but eventually they evolved into separate entities. The page content is stored in the DOM and may be accessed and manipulated via JavaScript, so that we may write this approximative equation:

API (HTML or XML page) = DOM + JS (scripting language)

The DOM was designed to be independent of any particular programming language, making the structural representation of the document available from a single, consistent API. Though we focus exclusively on JavaScript in this reference documentation, implementations of the DOM can be built for any language, as this Python example demonstrates:

# Python DOM example
import xml.dom.minidom as m
doc = m.parse("C:\\Projects\\Py\\chap1.xml");
doc.nodeName # DOM property of document object;
p_list = doc.getElementsByTagName("para");

For more information on what technologies are involved in writing JavaScript on the web, see JavaScript technologies overview.

Как получить доступ к DOM?

You don't have to do anything special to begin using the DOM. Different browsers have different implementations of the DOM, and these implementations exhibit varying degrees of conformance to the actual DOM standard (a subject we try to avoid in this documentation), but every web browser uses some document object model to make web pages accessible to script.

When you create a script–whether it's inline in a <script> element or included in the web page by means of a script loading instruction–you can immediately begin using the API for the document or window elements to manipulate the document itself or to get at the children of that document, which are the various elements in the web page. Your DOM programming may be something as simple as the following, which displays an alert message by using the alert() function from the window object, or it may use more sophisticated DOM methods to actually create new content, as in the longer example below.

This following JavaScript will display an alert when the document is loaded (and when the whole DOM is available for use).

<body onload="window.alert('Welcome to my home page!');"> 

This function creates a new H1 element, adds text to that element, and then adds the H1 to the tree for this document:

<html>
  <head>
    <script>
       // run this function when the document is loaded
       window.onload = function() {

         // create a couple of elements in an otherwise empty HTML page
         var heading = document.createElement("h1");
         var heading_text = document.createTextNode("Big Head!");
         heading.appendChild(heading_text);
         document.body.appendChild(heading);
      }
    </script>
  </head>
  <body>
  </body>
</html>

Важные типы данных

This reference tries to describe the various objects and types in as simple a way as possible. But there are a number of different data types being passed around the API that you should be aware of. For the sake of simplicity, syntax examples in this API reference typically refer to nodes as elements, to arrays of nodes as nodeLists (or simply elements), and to attribute nodes simply as attributes.

The following table briefly describes these data types.

document When a member returns an object of type document (e.g., the ownerDocument property of an element returns the document to which it belongs), this object is the root document object itself. The DOM document Reference chapter describes the document object.
element element refers to an element or a node of type element returned by a member of the DOM API. Rather than saying, for example, that the document.createElement() method returns an object reference to a node, we just say that this method returns the element that has just been created in the DOM. element objects implement the DOM Element interface and also the more basic Node interface, both of which are included together in this reference.
nodeList A nodeList is an array of elements, like the kind that is returned by the method document.getElementsByTagName(). Items in a nodeList are accessed by index in either of two ways:
  • list.item(1)
  • list[1]
These two are equivalent. In the first, item() is the single method on the nodeList object. The latter uses the typical array syntax to fetch the second item in the list.
attribute When an attribute is returned by a member (e.g., by the createAttribute() method), it is an object reference that exposes a special (albeit small) interface for attributes. Attributes are nodes in the DOM just like elements are, though you may rarely use them as such.
namedNodeMap A namedNodeMap is like an array, but the items are accessed by name or index, though this latter case is merely a convenience for enumeration, as they are in no particular order in the list. A namedNodeMap has an item() method for this purpose, and you can also add and remove items from a namedNodeMap.

Интерфейсы DOM

This guide is about the objects and the actual things you can use to manipulate the DOM hierarchy. There are many points where understanding how these work can be confusing. For example, the object representing the HTML form element gets its name property from the HTMLFormElement interface but its className property from the HTMLElement interface. In both cases, the property you want is simply in that form object.

But the relationship between objects and the interfaces that they implement in the DOM can be confusing, and so this section attempts to say a little something about the actual interfaces in the DOM specification and how they are made available.

Интерфейсы и Объекты

Many objects borrow from several different interfaces. The table object, for example, implements a specialized HTML Table Element Interface, which includes such methods as createCaption and insertRow. But since it's also an HTML element, table implements the Element interface described in the DOM element Reference chapter. And finally, since an HTML element is also, as far as the DOM is concerned, a node in the tree of nodes that make up the object model for an HTML or XML page, the table object also implements the more basic Node interface, from which Element derives.

When you get a reference to a table object, as in the following example, you routinely use all three of these interfaces interchangeably on the object, perhaps without knowing it.

var table = document.getElementById("table");
var tableAttrs = table.attributes; // Node/Element interface
for (var i = 0; i < tableAttrs.length; i++) {
  // HTMLTableElement interface: border attribute
  if(tableAttrs[i].nodeName.toLowerCase() == "border")
    table.border = "1"; 
}
// HTMLTableElement interface: summary attribute
table.summary = "note: increased border";

Ключевые Интерфесы в DOM

This section lists some of the most commonly-used interfaces in the DOM. The idea is not to describe what these APIs do here but to give you an idea of the sorts of methods and properties you will see very often as you use the DOM. These common APIs are used in the longer examples in the DOM Examples chapter at the end of this book.

Document и window объекты являются объектами, интерфейсы которых наиболее  часто используются при программировании DOM. Простыми словами, объект window  представляет собой что-то вроде браузера, а объект document корень самого документа. Element наследуется от общего интерфейса Node, и вместе эти два интерфейса предоставляют множество методов и свойств, которые вы используете для отдельных элементов. Эти элементы также могут иметь определенные интерфейсы для обработки данных, которые хранятся в этих элементах, как в примере объекта tableв предыдущем разделе.

Ниже приведен краткий список общих API-интерфейсов в сценариях веб-страниц и XML-страниц с использованием DOM.

Тестирование DOM API

This document provides samples for every interface that you can use in your own web development. In some cases, the samples are complete HTML pages, with the DOM access in a <script> element, the interface (e.g, buttons) necessary to fire up the script in a form, and the HTML elements upon which the DOM operates listed as well. When this is the case, you can cut and paste the example into a new HTML document, save it, and run the example from the browser.

There are some cases, however, when the examples are more concise. To run examples that only demonstrate the basic relationship of the interface to the HTML elements, you may want to set up a test page in which interfaces can be easily accessed from scripts. The following very simple web page provides a <script> element in the header in which you can place functions that test the interface, a few HTML elements with attributes that you can retrieve, set, or otherwise manipulate, and the web user interface necessary to call those functions from the browser.

You can use this test page or create a similar one to test the DOM interfaces you are interested in and see how they work on the browser platform. You can update the contents of the test() function as needed, create more buttons, or add elements as necessary.

<html>
  <head>
    <title>DOM Tests</title>
    <script type="application/javascript">
    function setBodyAttr(attr, value){
      if (document.body) eval('document.body.'+attr+'="'+value+'"');
      else notSupported();
    }
    </script>
  </head> 
  <body>
    <div style="margin: .5in; height: 400;"> 
      <p><b><tt>text</tt></b></p> 
      <form> 
        <select onChange="setBodyAttr('text',
        this.options[this.selectedIndex].value);"> 
          <option value="black">black 
          <option value="darkblue">darkblue 
        </select>
        <p><b><tt>bgColor</tt></b></p>
        <select onChange="setBodyAttr('bgColor',
        this.options[this.selectedIndex].value);"> 
          <option value="white">white 
          <option value="lightgrey">gray
        </select>
        <p><b><tt>link</tt></b></p> 
        <select onChange="setBodyAttr('link',
        this.options[this.selectedIndex].value);">
          <option value="blue">blue
          <option value="green">green
        </select>  <small>
        <a href="http://www.brownhen.com/dom_api_top.html" id="sample">
        (sample link)</a></small><br>
      </form>
      <form>
        <input type="button" value="version" onclick="ver()" />
      </form>
    </div>
  </body>
</html>

To test a lot of interfaces in a single page-for example, a "suite" of properties that affect the colors of a web page-you can create a similar test page with a whole console of buttons, textfields, and other HTML elements. The following screenshot gives you some idea of how interfaces can be grouped together for testing.

Схема 0.1 Sample DOM Test Page
Image:DOM_Ref_Introduction_to_the_DOM.gif

In this example, the dropdown menus dynamically update such DOM-accessible aspects of the web page as its background color (bgColor), the color of the hyperlinks (aLink), and color of the text (text). However you design your test pages, testing the interfaces as you read about them is an important part of learning how to use the DOM effectively.

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