XMLHttpRequest对象的属性

XMLHttpRequest对象提供了许多属性、方法和事件，因此Ajax脚本可以处理和控制Http请求和响应。本章的其余部分会对这些进行详细的讨论。

The XMLHttpRequest object exposes various properties, methods, and

events so Ajax scripts can process and control HTTP requests and

responses. The rest of this chapter discusses these in detail.

1、 The readyState Property

The XMLHttpRequest object cycles through several states as it sends an

HTTP request to the server, waits while the request is processed, and when

it receives a response. So that scripts can respond appropriately to the

various states, the object exposes a readyState property that represents

the object's current state, as shown in Table 1.1.

Table 1.1 ReadyState Property Values

ReadyState Property Value Description

0

Represents an “uninitialized” state in which an

XMLHttpRequest object has been created, but

not initialized.

1

Represents a “sent” state in which code has

called the XMLHttpRequest open() method

and the XMLHttpRequest is ready to send a

request to the server.

2

Represents a “sent” state in which a request

has been sent to the server with the send()

method, but a response has not yet been

received.

3

Represents a “receiving” state in which the

HTTP response headers have been received,

but message body has not yet been completely

received.

4

Represents a “loaded” state in which the

response has been completely received.

2、 The onreadystatechange Property

The XMLHttpRequest object generates a readystatechange

event whenever the readyState value changes. The

onreadystatechange property accepts an EventListener value,

specifying the method that the object will invoke whenever the

readyState value changes.

3、 The responseText Property

The responseText property contains the text of the HTTP response

received by the client. When the readyState value is 0, 1, or 2

responseText contains an empty string. When the readyState

value is 3 (Receiving), the response contains the incomplete response

received by the client. When readyState is 4 (Loaded) the

responseText contains the complete response.

4、 The responseXML Property

The responseXML property represents the XML response when the

complete HTTP response has been received (when readyState is 4),

when the Content-Type header specifies the MIME (media) type as

text/xml, application/xml, or ends in +xml. If the Content-Type

header does not contain one of these media types, the responseXML

value is null. The responseXML value is also null whenever the

readyState value contains any value other than 4. The responseXML

property value is an object of type Document interface, and represents

the parsed document. If the document cannot be parsed (for example if the

document is malformed or the character encoding of the document is not

supported) the responseXML value is null.

5、 The status Property

The status property represents the HTTP status code2 and is of type

short. The status attribute is available only when the readyState

value is 3 (Receiving) or 4 (Loaded). Attempting to access the status

value when readyState is less than 3 raises an exception.

6、 The statusText Property

The statusText attribute represents the HTTP status code text and is

also available only when the readyState value is 3 or 4. Attempting to

access the statusText property for other readyState values raises

an exception.

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