javascript

Chapter 17: Ajax and JSON
alert(xhr.responseText);
} else {
alert(“Request was unsuccessful: “ + xhr.status);
}
}
};
xhr.open(“get”, “http://www.somewhere-else.com/page/”, true);
xhr.send(null);
Unlike the XDR object in IE, the cross - domain XHR object allows access to the status and statusText
properties as well as allowing synchronous requests. There are some additional limitations on a cross -
domain XHR object that are necessary for security purposes. They are as follows:
❑
❑
❑
Custom headers cannot be set using setRequestHeader() .
Cookies are neither sent nor received.
The getAllResponseHeaders() method always returns an empty string.
Since the same interface is used for both same - and cross - domain requests, it ’ s best to always use a
relative URL when accessing a local resource, and an absolute URL when accessing a remote resource.
This disambiguates the use case and can prevent problems such as limiting access to header and/or
cookie information for local resources.
Like the IE approach to cross - domain requests, the Firefox implementation hasn ’ t been around long
enough to establish best practices around it.
JSON
Even though XML was a large part of the Ajax movement, it was quickly met with disdain from
JavaScript developers. As discussed in Chapter 15 , XML manipulation in JavaScript is quite different
from browser to browser, and extracting data from an XML structure requires walking a DOM
document, which takes a fair amount of code. Douglas Crockford introduced a data format called
JavaScript Object Notation (JSON) to try to ease some of the data - access concerns of XML.
JSON is based on a subset of the JavaScript syntax, most notably object and array literals. Using these
constructs, it ’ s possible to create a structured data format capable of representing the same types of data
as XML. For example, a collection of name - value pairs can be represented as an object containing named
properties as shown here:
{
}
“name”: “Nicholas C. Zakas”,
“title”: “Software Engineer”,
“author”: true,
“age”: 29
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