javascript

Chapter 17: Ajax and JSON
Headers can be used to pass additional, structured data from the server to the browser. The
getAllResponseHeaders() method typically returns something along the lines of the following:
Date: Sun, 14 Nov 2004 18:04:03 GMT
Server: Apache/1.3.29 (Unix)
Vary: Accept
X-Powered-By: PHP/4.3.8
Connection: close
Content-Type: text/html; charset=iso-8859-1
This output allows you to parse the response headers to find all of the header names that were sent
rather than checking for the existence of each one individually.
GET Requests
The most common type of request to execute is a GET, which is typically made when the server is being
queried for some sort of information. If necessary, query - string arguments can be appended to the end of
the URL to pass information to the server. For XHR, this query string must be present and encoded
correctly on the URL that is passed into the open() method.
One of the most frequent errors made with GET requests is to have an improperly formatted query
string. Each query - string name and value must be encoded using encodeURIComponent() before being
attached to the URL, and all of the name - value pairs must be separated by an ampersand, as in this
example:
xhr.open(“get”, “example.php?name1=value1 & name2=value2”, true);
The following function helps to add query - string arguments to the end of an existing URL:
function addURLParam(url, name, value) {
url += (url.indexOf(“?”) == -1 ? “?” : “ & ”);
url += encodeURIComponent(name) + “=” + encodeURIComponent(value);
return url;
}
The addURLParam() function takes three arguments: the URL to add the parameters to, the parameter
name, and the parameter value. First the function checks to see if the URL already contains a question
mark (to determine if other parameters already exist). If it doesn ’ t, then the function appends a question
mark; otherwise it adds an ampersand. Next the name and value are encoded and appended to the end
of the URL. The last step is to return the updated URL.
This function can be used to build up a URL for a request as shown in the following example:
var url = “example.php”;
//add the arguments
url = addURLParam(url, “name”, “Nicholas”);
url = addURLParam(url, “book”, “Professional JavaScript”);
//initiate request
xhr.open(“get”, url, false);
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