Home > Articles > Programming > Java

Getting Started with AJAX and the XMLHttpRequest Object

  • Print
  • + Share This
This chapter introduces you to the XMLHttpRequest object, showing you how to work around its implementation differences between browsers. Also learn how to make some actual page requests, both in a synchronous fashion and in an asynchronous fashion. This chapter finishes with some various fallback approaches that can be used if a browser doesn't support XMLHttpRequest, including how to use IFrames and cookies as your communication channel.
This chapter is from the book

In this chapter

2.1 XMLHttpRequest Overview

2.2 Cross-Browser XMLHttpRequest

2.3 Sending Asynchronous Requests

2.4 AJAX Without XMLHttpRequest

2.5 Fallback Option 1: Sending a Request Using an IFrame

2.6 Fallback Option 2: Sending a Request Using a Cookie

2.7 Summary

The foundation that makes AJAX possible is the communication layer with the server. The most complete option for performing this communication is the JavaScript XMLHttpRequest object. If XMLHttpRequest is not suitable to you, hidden IFrames and cookies can also be used. We will examine both options later in this chapter.

This chapter introduces you to the XMLHttpRequest object, showing you how to work around its implementation differences between browsers. After that, we make some actual page requests, both in a synchronous fashion and in an asynchronous fashion. This chapter finishes with some various fallback approaches that can be used if a browser doesn't support XMLHttpRequest, including how to use IFrames and cookies as your communication channel.

2.1 XMLHttpRequest Overview

Originally, Microsoft designed XMLHttpRequest to allow Internet Explorer (IE) to load XML documents from JavaScript. Even though it has XML in its name, XMLHttpRequest really is a generic HTTP client for JavaScript. With it, JavaScript can make GET and POST HTTP requests. (For POST requests, data can be sent to the server in a format of your choosing.) The main limitations to XMLHttpRequest are due to the browser security sandbox. It can make only HTTP(S) requests (file URLs, for example, won't work), and it can make requests only to the same domain as the currently loaded page.

The security limitations of XMLHttpRequest do limit the ways in which you can use it, but the trade-off in added security is well worth it. Most attacks against JavaScript applications center around injecting malicious code into the Web page. If XMLHttpRequest allowed requests to any Web site, it would become a major player in these attacks. The security sandbox reduces these potential problems. In addition, it simplifies the programming model because the JavaScript code can implicitly trust any data it loads from XMLHttpRequest. It can trust the data because the new data is just as secure as the page that loaded the initial page.

Despite the fact that XMLHttpRequest provides only a small API and just a handful of methods and properties, it has its differences between browsers. These differences are mainly in event handling and object instantiation (in IE, XMLHttpRequest is actually an ActiveX object), so they aren't hard to work around. In the following overview of the XMLHttpRequest API, the Mozilla syntax for XMLHttpRequest instantiation is used. If you want to run the examples in IE, you need to replace new XMLHttpRequest(); with either new ActiveXObject("MSXML2.XMLHTTP.3.0"); or the full cross-browser instantiation method shown in the "Cross-Browser XMLHttpRequest" section of this chapter.

XMLHttpRequest is the most-used method for AJAX communications because it provides two unique features. The first feature provides the ability to load new content without that content being changed in any way, which makes it extremely easy to fit AJAX into your normal development patterns. The second feature allows JavaScript to make synchronous calls. A synchronous call stops all other operations until it's complete, and while this isn't an option that is usually used, it can be useful in cases in which the current request must be completed before further actions are taken.

2.1.1 XMLHttpRequest::Open()

The open method is used to set the request type (GET, POST, PUT, or PROPFIND), the URL of the page being requested, and whether the call will be asynchronous. A username and password for HTTP authentication can also be optionally passed. The URL can be either a relative path (such as page.html) or a complete one that includes the server's address (such as http://blog.joshuaeichorn.com/page.html). The basic method signature is:

open(type,url,isAsync,username,password)

In the JavaScript environment, security restrictions are in place. These security restrictions cause the open method to throw an exception if the URL is from a different domain than the current page. The following example uses open to set up a synchronous GET request to index.html:

1 var req = new XMLHttpRequest();
2 req.open('GET', 'index.html', false);
3 req.send(null);
4 if(req.status == 200)
5 alert(req.responseText);

2.1.2 XMLHttpRequest::Send()

The send method makes the connection to the URL specified in open. If the request is asynchronous, the call will return it immediately; otherwise, the call will block further execution until the page has been downloaded. If the request type is POST, the payload will be sent as the body of the request that is sent to the server. The method signature is:

send(payload)

When you make a POST request, you will need to set the Content-type header. This way, the server knows what to do with the uploaded content. To mimic sending a form using HTTP POST, you set the content type to application/x-www-form-urlencoded. URLencoded data is the same format that you see in a URL after the "?". You can see an example of this encoded data by making a form and setting its method to GET. The following example shows a synchronous POST request to index.php that is sending a URLencoded payload. If index.php contains <?php var_dump($_POST); ?>, you can see the submitted data translated as if it's a normal form in the alert:

1 var req = new XMLHttpRequest();
2 req.open('POST', 'index.php', false);
3 req.setRequestHeader('Content-type',
4            'application/x-www-form-urlencoded;charset=UTF-8;');
5 req.send('hello=world&XMLHttpRequest=test');
6 if(req.status == 200)
7   alert(req.responseText);

2.1.3 XMLHttpRequest::setRequestHeader()

There are many different cases in which setting a header on a request might be useful. The most common use of setRequestHeader() is to set the Content-type, because most Web applications already know how to deal with certain types, such as URLencoded. The setRequestHeader method signature takes two parameters: the header to set and its value:

setRequestHeader(header,value)

Because requests sent using XMLHttpRequest send the same standard headers, including cookie headers and HTTP authentication headers, as a normal browser request, the header name will usually be the name of the HTTP header that you want to override. In addition to overriding default headers, setRequestHeader is useful for setting custom, application-specific headers. Custom headers are generally prefixed with X- to distinguish them from standard ones. The following example makes a synchronous GET request adding a header called X-foo to test.php. If test.php contains <?php var_dump($_SERVER); ?>, you will see the submitted header in the alert:

1 var req = new XMLHttpRequest();
2 req.open('GET', 'test.php', false);
3 req.setRequestHeader('X-foo','bar');
4 req.send(null);
5
6 if(req.status == 200)
7      alert(req.responseText);

2.1.4 XMLHttpRequest::getResponseHeader() and getAllResponseHeaders()

The getResponseHeader method allows you to get a single header from the response; this is especially useful when all you need is a header like Content-type; note that the specified header is case-insensitive. The method signature is as follows:

getResponseHeader(header)

getAllResponseHeaders returns all the headers from the response in a single string; this is useful for debugging or searching for a value. The following example makes a synchronous GET request to test.html. When the client receives a response, the Content-type is alerted and all the headers are alerted:

1 var req = new XMLHttpRequest();
2 req.open('GET', 'test.html', false);
3 req.send(null);
4
5 if(req.status == 200) {
6     alert(req.getResponseHeader('Content-type'));
7       alert(req.getAllResponseHeaders());
8 }

2.1.5 Other XMLHttpRequest Methods

All browsers implement an abort() method, which is used to cancel an in-progress asynchronous request. (An example of this is shown in the "Sending Asynchronous Requests" section in this chapter.) Mozilla-based browsers also offer some extra methods on top of the basic API; for instance, addEventListener() and removeEventListener() provide a way to catch status events without using the on* properties. There is also an overrideMimeType() method that makes it possible to force the Content-type to text/xml so that it will be parsed into a DOM document even if the server doesn't report it as such. The Mozilla-specific methods can be useful in certain circumstances, but in most cases, you should stay away from them because not all browsers support them.

2.1.6 XMLHttpRequest Properties

XMLHttpRequest provides a number of properties that provide information or results about the request. Most of the properties are self-explanatory; you simply read the value and act on it. The on* properties are event handlers that are used by assigning a function to them. A list of all the properties follows:

  • status. The HTTP status code of the request response.
  • statusText. The HTTP status code that goes with the code.
  • readyState. The state of the request. (See Table 2-1 in the next section of this chapter for values.)
  • responseText. Unparsed text of the response.
  • responseXML. Response parsed into a DOM Document object; happens only if Content-type is text/xml.
  • onreadystatechange. Event handler that is called when readyState changes.
  • onerror. Mozilla-only event handler that is called when an error happens during a request.
  • onprogress. Mozilla-only event handler that is called at an interval as content is loaded.
  • onload. Mozilla-only event handler that is called when the document is finished loading.

2.1.7 readyState Reference

Table 2-1 shows the possible values for the readyState variable. It will return a number representing the current state of the object. Each request will progress through the list of readyStates.

Table 2-1. readyState Levels

readyState Status Code

Status of the XMLHttpRequest Object

(0) UNINITIALIZED

The object has been created but not initialized. (The open method has not been called.)

(1) LOADING

The object has been created, but the send method has not been called.

(2) LOADED

The send method has been called, but the status and headers are not yet available.

(3) INTERACTIVE

Some data has been received. Calling the responseBody and responseText properties at this state to obtain partial results will return an error, because status and response headers are not fully available.

(4) COMPLETED

All the data has been received, and the complete data is available in the responseBody and responseText properties.

The readyState variable and the onreadystatechange event handler are linked in such a way that each time the readyState variable is changed, the onreadystatechange event handler is called.

  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020