Home > Articles > Open Source > Ajax & JavaScript

  • Print
  • + Share This
This chapter is from the book


Scope is a programming concept that exists to reduce the amount of variable and function collisions in your code. It controls how far information can travel throughout your JavaScript document. Earlier on, we briefly mentioned global variables. “Global” is a type of scope; the global scope for a variable means that the variable can be accessed and used anywhere in the document. Global variables are generally a bad thing, especially in larger files where naming collisions are more likely. Try to keep things out of the global scope if possible. Listing 6.3 shows how to declare a basic anonymous function and keep variables out of the global scope.

Listing 6.3. Defining an Anonymous Function

/* set up your anonymous function */
(function () {

    /* define a variable inside the function */
     var greeting = "Hello Tim";

     /* access the variable inside the function */
     alert("in scope: " + greeting);

})(); // end anonymous function

For the most part, you will be dealing in function-level scope. This means that any variable defined inside a function cannot be used outside that function. This is a great benefit of using anonymous functions. If you wrap a code block in an anonymous function, the contents of that function, which would normally default to the global scope, will now be contained within the scope of that anonymous function.

Listing 6.3.1 defines a variable inside an anonymous function, alerts the variable, and then tries to alert the variable again, outside the function (it won’t end well).

Listing 6.3.1. Showing Scope Inside an Anonymous Function

/* set up your anonymous function */
(function () {

    /* define a variable inside the function */
     var greeting = "Hello Tim";

     /* access the variable inside the function */
     alert("in scope: " + greeting);

})(); // end anonymous function

/* try and access that variable outside the function scope */
alert("out of scope: " + typeof(greeting)); // alerts "undefined"

As you can see, the variable alert is undefined, even though you can see it’s clearly defined within the anonymous function. This is because the function scope will not allow the variable to leave the function.

Calling a Function with a Function

When you have a function that calls another function, the second function is referred to as a callback. The callback function is defined as a normal function with all the others but is executed inside another function. They’re a little different because instead of you having to do something to execute the function, another function does something. It’s like having robots that are built by other robots—total madness, I know.

Callback functions are a great way to separate out the levels of functionality in your code and make parts more reusable. Often you will see callback functions passed as arguments to other functions. We’ll get more into that in the next chapter when we talk about JavaScript events, and they’re especially important when dealing with server communications like Ajax. Listing 6.3.2 shows our sayHello() function being defined and then called inside the anonymous function. In this case, sayHello() is a callback function (calling it twice).

Listing 6.3.2. Using a Callback Function

function sayHello(message) {

(function (){

    var greeting = "Welcome",
        exitStatement = "ok, please leave.";



Returning Data

Every function you create will not result in a direct output. Up to this point you’ve been creating functions that do something tangible, usually alerting a piece of data into the browser. You won’t always want to do that, though; from time to time you will want to create a function that returns information for another function to use. This will make your functions a little smaller, and if the function that gathers information is general enough, you can reuse it to pass the same (or different) information into multiple functions.

Being able to return data and pass it into another function is a powerful feature of JavaScript.

Returning a Single Value

Going back to the sayHello() function that was defined in Listing 6.2, we’re going to remove the alert() action that was previously being executed when the function was called, and we’ll replace it with a return statement. This is depicted in Listing 6.3.3.

Listing 6.3.3. Returning Data with a Function

function sayHello(message){
    return message + "!"; // add some emotion too

You’ll probably notice that the sayHello() function doesn’t do anything in the browser anymore. That’s a good thing (unless you’re getting an error—that’s a bad thing). It means the function is now returning the data but it’s just sitting there waiting to be used by another function.

Returning Multiple Values

Sometimes returning a single value isn’t enough for what you’re trying to accomplish. In that case you can return multiple values and pass them in an array format to other functions. Remember how I mentioned that arrays are really important? They creep up a lot when dealing in data storage and flow in JavaScript. In Listing 6.3.4 you can see the sayHello() function taking two arguments. Those arguments get changed slightly and are resaved to variables; then they are returned in an array format to be accessed later.

Listing 6.3.4. Returning Multiple Data Values with a Function

function sayHello(greeting, exitStatement){

    /* add some passion to these dry arguments */
    var newGreeting = greeting + "!",
        newExitStatement = exitStatement + "!!";

    /* return the arguments in an array */
    return [newGreeting, newExitStatement];


Passing Returned Values to Another Function

Now that you’re returning variables, the next step is to pass those variables into another function so they can actually be used. Listing 6.3.5 shows the sayHello() function from Listing 6.3.1 returning an array of information and a new function called startle(), taking two arguments, passing them through the original sayHello() function, and alerting the results.

Listing 6.3.5. Using Returned Function Values Passed into Another Function

function sayHello(greeting, exitStatement){

    /* add some passion to these dry arguments */
    var newGreeting = greeting + "!",
        newExitStatement = exitStatement + "!!";

    /* return the arguments in an array */
    return [newGreeting, newExitStatement];


function startle(polite, rude){

    /* call the sayHello function, with arguments and same each response to a variable */
    var greeting = sayHello(polite, rude)[0],
        exit = sayHello(polite, rude)[1];

    /* alert the variables that have been passed through each function */
    alert(greeting + " -- " + exit);


/* call the function with our arguments defined */
startle("thank you", "you stink");

A Function as a Method

Just as you can group variables and data into objects, you can also do it with functions. When you group functions into objects, they’re not called functions anymore; they’re called “methods.”

When I first started out with JavaScript, I came in from a design background rather than as a developer. This meant that I wasn’t familiar with common programming terms such as object, function, method, loop, and so on. I quickly learned what a function was and how to work with them through a lot of Googling. But I would hear people talk about the alert() method and other methods native to JavaScript, and I wouldn’t really get it because they look the same as functions. Why isn’t it the “alert function”? I had no idea. This comes up a lot when you’re dealing with JavaScript libraries as well (we get into that later in the book); everything is a method and nothing is a function, even though they all look and act the same.

Here’s what’s going on. In Chapter 5, “Storing Data in JavaScript,” you learned about storing information in objects. I mentioned that you could also store functions in objects. When you do that, they’re called methods instead of functions, but they work the same way. It’s weird, I know, and it’s not even an important distinction while you’re coding. It’s more about organizing your functions in groups to make them easier to maintain. The alert() method lives inside a global object (you never see it), which is why it’s called a method.

Now that we’re past that ordeal, organizing your functions into meaningful objects can clean up a lot of your code, especially on larger projects where you need the code organization help to keep your sanity. Listing 6.4 should look a little familiar; it shows how to organize our two functions (sayHello and startle) inside an object called “addressBookMethods.” If we were building a large-scale application with many features, this would be a great way to section off the functionality meant only for the address book feature.

Listing 6.4. Grouping Similar Functions

var addressBookMethods = {

    sayHello: function(message){

        return message;

    startle: function(){

        alert(addressBookMethods.sayHello("hey there, called from a method"));



/* call the function */

Calling a method is a little different from calling a function. You’ll notice in Listing 6.4 that instead of calling startle() by itself, you have to call addressBookMethods.startle(). This is because before you can access the method, you have to access the object and drill down to the method.

Performance Considerations

Nesting functions in objects has the same performance implications that we spoke of when nesting variables in objects. The deeper a function is nested inside an object (addressBookMethods), the more resources it takes to extract. This is another place in your code where you will have to balance performance with maintainability. We’re not talking a ton of time here—maybe a few milliseconds difference—but it can add up. Most of the time it won’t matter, but if you find yourself needing a performance boost, function objects would be a place to look for a bottleneck. I probably wouldn’t go more than a few levels deep when creating these objects. Listing 6.4 goes only one level deep, which is a nice balance between performance and maintainability.

  • + 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.


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.


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.


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.


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


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


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.


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.


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