Home > Articles > Open Source > Ajax & JavaScript

Learning JavaScript: Variables, Functions, and Loops

Tim Wright expands on variables, then moves on to creating functions, and last, goes over how to loop through data to autoexecute the same code block over and over.
This chapter is from the book

This is one of the more important chapters in the book because you learn some of the core features in JavaScript. We expand on the variables that were mentioned in the previous chapter, then move on to creating functions, and last, we go over how to loop through data to autoexecute the same code block over and over. Using variables, functions, and loops are often the only thing a person knows how to do in JavaScript, and they usually get along just fine. You’re already past that part and on your way to becoming an elite JavaScript developer, so no worries there. You’ll be coding while all the others are looking up how to do something.

Now that you have a solid base in how to work with a lot of the common things in JavaScript, you can start building an application and producing something tangible. Up to this point in the book, the examples have been pretty specific, but also a little abstract. You’ve been manipulating content and data, then alerting or observing the result. In this chapter we expand on what you’ve learned already and begin building a simple JavaScript application that will get more robust as you step through the subsequent chapters.

As you progress though this chapter, you notice that an address book application should be starting to form. Some of the methods that we go over repeat in their core functionality but have very different use-cases. Although they may not necessarily all live in the same application, this is the chapter where you start building that tangible knowledge that can be directly transferred into a project.

Defining Variables

For the most part, you learned about variables within the context of data storage, but they also have an integral part in your application when it comes to functionality.

When considering variable and function naming, it’s best to make them meaningful and speak to their contents or purpose. For example, using a variable name of “myBestFriend” would be much more helpful than something like, “firstVariableName.” Something else to consider when naming variables is that they can’t start with a number. They can contain numbers, such as “dogs3” or “catsStink4Eva,” but they can’t begin with a number, such as “3dogs.”

Grouping Variables

When you’re writing an application, it’s best to try to group all variables at the top of your JavaScript file or function (when possible) so they can all be immediately cached for later reference. Some people find this method a little unnatural because functions are defined throughout the document, and it’s a little easier to maintain when variables are right there with the function they belong to; but grouping variables at the top is one of those small performance boosts you can give to your application. It helps to think of it as one large file containing JavaScript for an application versus thinking of the file as a collection of one-off actions that get executed. When thinking of it as a single unit, it feels a little better (to me) when I’m grouping all variables together at the top.

You can group variables in your document in two ways. Up to this point we have been using a new var declaration for each variable; a lot of people prefer this method, and it’s perfectly fine to use. An alternative method is to use a single var declaration, using commas to separate the individual variables and a semicolon at the very end. Listing 6.1 shows an example of grouping variables with a single var declaration. Note the commas at the end of each line.

Listing 6.1. Grouping Variables with a Single var Declaration

var highSchool = "Hill",
    college = "Paul",
    gradSchool = "Vishaal";

There’s no difference in the way you access these variables compared to how you access variables declared with individual var declarations. At the variable level, it’s purely a way to group. It isn’t good or bad at this point—it’s only personal preference. You’ll see both methods in looking through JavaScript others have written, so it’s good to know what’s going on.

You see this style of variable declaration a lot more when getting into objects, methods, and grouping functions together. I prefer it because it feels cleaner and a little more consistent, but as you progress you will settle on a preference of your own. Both are certainly valid methods.

Reserved Terms

JavaScript contains a lot of core functionality. We’ve been over quite a bit of it so far. Beyond that core functionality you will be defining a lot of your own custom code. If the names of your custom JavaScript match up with anything built into the language, it can cause collisions and throw errors. It’s the same as if you’re writing a large JavaScript file—you want to make sure all the function and variable names are as unique as possible to prevent problems and confusion while parsing the information. If you have two functions with the same name, it’s difficult to tell the browser which one to use, so it’s just not allowed.

To prevent these issues with native JavaScript, there are some reserved words (keywords) that you can’t use when defining variables, functions, methods, or identifiers within your code. Following is a list of the reserved words:

  • break
  • case
  • catch
  • continue
  • debugger
  • default
  • delete
  • do
  • else
  • finally
  • for
  • function
  • if
  • implements
  • in
  • instanceof
  • interface
  • new
  • package
  • private
  • protected
  • public
  • return
  • static
  • switch
  • this
  • throw
  • try
  • typeof
  • var
  • void
  • while
  • with

Most of these are no-brainers, like function and var, and under normal circumstances you probably would never come across a situation where something like “implements” would be a reasonable name for a variable or function. If you end up using any of these terms in your code, the console will throw an error and let you know that you’re using a reserved word. With that in mind, I think the value in this list is not so much memorizing it, but rather recognizing that these words map to native actions in the language. It will help you write better code and also aid in learning more advanced JavaScript down the road if you choose to research some of those terms that are beyond the scope of this book, such as public, private, and protected.

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