Common pitfalls with JavaScript scope

I’m a firm believer that JavaScript’s flexibility is great for small projects with a single developer. But, its flexibility can become a seriously liability in medium to large projects if not managed properly. The ‘scope’ of a variable can cause some of the largest headaches for developers. And, in big projects tracking down an improperly scoped variable will take time, and it will try your patience.

In most cases, variables in JavaScript have two scopes: global and private. Scope is the context in which a variable is used. A global variable is defined outside of any function() in a JavaScript text block, and it can be accessed from inside any function(). Any change to a global variable will be reflected where ever else that variable is used. A private variable is defined only inside a function() and private variables are not accessible by other function()’s by definition. So a change to a private variable only changes it’s own value and doesn’t affect any other variable outside the scope of the function(). If you are new to scope, you may want to re-read this paragraph twice.

If you’ve been using JavaScript for any amount of time you’ll discover that simply misplacing a “var” statement in front of a variable causes it become global in scope. And, if you happen to already have a global variable somewhere else by the same name then these values can overwrite each other. If you are working on a project with thousands of lines of code and multiple .js libraries then your problems can get larger. I’ve accidentally deleted a “var” keyword in several cases and then spent a considerable head banging tracking it down.

To demonstrate these pitfalls, I’d rather show you in code what the problems are. Hopefully by reading this, and understanding a bit more about scope and by using best practices, you’ll avoid the common pitfalls as much as possible.

Scenario 1– properly defined private variables. This scenario demonstrates best practices for defining local variables within a function(). You can have privately scoped variables with the same name as global variables because of JavaScript obeying adherence to scope. Click here to try out this scenario.

  //This sets a global variable scope
  var color = "blue";

  function init() {

	var me = new Person("Andy");
	alert(" Private scoped name: " + me.name +
	   "\r\n Private scoped color: " + me.favoriteColor +
	   ", \r\n Global scoped color: " + color
	);

  }

  function Person(myname)
  {
	  //This creates a privately scoped variable
	  //Does not affect or change the globally scoped
	  //variable of same name
	  var color = "red";

	  //myname exists within the private scope of the function
	  //color is privately scoped
	  this.name = myname;
	  this.favoriteColor = color;
  }

Scenario 2 – improper use of a global variable. This scenario demonstrates forgetting to set “var” on the variable color. The value of the global variable named color is changed. If you use this pattern for manipulating global variables you are asking for trouble as your project grows larger. Click here to try out this scenario.

  //This sets a global variable scope
  var color = "blue";

  function init() {

	var me = new Person("Andy");
	alert(" Private scoped name: " + me.name +
	   "\r\n Private scoped color: " + me.favoriteColor +
	   ", \r\n Global scoped color: " + color
	);

  }

  function Person(myname)
  {
	  //This changes the globally scoped
	  //variable of same name
	  color = "red";

	  //myname exists within the private scope of the function
	  //color exists within the global scope of the application
	  this.name = myname;
	  this.favoriteColor = color;
  }

Scenario 3 – this scenario shows the best practice for passing in a global variable to a function(). By passing the global variable into someColor you protect the scope of it within the function(). Click here to try this scenario.

  //This sets a global variable scope
  var color = "blue";

  function init() {

	var me = new Person("Andy", color);
	alert(" Private scoped name: " + me.name +
	   "\r\n Private scoped color: " + me.favoriteColor +
	   ", \r\n Global scoped color: " + color
	);

  }

  function Person(myname, someColor)
  {
	  //myname exists within the private scope of the function
	  //someColor is a private scope, but it inherits the value
	  //of the variable passed to it.
	  this.name = myname;
	  this.favoriteColor = someColor;
  }

References:

Scope in JavaScript by Mike West
Variable Scope for New Programmers by Jonathan Snook

The Largest Conference For Mapping and Geospatial Developers – Esri DevSummit 2012

I’ll be presenting at the Esri DevSummit next week so if you are attending please swing by my sessions and say “hi”. If you aren’t familiar with Esri or the conference, about 1400 developers and other technical experts converge on Palm Springs, California every Spring to learn all things technical about building commercial and enterprise geographic information systems. There will be everything from introductory Dojo workshops to deep dives into the heart of our APIs.

If you’re around here’s my schedule. I’d be very interested to hear about what you are working on:

Monday,  March 26

Getting Started with the ArcGIS Web APIs – 8:30am – 11:45am, Pasadena Room. I’ll be presenting the portion related to our ArcGIS API for JavaScript.

Gettings Started with Smartphone and Tablet ArcGIS Runtime SDKs – 1:15pm – 4:45pm, Pasadena Room. In this session, I’ll be presenting on our ArcGIS Runtime SDK for Android.

Wednesday, March 28

Flex the World – 10:30am, Demo Theater 2. I’ll be presenting with my esteemed colleague Sajit Thomas on Apache Flex.

7 required improvements for the Web, HTML and JavaScript

Here’s my 2012 web developer wish list for improvements that I’d like to see happen in the web developer world. If HTML and JavaScript want to be considered enterprise ready for commercial-grade deployments then here’s some things that are needed today.

For clarity, I consider a commercial software deployment to be one that contains over one thousand lines of code, at least two custom .js libraries and involves at least two developers and some sort of code versioning system.

  1. Refactoring. Not having this capability continues to be a huge productivity issue for large projects. Try refactoring across six JavaScript libraries and 1200 lines of code using Notepad++.
  2. Even stronger scope enforcement in JavaScript classes. One wrong misspelling and you can spend fun filled hours (or days) tracking down a private variable that turned itself into a global variable.
  3. Built-in support for code comments. Visual Studio does a fine job, for example. But, it’s still kind of a hack to make it work. I’d like the built-in ability to create comments for methods and classes directly and then be able to access those comments via intellisense throughout any file in the project. Again, this is all about productivity by having this information accessible at your fingertips.
  4. Better built-in JavaScript checking for IDEs. I’d like to see built-in JSLint-like capabilities that have been updated to the latest HTML, JavaScript and CSS3 versions, and not some third party plug-in that’s optional.
  5. Best practice whitepapers. These would be whitepapers written by the browser vendors that provide guidelines on the correct patterns to use when building apps against their browsers. Seriously, it’s been roughly 21 years since we started using browsers and there’s no guidance at all from the powers that be.  Honestly, I’m stunned that these don’t exist. That would be similar to Microsoft publishing .NET and then not providing any conceptual help documentation.
  6. Official tools for browser certification and testing. The folks that build the browsers don’t give us a way to verify if we are building our apps in the best way possible. If these items existed, then quality could get a lot better, and we’d all learn a lot too.
  7. Slower browser release cycles. A slower release cycle for browsers and more improved security and stability. I already blogged about this here.

HTML 5 and Web Maps Seminar

@derekswingley and I are presenting several one hour Live Training Seminars on HTML5 and ArcGIS for developers on Thursday, February 23, 2012. ArcGIS is the flagship mapping and geo-spatial product line for the company I work for: Esri. Now, even if you aren’t currently using anything geo I still encourage you to listen in for an hour and hear what we have to say about HTML5 from a web app development perspective.

We’ve aimed the content at both new developers and experience developers that haven’t had a chance to play with HTML5 yet. Our goal is to help you understand what HTML5 really is and give some concrete uses cases and tips on how to effectively use it. So, sign on up, hopefully gain some new insight and ask some questions!

Click here to find out more details.

Simple Native JavaScript Classes – So simple a caveman could do it.

Among the many things I severely dislike about JavaScript is working with applications that have oodles of global variables, multiple .js libraries and dozens of loosely organized, individual functions. This is a very common pattern (or perhaps an anti-pattern). But it’s a terrible way to code for medium to large projects, especially where you have to share your code. Here’s a highly simplified example:


var someNumber = 2; //Global variable

function add(number){
    return someNumber + number;
};
alert(add(4)); //displays "6"

It’s unfortunate that this pattern is reinforced in authoritative books, blogs and articles. It’s completely pervasive in the majority of examples you see on the web. The downside is modifying, testing and troubleshooting this pattern can be an absolute nightmare. I compare it to building a fragile, multi-level house of cards: one wrong bump and it all falls down.

There is a better way!

So, I offer an easy-to-implement solution: where possible place your functions and properties in Class-like objects and all will be so much better. It’s not quite what you get with strongly-typed object oriented languages like C# or Java, but it works. Besides, if you use this pattern in your project then cats and dogs will live peacefully side-by-side and the universe will be in balance. The best news: this works perfectly fine with plain old JavaScript (POJO). And, if you are using something like jQuery use can use some version of Classes with those too, and I highly recommend it.

Here’s the fundamental pattern of a JavaScript Class:

function Add(){};
var someMath = new Add();

That’s very easy…right?? The advantages are many and include the following:

  • You get a powerful framework for logically grouping functionality. This lends to scalability and ultimately stability in your projects.
  • You can easily extend this pattern using prototypal inheritance.
  • You can take advantage of encapsulation.
  • You can implement inheritance and polymorphism.
Now let’s put this pattern to use. I’m providing two examples here. There are several other syntactical ways of doing this, but for brevity I’m sticking with two. The first example uses a very basic Object to implement namespace-like behavior. I say namespace-like because it’s not a true namespace like in C# or Java. The second example uses the built-in windows Object as a way of passing the namespace information. Click here to download a sample app that demonstrates these concepts.

POJO Class Pattern using Object Namespaces

Here’s an expanded example of the pattern with two levels of namespace separation using a standard Object :

if (!com) var com = {};    //1st level namespace
if (!com.ag) com.ag = {};  //2nd level namespace
if (!com.ag.Add) {
    com.ag.Add = function (value) {
        /// <summary>Demonstrates the plain old JavaScript pattern for classes.</summary>
        /// <param name="value" type="Number">Any number.</param>

        this.getValue = function () {
            /// <summary>Returns the property passed to the constructor.</summary>
            /// <returns type="Number">A number that was passed to the constructor.</returns>
            return value;
        },

        this.add = function (number) {
            /// <summary>This method adds value property + number</summary>
            /// <param name="number" type="Number">The number we want to add.</param>
            /// <returns type="Number">The number passed to the contructor plus this number</returns>
            return value + number;
        }

        //For Visual Studio intellisense cues
        com.__namespace = true;
        com.ag.__namespace = true;
        com.ag.Add.__class = true;
    }
}

And, here’s how to use this class:

var test = new com.ag.Add(2);
alert(test.add(4)); //displays "6"

POJO  Class Pattern using window[] Namespaces

Here is a slightly different version of the pattern that uses the window object, the results are the same:

if (!window["NS"]) window["NS"] = {};

window["NS"].Add = function (value) {
    /// <summary>This class uses addition</summary>
    /// <param name="value" type="Number">The number we pass to the contructor.</param>

    this.getValue = function () {
        /// <summary>Returns the private property called value.</summary>
        /// <returns type="Number">The number passed to the contructor.</returns>
        return value;
    },

            this.add = function (number) {
                /// <summary>This method adds value + number</summary>
                /// <param name="number" type="Number">The value we want to add.</param>
                /// <returns type="Number">The number passed to the contructor plus this number</returns>
                return value + number;
            }

    //For Visual Studio intellisense cues
    window["NS"].__namespace = true;
    window["NS"].Add.__class = true;
}

Here’s an example of how to use this class:

var test = new NS.Add(2);
alert(test.add(4)); //displays "6"

You are probably wondering about the funky xml comments. That’s for Visual Studio 2010’s built-in intellisense. I think but I’m not 100% certain that these work with Visual Studio Express as well. If you know for sure then please drop a comment. Notepad++ and other tools are okay for small projects but you can thank me later when you use this Class pattern along with built-in intellisense for any project that involves more than a dozen or so functions. And that’s not all – you can also see intellisense across different .js libraries. It’s all about productivity and ease-of-debugging. And, everyone will thank you when they have to re-use your code.

I’ve also attached a screenshot below to show you what the Visual Studio intellisense looks like. Also note in this example, the physical file, Add.js, in in the directory /scripts/com/ag/Add.js and I’m writing code in index.html which is at the root directory. How cool is that?!

 References

Sample application that demonstrates the Class concepts

Douglas Crockford’s [awesome] JavaScript Blog

Visual Studio JavaScript Intellisense Revisited

Creating Advanced [JavaScript] Web Applications with Object Oriented Techniques

Object Oriented Programming in JavaScript

Write Object Oriented JavaScript Part 1

The Format for JavaScript Doc Comments (Visual Studio)