Javascript application development in Mobile devices

Build Native Applications using Javascript

Palm’s new OS is the first mobile platform to be built from the ground up to combine standard technology, innovation and integration. At its core, webOS leverages several industry-standard technologies, including web technologies such as CSS, XHTML and JavaScript. You can think of webOS applications as native applications, but built from the same standard HTML, CSS and JavaScript that you’d use to develop web applications. Palm has extended the standard web development environment through a JavaScript framework that gives standardized UI widgets, and access to selected device hardware and services.

Javascript can be used on the iphone too.  The HTML 5 specification provides a new mechanism for client-side data storage: JavaScript database support. This feature shipped originally with iPhone OS 2.1. When you use “Add to Home Screen” from the “+” button on Safari for iPhone, a web application with a manifest defined (per the HTML5 spec) will be saved with any cached resources.  It is also worth noting that these features have not been added to Safari on Mac OS X or Windows.

Desktop Application development using Javascript

Adobe AIR is a cross-platform desktop runtime created by Adobe that allows web developers to use web technologies to build and deploy Rich Internet Applications (RIAs) and web applications to the desktop. This means what you can write you application using Javascript and then let AIR to figure out how to run your code in Windows, Linux, Mac and future operating systems.

Desktop App using Javascript

The application shown in the pic is written using Javascript, HTML and CSS. It uses the ExtJS library. Javascript can be used to dock the application in the taskbar, delete and modify files on the hard drive, communicate to different server, use AJAX, etc. You can even use flash within your app to make the application look and feel much better. This gives the web developer good access to a desktop.

Server side Scripts using Javascript

Server side JavaScript has been around for a long time and potentially offers some unique and interesting advantages over other languages (like PHP, ASP, etc) because the same language is spoken by both client and server. There are numerous attempts to bring server side javascript to the masses.  Server side Javascript has to do much more than what it has to do at the browser level with access to databases, files, and networking, as well as logging, process management, scalability, security, integration APIs, and extensibility. You can even use your own javascript libraries on the server jQuery, dojo, Ext JS, prototype, etc.

Javascript in other places

- Adobe reader (from v3.02) supports Javascript for forms and basic operations. SOAP support has also been included since version 7.0. There is also access to the entire 3D Javascript API. More details on using Javascript is available here
- Open office application suite supports Javascript to write macros. This move looks like a sensible addon given the usage of VBScript in MS Office suite.
- Apple’s Dashboard Widgets, Microsoft’s Gadgets, Yahoo! Widgets, Google Desktop Gadgets are implemented using JavaScript.
- The oddest one in this list will have to be the  Philips Remote Control which uses Javascript

ECMAScript 4 (ECMAScript Harmony)

Javascript has been relatively stable since the third edition of ECMAScript published in 1999. Features under discussion for a future edition originally ECMAScript 4 now ECMAScript Harmony include Classes, a module system, static typing and Optional type annotations, Generators, Iterators, Destructuring assignment, algebraic data types. Well if you think this is ambitious you should see the original ECMAScript 4 which had more semantic and syntactic innovation. Packages, namespaces and early binding from ECMAScript 4 are no longer included for planned releases with ECMAScript Harmony. The intent of these features is partly to better support “programming in the large”, and to let programmers sacrifice some of the script’s ability to be dynamic for performance.

So yes as a web developer who uses Javascript on a daily basis, I am excited to watch Javascript grow and be available in more places.

1. Floating point arithmetic

This can be a major cause of frustration for people who are new to Javascript and are trying to perform some kind of mathematical computation.

<br /><br />
<script><br /><br />
alert(0.02 / 0.1);  //0.19999999999999998 <br /><br />
alert(1.14 * 100);  //113.99999999999999    ;)<br /><br />
</script><br /><br />

This is where the Math.round() functions come in handy

2. Plus operator overloading 

The plus operator stands for both arithmetic operations and also string concatenation. This is convenient if used correctly. Lets take a look

<br /><br />
<script><br /><br />
var msg, one="1";<br /><br />
msg = 2 + "1"; // msg = "21"<br /><br />
msg = 2 + one; // msg = "21"<br /><br />
msg = 1 + 1 + 1 + " musketeers"; // msg = "3 musketeers"<br /><br />
msg = "Bond " + 0 + 0 + 7; //msg = "Bond 007"  <br /><br />
</script><br /><br />

 
The above behaviour is because the operations are performed left to right. If we use parantheses the behaviour will change based on the order of the string or number in it.

3. Semicolon insertion at line feed

Javascript automatically inserts ; at a line feed. Lets see this in action with a simple example:

<br /><br />
<script><br /><br />
function returnSame(a){<br /><br />
   return                 //Inserts semi-colon to convert to return;<br /><br />
   a                      //a becomes a; - Unreachable<br /><br />
}<br /><br />
alert(returnSame(2));  //Output is undefined<br /><br />
</script><br /><br />

The magical semicolon insertion can get things complicated while creating objects using object literals

4. typeof operator

typeof is a unary operator. The results are not one would normally expect. It suprisingly evaluates null to ”object”. 

<br /><br />
<script><br /><br />
var obj={};  //object created using object literal<br /><br />
var arr=[];  //array created by array literal<br /><br />
alert(typeof(obj));   //object  - Good<br /><br />
alert(typeof(arr));   //object  - Bad<br /><br />
alert(typeof(null));  //object  - Ugly!  ;)<br /><br />
</script><br /><br />

 
It should only be used to distingush objects from other primitive types.

5. false, null, undefined, NaN, Infinity

They stand for different things although they might look similar or at times look redundant. Javascript uses three primitive datatypes numbers, strings and boolean. In addition it has two trivial datatypes undefined and null. Both null and undefined are equal according to the equality operator (==)

 <br /><br />
<script><br /><br />
var a;<br /><br />
alert (a);    //undefined<br /><br />
alert (1/0);  //Infinity<br /><br />
alert (0/0);  //NaN<br /><br />
0/0 == 0/0;   //false - a NaN != NaN<br /><br />
alert (b);    //error<br /><br />
</script><br /><br />

6. String replace only replaces first occurence

Unlike PHP or other languages, string replace in Javascript only replaces the first occurence of a string by default.

<br /><br />
<script><br /><br />
var nospace = "I dont need spaces".replace(" ","_");<br /><br />
alert(nospace);    //I_dont need spaces   - Only first occurence<br /><br />
var nospace = "I dont need spaces".replace(/ /g,"_");<br /><br />
alert(nospace);    //I_dont_need_spaces<br /><br />
</script><br /><br />

7. ParseInt function

This is used to convert a string into an integer. The function does accept two arguments and the second argument specifies the radix. The radix here for decimal is10. If no radix argument is passed the parseInt function tries to guess the base and in this case because the string starts with 0, it is parsed as an octal number.

<br /><br />
<script><br /><br />
var str = "017";<br /><br />
var strInt = parseInt(str);      //strInt = 15  ;)<br /><br />
var strInt = parseInt(str,10);   //strInt = 17<br /><br />
</script><br /><br />

Please share your Javascript Gotcha’s using the form below:
(note use &lt; and &gt; to get past the comment filter for < and >)

Standard Javascript file include method

<SCRIPT src="A.JS" language="JavaScript/text"></SCRIPT>
<SCRIPT src="B.JS" language="JavaScript/text"></SCRIPT> 

<IMG src="1.GIF" />
<IMG src="2.GIF" />
<IMG src="3.GIF" />

Javascript blocks the other elements from loading (Example)

Script DOM element method

var p = g.getElementsByTagName("HEAD")[0];
var c = g.createElement("script");
c.type= "text/javascript";
c.onreadystatechange = n;
c.onerror = c.onload = k;
c.src = e;
p.appendChild(C);

Javascript executed without blocking any element from loading

This method creates a DOM element for each Script and then adds the element to the HTML. 

NOTE: 

1) Only when the appendChild function is called the Javascript will be executed.
2) For inlined code that depends on the Javascript and also for a similar method of Asynchronous Script Loading – See Steve Souders blog
3) If you go to MSN.com (Alexa top 10 site based on traffic) and hit view source you can see the javacript elements are included by using the script dom element to load the web pages faster. This is a good example of where you can use the script dom element method for certain js files that do not have inlined code dependency. Let us run a Pagetest waterfall report and you can see the following for MSN.COM

Pagetest Waterfall report of MSN.COM shows no blocking during page load