Monday, January 7, 2008

Javascript Best Practices

I found two links that is really useful for javascript developers: and
These two links talk about best practices when developing javascript code.

1. Always Use 'var'
Variables in javascript either have global scope or function scope, and using the 'var' keyword is vital to keeping them straight. When declaring a variable for use either as a global variable or as a function-level variable, always prefix the declaration with the 'var' keyword.

2. Avoid 'with'

3. Use onclick In Anchors Instead Of javascript: Pseudo-Protocol

When you want to trigger javascript code from an anchor <A> tag, the onclick handler should be used rather than the javascript: pseudo-protocol. The javascript code that runs within the onclick handler must return true or false (or an expression than evalues to true or false) back to the tag itself - if it returns true, then the HREF of the anchor will be followed like a normal link. If it returns false, then the HREF will be ignored. This is why "return false;" is often included at the end of the code within an onclick handler.

Correct Syntax

<a href="javascript_required.html" onclick="doSomething(); return false;">go</a>

What Not To Do

<a href="javascript:doSomething()">link</a>
<a href="#" onClick="doSomething()">link</a>
<a href="#" onClick="javascript:doSomething();">link&;lt;/a>
<a href="#" onClick="javascript:doSomething(); return false;">link</a>

4. Avoid document.all
Only Use document.all As A Last Resort
There is never a reason to use document.all in javascript except as a fall-back case when other methods are not supported and very early IE support (<5.0) is required.
if (document.getElementById) {
var obj = document.getElementById("myId");
else if (document.all) {
var obj = document.all("myId");

5. Use Correct <script> Tags
The LANGUAGE attribute is deprecated in the <script> tag. The proper way to create a javascript code block is:
<script type="text/javascript">
// code here

6. Use JSON

When storing data structures as plain text or sending/retrieving data structures via Ajax, use JSON instead of XML when possible. JSON (JavaScript Object Notation) is a more compact and efficient data format, and is language-neutral.

7. Avoid Cluttering The Global Namespace
Global variables and functions are rarely required. Using globals may cause naming conflicts between javascript source files and cause code to break. For this reason, it is a good practice to encapsulate functionality within a single global namespace.
var MyLib = {}; // global Object cointainer
MyLib.value = 1;
MyLib.increment = function() { MyLib.value++; } = function() { alert(MyLib.value); }

MyLib.increment();; // alerts 7

8. Avoid 'eval'
The eval() function in javascript is a way to run arbitrary code at run-time. In almost all cases, eval should never be used. If it exists in your page, there is almost always a more correct way to accomplish what you are doing. For example, eval is often used by programmers who do not know about using Square Bracket Notation.

The rule is, "Eval is evil." Don't use it unless you are an experienced developer and know that your case is an exception.

9. Use Square Bracket Notation

When accessing object properties that are determined at run-time or which contain characters not compatible with dot notation, use square bracket notation. If you are not an experienced javascript programmer, it's not a bad practice to use square bracket notation all the time.

Objects properties in javascript can be accessed primarily in two ways: Dot notation and Square bracket notation.

Dot notation

Square bracket notation


With dot notation, the property name is hard-coded and cannot be changed at run-time. With bracket notation, the property name is a string which is evaluated to resolve the property name. The string can be hard-coded, or a variable, or even a function call which returns a string property name.

The recommendation for using square bracket notation is to always use it when it is required (obviously). Using it when not strictly required is a matter of personal preference and convention. One good rule of thumb is to use dot notation to access standard properties of objects, and square bracket notation to access properties which are defined as objects in the page.

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