Bitwise AND operator

To alternate the table background color:

rows [i].className = ((rows [i].index & 1) == 1) ? "alt" : "";

Javascript RegExp

- Example 1

var regex = /<title[^>]*>([\s\S]*?)<\/title>/i;
while(match = regex.exec(httpResp)){                     
   if ('Login Page' == match[1]) {    
     ...

"?" is important in the above regexp. By default it is greedy mode, with ?, it means the group will stop at the first encountered '<';

exec return ["<title>Login Page</title>", "Login Page"] since group () is used in regex which is useful to extract interested content.

-Example 2

 html += ' <a href="javascript:showManyToMany (groupsForUser, \'' + 
rowData.name.replace(/\'/g, "\\'").replace(/\"/g, "&.quot;") +  '\');">Edit</a>';

In this case, single quote has to be escaped by javascript \\'; however Double quotes has to be escaped by html as &.quot; rather than \\", otherwise, "unterminated string literal" error occur. It looks like browser can understand \', but it doesn't
understand \".

- Example 3
cartName.replace(" ", "_"); will only replace first occurrence of space character. It should be cartName.replace(/\s/g, "_"). 1st parameter is regExp, rather than string, and /g means global search for all occurrences of a pattern. Note that the regular expression doesn't need to be quoted.

Session timeout handing in AJAX call

DWR has a global handler textHtmlHandler for this purpose. When a DWR request receives a response that isn't Javascript. It indicates a server session has timed out, and we can redirect user to logon screen in this handler. So any DWR call doesn't need to worry about session timeout.

dwr.engine.setTextHtmlHandler(function() {
   document.location.reload();
});

Unfortunately, Dojo doesn't have such mechanism, so for each dojo.io.bind or FormBind, we have to deal with it individually as shown below.

     load: function(type, data, evt) {
          sessionTimeoutCheck(data, evt);
          ... 
      }

I have modified dojo source code (doLoad method in BrowserIO.js) to support a global/transparent session timeout for dojo. I don't think Dojo itself will support this feature in the future. This is because dojo's response could be 'text/html' mimeType, which is typically used in setURL of contentPane. And the server normally returns the logon page when session timeout occurs. So dojo cannot know whether it is a requested page or timeout page. DWR is simplier since it doesn't support text/html and we can treat a html response as timeout.


** More on DWR's error handling - It has global/batch/call three levels of fine-grained support.

global error handler:

dwr.engine.setErrorHandler(handler);

call level error handlers:

Remote.method(params, {
  callback:function(data) { ... },
  errorHandler:function(errorString, exception) { ... }
});

or, in batch level:

dwr.engine.beginBatch();
Remote.method(params, function(data) { ... });
// Other remote calls
dwr.engine.endBatch({
   errorHandler:function(errorString, exception) { ... }
});

Javascript undefined vs. null

In JavaScript, undefined means a variable has been declared but has not yet been assigned a value, such as:

var TestVar;
alert(TestVar); //shows undefined
alert(typeof TestVar); //shows undefined

null is an assignment value. It can be assigned to a variable as a representation of no value:

var TestVar = null;
alert(TestVar); //shows null
alert(typeof TestVar); //shows object

From the preceding examples, it is clear that undefined and null are two distinct types: undefined is a type itself (undefined) while null is an object.

Unassigned variables are initialized by JavaScript with a default value of undefined.

JavaScript never sets a value to null. That must be done programmatically. As such, null can be a useful debugging tool. If a variable is null, it was set in the program, not by JavaScript.

null values are evaluated as follows when used in these contexts:

Boolean: false
Numeric: 0
String: “null”

undefined values are evaluated as follows when used in these contexts:

Boolean: false
Numeric: NaN
String: “undefined”

.........................................................................................................................................................

null is an object. It's type is null. undefined is not an object, it's type is undefined.

Use if (SomeObject) to check value existence is not robust. It will also catch the cases of SomeObject having numeric value of 0, or string value of empty string. Because their boolean coercion is also false. See, in JavaScript, null and undefined are actually equal according to the == and != operator! (ECMA-262 standard, section 11.9.3 tells us so.) So we have null == undefined. In vast majority of cases, you don't care about the difference at all, so using someExpr != null is good enough.

If we really need to differentiate null and undefined, use

typeof(SomeObject) == 'undefined'