-C-O-N-C-E-P-T- Series : Javascript eval() Function

The eval() function executes a string as through it were a JavaScript statement.

Syntax: eval(string)

where string is any string representing a JavaScript expression, statement, or sequence of statements. The expression can include variables and properties of existing objects.

Description

The argument of the eval() function must be a string that can also be executed as a valid JavaScript statement. If the string represents an expression, eval() evaluates the expression. If the string is a numeric sring character, eval() returns the number. If the argument represents one or more JavaScript statements, eval() performs the statements.

Example

When executed, this script






displays this on the screen:

eval(string1) = 50

eval(executable) produces

Please feel free to correct me at any point :)

-C-O-N-C-E-P-T- Series : Making simultaneous AJAX calls to the same servlet

AJAX calls are "asynchronous" in nature. So what does this mean? You can have multiple requests to same (or different) servlets with each request processed asynchronously. Right, so are you speaking anything new?? Yes I am :) When you make simultaneous AJAX calls to the same servlet, the last call overlaps the response text with the previous, so as a result you get only the last requet's response text i.e. HTTP requests are overlaped. I faced this this problem in one of our Project, so I thought to share some views on it.

Examples:

Traditional code:


function ajaxRequest() {
...
myRequest.onreadystatechange = callBack; // assign callback function
...
}

function callBack() {
if (myRequest.readyState == 4) {
if (myRequest.status == 200) {
// Update DOM
}
}
}


Rearranging the above code:

Now we make some rearrangement of the AJAX code so that multiple HTTP requests can go on simultaneously. To do this, the XMLHTTP request object must be instantiated inside the Javascript function that calls the server. Furthermore, all code related to the request must be inside this function. The callback function must be an anonymous inner function within the ajaxRequest() function, and the code which updates the page after the callback must also be within the anonymous inner function.


function ajaxRequest() {
...
myRequest.onreadystatechange = function()
{
if (myRequest.readyState == 4)
{
if (myRequest.status == 200)
{
// update DOM with results
}
}
} // end anonymous inner function (callback)
...
}


Doing this I was able to handle multiple AJAX calls simultaneously.

Please feel free to correct me at any point :)