How to store and access ajax data in javascript without using global variables?

Question

I may be missing something obvious here, but how could I rewrite this code so that it doesn't need the theVariable to be a global variable ?

<script language="javascript">  
theVariable = "";  
function setValue()  /* called on page load */ 
{    
   /* make ajax call to the server here */
   theVariable = "a string of json data waiting to be eval()'d"; 
}  
function getValue()  
{  
    alert(theVariable);  
}  
</script>     


<input type="button" onClick="javascript:getValue()" value="Get the value">

In my actual situation, the setValue function makes an ajax call to the server, receives a json string and the data from that is accessed when you mouseover various parts of the page. I end up using several global variables which works fine, but is messy and I'd like to know if there's a better and more elegant way of doing it ?

Answer 1

I would do something like:

<script language="javascript">
var myApp = function () {

    /* theVariable is only available within myApp, not globally 
     * (unless you expose it) 
     */
    var theVariable = "";  

    /* called on page load */
    var setValue = function setValue(){

       /* make ajax call to the server here */
       theVariable = "a string of json data waiting to be eval()'d"; 

    };

    var getValue = function getValue() {

        alert(theVariable);

        // call useless private function
        pFunc();

    };

    var pFunc = function pFunc(){
        // this is a hypothetical private function
        // it's only here to show you how to do it
        // in case you need it

    };

    // now expose anything you need to be globally available
    // basically anything that will be called outside of myApp

    return { setValue: setValue, getValue: getValue}; 
}(); 
</script>     


<input type="button" onClick="myApp.getValue()" value="Get the value">

Then somewhere you would add an event listener or whatever for myApp.setValue() to run it on page load.

If you did:

return this;

Or just left the return statement out completely (which would imply return this)...

Then everything would be globally available as myApp.whatever or myApp[whatever].

Answer 2

If you use jQuery (which you may use for the ajax portion), you can use the .data() method which allows you to assign arbitury data to document elements by key/value.

Becuase javascript is dynamically typed, you can also get an element by name/id and then add a property to that element eg

document.myData = 'xyz';

Finally, you can limit the scope of your variables using something called a closure.