Using jQuery to get multiple checkbox's value and output as comma separated String.

Use the .map() function:

$('.Checkbox:checked').map(function() {return this.value;}).get().join(',')

Breaking that down:

$('.Checkbox:checked') selects the checked checkboxes.

.map(function() {return this.value;}) creates a jQuery object that holds an array containing the values of the checked checkboxes.

.get() returns the actual array.

.join(','); joins all of the elements of the array into a string, separated by a comma.

Working DEMO

var areaofinterest = '';

$('[name="areaofinterest"]')​.each(function(i,e) {
    var comma = areaofinterest.length===0?'':',';
    areaofinterest += (comma+e.value);
})​;

To get only checked boxes value :

var areaofinterest = '';

$('[name="areaofinterest"]')​.each(function(i,e) {
    if ($(e).is(':checked')) {
        var comma = areaofinterest.length===0?'':',';
        areaofinterest += (comma+e.value);
    }
})​;

FIDDLE

You could also do:

var areaofinterest = [];
$('[name="areaofinterest"]:checked').each(function(i,e) {
    areaofinterest.push(e.value);
});

areaofinterest = areaofinterest.join(',');

And there's probably a bunch of other ways to do this ?