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AJAX building query string from checkbox array

Hi all,

I'm trying to build a querystring from a check box array.

Here's my AJAX code - the var options and var querystring are the areas I'm guessing I need to flesh out but not quite sure how to do it:

<script language="javascript" type="text/javascript">
//Browser Support Code
function ajaxFunction(){
	var ajaxRequest;  // The variable that makes Ajax possible!
		// Opera 8.0+, Firefox, Safari
		ajaxRequest = new XMLHttpRequest();
	} catch (e){
		// Internet Explorer Browsers
			ajaxRequest = new ActiveXObject("Msxml2.XMLHTTP");
		} catch (e) {
				ajaxRequest = new ActiveXObject("Microsoft.XMLHTTP");
			} catch (e){
				// Something went wrong
				alert("Your browser broke!");
				return false;
	// Create a function that will receive data sent from the server
	ajaxRequest.onreadystatechange = function(){
			if(ajaxRequest.readyState == 4){
			document.getElementById('result').innerHTML = ajaxRequest.responseText;
	//Need to grab query string and append checked checkboxes array
	var options = document.get......
	var queryString = "?filter_options=" + options....;
	ajaxRequest.open("GET", "filter_query.php" + queryString, true);


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Here's some checkboxes:

<label for="1"><input type="checkbox" onClick="ajaxFunction();" name="filter_option[]" value="1" id="1">Option 1</label>
<label for="2"><input type="checkbox" onClick="ajaxFunction();" name="filter_option[]" value="2" id="2">Option 2</label>
<label for="3"><input type="checkbox" onClick="ajaxFunction();" name="filter_option[]" value="3" id="3">Option 3</label>

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There can be loads of these and I was thinking of sending the string of id's to the server like:
filter_option=1-2-3-4-5-45-34-67-89 and doing an explode() on them before querying.

Lastly, the user arrives at this page with variables in the URL obtained via GET already.

e.g. search.php?qs=My+Search&date=3m

So I have a few questions around this:
A) How can this be achieved?
B) Is this the best way and is this fully browser compliant?
C) How can I include the variables already in the URL in the AJAX call? Write them into it via php?

Huge thanks
2 Solutions
use :

var checkboxes = document.getElementsByName("filter_option[]");
var queryString = [];
for(var i=0;i<checkboxes.length;i++) {
    if( checkboxes[i].checked ) {
        queryString.push( checkboxes[i].value );
queryString = "?filter_options=" + queryString.join(",");

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test page : http://jsfiddle.net/XekX7/
In order to include the "variables already in URL" into the new URL you could add to leakim971 last line:

prevQS="<?php echo($_SERVER["QUERY_STRING"])?>";
queryString = "?"+prevQS+"&filter_options=" + queryString.join(",");

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I believe this should work.

dolythgoeAuthor Commented:
Huge thanks guys
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