Passing variable into xmlHTTPRequest as a URL parameter

I use a combination of PHP and JS.

I have a URL parameter in my parent URL called market.

http://example.com?market=stackoverflow

I store it to a variable.

<?php 
    $market = $_GET['market'];
?

Then in JS:

<script>
    var market;
    market = <?php echo $market; ?>;
</script> 

Then I have a function - xmlHTTPREQUEST. This is where the problem lies. I need to pass variable market into the function - append it to the script being called - mexx.php

<script>
    function test() {

        var ajax = new XMLHttpRequest();
        ajax.onreadystatechange = function() {
          if (ajax.readyState == 4) {
            xxx = (ajax.responseText);
            console.log(xxx);

          }
        };
        ajax.open("GET", "'mexx.php?market='+market", true);
        ajax.send(null);

    }
</script>

I can't see anything in console - all blank.

Where is the syntax error?