Why am I receiving with this AJAX script the status 404?

I got into AJAX and am using the MAMP web server and trying to implement a simple script which should show me whether this is working. I tried tracking down the problem by using the status property of XMLHttpRequest(); I found out that it was a problem of type 404 (Page not found) and this is astonishing as I am using JSON file to handle the script which is in the same directory as my ajax script.

Here is the AJAX script:

var xhr = new XMLHttpRequest();
xhr.onload = function() {

if (xhr.status === 200) {

    alert("Good");
}

else if(xhr.status === 304)
{
    alert("304");

}


else if(xhr.status === 404)
{
    alert("404");
} 

else
{
    alert("500");
}
xhr.open("GET", 'json.json', true);
xhr.send(null);

And here my file json.json:

{
"events" :
[
    {
        "location" : "San Francisco, CA"
    }

]
}

This is really frustrating. Thanks in advance,

Raul Rao