I'm using XMLHttpRequest, and I want to access a local variable in the success callback function.

Here is the code:

function getFileContents(filePath, callbackFn) {  
    var xhr = new XMLHttpRequest();
    xhr.onreadystatechange = function() {
        if (xhr.readyState == 4) {
            callbackFn(xhr.responseText);
        }
    }
    xhr.open("GET", chrome.extension.getURL(filePath), true);
    xhr.send();
}

And I want to call it like this:

var test = "lol";

getFileContents("hello.js", function(data) {
    alert(test);
});

Here, test would be out of the scope of the callback function, since only the enclosing function's variables are accessible inside the callback function. What is the best way to pass test to the callback function so the alert(test); will display test correctly?