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

alert(data);