Pass arguments into ajax onreadystatechange callback?

Javascript supports closures, so the anonymous function you wrote will be able to access xhttp and msg from the enclosing doRequest() scope.

If wanted to do this explicitly (say, if you want to define the callback function somewhere else in the code and reuse it), you could create a function that creates the callbacks. This also allows you to alias the variables to be accessible with different names (like x and m):

function createCallback(x, m) {
    return function() {
        /* Do whatever */
    };
}

and then in doRequest(), do xhttp.onreadystatechange = createCallback(xhttp, msg);

If all you wanted to do was ‘rename’ the variables, you can do this inline and anonymously:

xhttp.onreadystatechange = (function(x, m) {
    return function() {
        /* Do stuff */
    }
})(xhttp, msg);