gistfile1.js

function load(url, callback) {

    var xhr = new XMLHttpRequest();

    xhr.onreadystatechange = onReady;

    function onReady() {

        if(xhr.readyState < 4 || xhr.status !== 200) {
            return;
        }

        if(xhr.readyState === 4) {
            callback(xhr);
        }
    }

    xhr.open('GET', url, true);
    xhr.send('');
}