JS 异步加载script

function loadScript(url, callback) {
    var oscript = document.createElement('script');
    if (oscript.readyState) { // ie8及以下版本
        oscript.onreadystatechange = function () {
            if (oscript.readyState === 'complete' || oscript.readyState === 'loaded') {
                callback();
            }
        }
    } else {
        oscript.onload = function () {
            callback()
        };
    }
    oscript.src = url;
    document.body.appendChild(oscript);
}