按需加载的模拟

require

function require(name, callback) {
        var newJs = document.createElement('script');
        // ie
        newJs.onreadystatechange = function(){
            if(newJs.readyState === 'loaded' || newJs.readyState === 'complete'){
                newJs.onreadystatechange = null;
                callback && callback();
            }
        }
        // 正常
        newJs.onload = function(){
            callback && callback();
        }
        newJs.src = name;
        // document.documentElement 特指 head
        document.documentElement.firstChild.appendChild(newJs);
    }

借鉴学习来自 O'Reilly JavaScript Patterns