前端异步加载js且判断js是不是加载完成

/**
     * 异步加载依赖的javascript文件
     * src：script的路径
     * callback：当外部的javascript文件被load的时候，执行的回调
     */
    function loadAsyncScript(src, callback) {
		var head = document.getElementsByTagName("head")[0];
		var script = document.createElement("script");
		script.setAttribute("type", "text/javascript");
		script.setAttribute("src", src);
		script.setAttribute("async", true);
		script.setAttribute("defer", true);
		head.appendChild(script);
		
		//fuck ie! duck type
		if (document.all) {
			script.onreadystatechange = function() {
				var state = this.readyState;
				if (state === 'loaded' || state === 'complete') {
					callback();
				}
			}
		} else {
			//firefox, chrome
			script.onload = function() {
				callback();
			}
		}
    }