简易计算JS运行用时

var runtime = {
    start : function() {
        this.startTime = new Date();
    },
    end : function() {
        return (new Date() - this.startTime);
    }
};
runtime.start();
var html = '';
for (var i=0; i<1000; i++) {
    html += i+'cxcxcxcxcxcxcxc<br />';
}
document.body.innerHTML += html;
alert(runtime.end()); //单位ms