(精华)2020年8月18日 js同异步和Performance页面性能分析<script src="js/main1.

js同异步

<!DOCTYPE html>
    <head>
        <meta charset="utf-8">
        <meta http-equiv="X-UA-Compatible" content="IE=edge">
        <title></title>
        <meta name="description" content="">
        <meta name="viewport" content="width=device-width, initial-scale=1">
        <link rel="stylesheet" href="">
    </head>
    <body>
        <div id="testdom"></div>
        <script src="js/main1.js" defer ></script>
        <script src="js/main2.js" defer ></script>
        <script src="js/main3.js" defer ></script>

        <!-- async:异步的，都不会阻塞DOM的渲染， 谁先加载完 就先执行哪个文件 -->
        <!-- defer：异步的，都不会阻塞DOM的渲染， 但是不会马上执行，按照顺序执行 ， 
            执行要在所有元素解析完成之后，DOMContentLoaded 事件触发之前完成-->


            <!-- dom文档加载顺序
                1> 解析<HTML>
                2>加载样式文件和脚本
                3>解析并执行脚本代码
                4> DOMContentLoaded执行点
                5> 加在外部图片
                6> load -->

                <!-- window.onload = function(){
                    
                } -->
        
        <!-- <script src="js/main1.js" defer ></script>
        <script src="js/main2.js" defer ></script>
        <script src="js/main3.js" defer ></script> -->


        <!-- async 对于应用脚本的用处不大，因为它完全不考虑依赖（哪怕是最低级的顺序执行），
        不过它对于那些可以不依赖任何脚本或不被任何脚本依赖的脚本来说却是非常合适的，最典型的例子：Google Analytics -->

<!-- 
        <script src="js/main1.js" defer async></script>
        <script src="js/main2.js" defer async></script>
        <script src="js/main3.js" defer async></script> -->

    </body>
</html>

Performance页面性能分析

<!DOCTYPE html>
    <head>
        <meta charset="utf-8">
        <meta http-equiv="X-UA-Compatible" content="IE=edge">
        <title></title>
        <meta name="description" content="">
        <meta name="viewport" content="width=device-width, initial-scale=1">
        <link rel="stylesheet" href="">
    </head>
    <body>
        <div id="testDom">
            test 
            session cookie
        </div>
        <script>
            var testDom = document.getElementById('testDom');
            // testDom.onclick = function(){
            //     console.log('111');
            // }

            // testDom.onclick = function(){
            //     console.log('222');
            // }

            testDom.addEventListener('click',function(){
                console.log('kkk');
            })

            testDom.addEventListener('click',function(){
                console.log('mmmm');
            })

            function getTiming(){
                var time = performance.timing;
                var timingObj = {};
                    timingObj['重定向时间'] = (time.redirectEnd - time.redirectStart)/1000;    
                    timingObj['DNS解析时间'] = (time.domainLookupEnd - time.domainLookupStart)/1000;
                    timingObj['TCP链接的时间'] = (time.connectEnd - time.connectStart)/1000;
                    timingObj['HTTP请求响应完成时间'] = (time.responseEnd  -  time.requestStart)/1000;
                    timingObj['解析DOM树解析的时间'] = (time.domComplete - time.domLoading)/1000;
                    timingObj['脚本的时间'] = (time.domContentLoadedEventEnd - time.domContentLoadedEventStart)/1000;
                    timingObj['onload事件时间'] = (time.loadEventEnd - time.loadEventStart) / 1000;
                
                    var totalTime = (time.loadEventEnd - time.fetchStart)/1000;
           
                for(item in timingObj) {
                    console.log(item+':'+timingObj[item]) +'秒'
                }
                console.log('总共的时间:'+totalTime);
            }

            window.addEventListener('load',getTiming)

            // window.onload  = function(){
            //     getTiming()
            // }

        var arr= [];
        for(var i=0;i<1000;i++) {
            arr.push(i);
        }

        </script>
    </body>
</html>