<! DOCTYPE HTML > < HTML lang = "EN" > < head > < Meta charset = "UTF-. 8" > < title > debug mode </ title > ! <- debugging method is a method of displaying variables, there are three: alert (variable) console.log (variable) document.title = variable --> <script> // the window.onload = function () { // var oBody = document.getElementById ( 'the body1'); // iNum02 = 123; // alert (iNum02); // // alert by testing, the program will then blocked code. // oBody.style.backgroundColor = 'Violet'; // } the knob = 0 ; setInterval(function(){ console.log (iNum ++ ); // use console.log debugging js, you need to use console.log elements (elements) package to be tested and then view the console by console in your browser. document.title = iNum; // use document.title test // the syntax is: document.title = element; // will change the position of elements displayed in the label name browser. },10); </script> </head> <body id="body1"> </body> </html>