JavaScript Select cascade operation

  var s1=    document.getElementById ("S1");    var s2=   document.getElementById ("S2");    var schoolterm =["First semester", "second semester", "Third semester", "fourth semester", "fifth semester"];    var content=[        ["HTML", "CSS", "JavaScript"],  ["C", "C + +"],        ["MySQL", "Oracle"],   ["Java"],        [" Jqury "," Bootstrap "," CSS3 "," HTML5 "  ]    ;    Initialize   for (var i=0; i<schoolterm.length; i++) {       s1.add (new Option (Schoolterm[i],schoolterm[i])  , NULL);   }    for (var i=0; i<content[0].length; i++) {        s2.add (new Option (Content[0][i],content[0][i])  , null);    }    /* SELECT *    /s1.onchange = function () {        var index =   s1.selectedindex;        s2.innerhtml = "";        for (var i=0; i<content[index].length; i++) {            s2.add (new Option (Content[index][i],content[index][i])  , NULL);        }    };

　　

<select name= "S1" id= "S1" ></select><select name= "s2" id= "S2" ></select>

　　

JavaScript Select cascade operation