Checkbox selected and unchecked judgment example

$(document).ready(function() {  var flag = $("#protTuanFlag").val();  if(flag==1){    document.getElementById("tuan").checked=true;  };  $("#tuan").click(function() {    if(document.getElementById("tuan").checked){      //alert("选中");       document.getElementById("tuan").value = 1;       // alert(document.getElementById("tuan").value);    }else{      //alert("未选中");       document.getElementById("tuan").value = 0;       //alert(document.getElementById("tuan").value);    }  });})