<script>

$(function(){

var num=0;

$("#btn").on('click',function(){

if(this.checked){

$("#box").children().attr('checked',true);

num=$("#box").children().length;//赋个值

}else{

$("#box").children().attr('checked',false);

num=0;//赋个值

}

});

$("#box").children().on('click',function(){

if(this.checked){

num++;

}else{

num--;

}

if(num==$("#box").children().length){

$("#btn").attr('checked',true);

}else{

$("#btn").attr('checked',false);

}

});

});

</script>

=======

<script>

window.onload = function(){

var aInput = document.getElementsByTagName('input');

//上面的全选按钮:

aInput[0].onclick = function(){

if(this.checked == true){

for(var i = 0; i < aInput.length; i++){

aInput[i].checked = true;

}

num = aInput.length - 1;

}else{

for(var i =0; i < aInput.length; i++){

aInput[i].checked = false;

}

num = 0;

}

}

var num = 0;

for(var i = 1; i < aInput.length; i++){

aInput[i].onclick = function(){

if(this.checked == true){

num++;

}else{

num--;

}

if(num == aInput.length - 1){

aInput[0].checked = true;

}else{

aInput[0].checked = false;

}

}

}

}

</script>