code show as below:
<html>
<head>
<script type="text/javascript">
function checkAll(){
var hobbies=document.getElementsByName("hobby");
for(var i=0;i<3;i++){
hobbies[i].checked=true;
}
}
function checkNo(){
var hobbies=document.getElementsByName("hobby");
for(var i=0;i<3;i++){
hobbies[i].checked=false;
}
}
function checkRe(){
var hobbies=document.getElementsByName("hobby");
for(var i=0;i<3;i++){
if(hobbies[i].checked){
hobbies[i].checked=false;
}else{
hobbies[i].checked=true;
}
}
}
</script>
</head>
<body>
<input type="checkbox" name="hobby" value="java"/>java
<input type="checkbox" name="hobby" value="c"/>c
<input type="checkbox" name="hobby" value="js"/>js
<button οnclick="checkAll()">Select All</button>
<button οnclick="checkNo()">No check</button>
<button οnclick="checkRe()">反选</button>
</body>
</html>
Problem encountered : when using for loop traversal, the loop body is not executed during execution. Finally, it was discovered that i was not assigned an initial value.
The variables in js are weak types, that is, the types can be changed. The assigned value determines the type of the variable. A variable that has not been assigned an initial value is undefined.