function checkall(){
var hobby=document.getElementsTagName("input");
if(hobby==document.getElementsByName("checkbox")){
checkall();
}
} //任务一function checkall(){
var hobby = document.getElementsByTagName("input");
for(var i=0,len=hobby.length;i<len;i++){
if(hobby[i].type=="checkbox"){
hobby[i].checked=true;
}
}
}