Javascript Checkbox全选

function selectAll() {  
        var a = document.getElementsByTagName("input");   
        if(document.getElementsById("selectAll").checked == true) {   
            for (var i=0; i<a.length; i++)   
                   if (a[i].type == "checkbox") {  
                       a[i].checked = true;   
                   }  
        }   
        else {   
            for (var i=0; i<a.length; i++) {  
                if (a[i].type == "checkbox") {  
                    a[i].checked = false;   
                }  
            }      
        }   
    }