[JavaScript]截断字符串，解决中文1个字只算了1个字符的问题

function cutStr(str,num){
var val = str;
var length = 0;
var newVal="";
for (var i = 0; i < val.length; i++) {
if (val.charCodeAt(i) > 255){ //汉字还是字母
length = length + 2;
}else{
length = length + 1;
}
if(length > num){
newVal= str.substring(0,i);
return newVal;
}
}
return str;

}