随机数组找丢失数据

例：1-20的随机数组，找丢失的某几个数

function find(arr,count){
    var str= arr.toString();
    var newarr=[];
    for(var i=1;i<=count;i++){
        var reg=new RegExp("^"+i+",|,"+i+",|,"+i+"$","g");
        if(reg.test(str)) continue;
         newarr.push(i);
    }
    return newarr;
}
var arr=[11,2,4,7,8,3,10,13,14,19,18,16,12,20];
var result=find(arr,20);
console.log(result.toString());