问对求和数字使用递归，直到没有剩馀和为止

EN

function digital_root(n){
   return n < 10 ? n : digital_root(String(n).split('').reduce((acc,val) => acc+(val|0),0));
}

function digital_root(n) {
    // convert multiple digit number to String
    let a = n.toString();
    // split the String into an array of single digit Strings
    let ai = a.split('');
    // convert String array into int array  
    let bi = ai.map((item)=>{
        return parseInt(item);
    })
    
    // it looks like you are just copying the array here, 
    //I don't think there is a need for that
    //  let newArr = [];

    //  for(let i = 0; i < bi.length; i++){
    //    let c = bi[i];
    //    newArr.push(c);
    //  }
      
    // reduce the int array to the sum of its digits
    let d = bi.reduce((total, item)=>{
        return total + item;
    }, 0); 
   
    // That should be it, now just print it or recurse it
    if (d < 10) {
        return d;
    }
    // here is how you recurse, call the method from within the method
    return digital_root(d);

    // I'm not sure what you are doing here, you are repeating code
    // this is not how recursion works
//        function recursive(d){
//            if(d < 10){
//              return d
//            }

//            a = d.toString();
//            ai = a.split('');
      
//            bi = ai.map((item)=>{
//                return parseInt(item);
//            });

//            newArr = [];

//            for(let i = 0; i < bi.length; i++){
//                let c = bi[i];
//                newArr.push(c);
//            }
      
//            d = newArr.reduce((total, item)=>{
//                return total + item;
//            }, 0); 

//            return recursive(d);
//        }
//        return d;
}


console.log(digital_root(123));
console.log(digital_root(111));
console.log(digital_root(51024));

function sum(n) {
    if (n <= 9) {
        return n;
    } else {
        return sum((n % 10) + sum(n / 10));
    }
}

>>> sum(156)
... 3
>>> sum(9999)
... 9
>>> sum(10)
... 1