Adding two numbers JS

You can try this:

function digital_root(n) {
  var z = n.toString();
  //use length here
  if (z.length == 2) {
    //convert to int
    var x = parseInt(z[0]) + parseInt(z[1]);
    return x;
  } else {
    return "not possible!";
  }
}

console.log( digital_root(16) );
console.log( digital_root(99) );
console.log( digital_root(999) );

Use split to split the string in half and add the two using parseInt to convert to a number.

const sum = (s) => (''+s).split('').reduce((a,b) => parseInt(a)+parseInt(b))
       ↑             ↑        ↑         ↑
      our          coerce   split      sum
    function     to string  in two     both

Here a test :

const sum = (s) => (''+s).split('').reduce((a,b) => parseInt(a)+parseInt(b))

console.log(sum(12))

There are several approaches to sum digits of a number. You can convert it to a string but IDK if thats neccesary at all. You can do it with numerical operations.

var input = 2568,
    sum = 0;

while (input) {
    sum += input % 10;
    input = Math.floor(input / 10);
}

console.log(sum);

Here's a fun short way to do it:

const number = 99
const temp = number.toString().split('')
const res = temp.reduce((a, c) => a + parseInt(c), 0) // 18

1.) Convert number to string

2.) Separate into individual numbers

3.) Use reduce to sum the numbers.

Your way would be the iterational way to solve this problem, but you can also use a recursive way.

Iterative solution (Imperative)

1. n.toString() Create String from number.
2. .split("") split string into chars.
3. .reduce(callback, startValue) reduces an array to a single value by applying the callback function to every element and updating the startValue.
4. (s, d) => s + parseInt(d) callback function which parses the element to an integer and adds it to s (the startValue).
5. 0 startValue.

Recursive solution (Functional)

1. condition?then:else short-hand if notation.
2. n<10 only one digit => just return it.
3. n%10 the last digit of the current number (1234%10 = 4).
4. digital_root_recurse(...) call the function recursivly.
5. Math.floor(n / 10) Divide by 10 => shift dcimal point to left (1234 => 123)
6. ... + ... add the last digit and the return value (digital root) of n/10 (1234 => 4 + root(123)).

function digital_root_string(n) {
  return n.toString().split("").reduce((s, d) => s + parseInt(d), 0);
}

function digital_root_recurse(n) {
  return n < 10 ? n : n % 10 + digital_root_recurse(Math.floor(n / 10));
}

console.log(digital_root_string(16));
console.log(digital_root_string(99));
console.log(digital_root_recurse(16));
console.log(digital_root_recurse(99));

The issue in your code is that you stored the length of n into z. The length is an integer, so both z[0] and [1] are undefined. The solution is to store the string into another variable and use that instead of z.

function digital_root(n) {
  n = n.toString();
  var l = n.length;
  if (l === 2) {
    return parseInt(n[0], 10) + parseInt(n[1], 10);
  }
}

console.log( digital_root(16) );

Simply use var x = parseInt(n/10) + (n%10); and it will work for you.

function digital_root(n) {
  var z = n.toString().length;
  if (z == 2) {
    var x =  parseInt(n/10) + (n%10);
    return x;
  }
}

console.log( digital_root(16) );
console.log( digital_root(99) );
console.log( digital_root(62) );

Convert input to string, split it, convert each item back to number and sum them all:

function digital_root(n) {
    return String(n).split('').map(Number).reduce((a,b) => a + b)
}

const result = digital_root(99);

console.log(result);