Determine Happy Numbers Using Recursion in JavaScript

Happy Number

A happy number is a number which eventually reaches 1 when replaced by the sum of the square of each digit. Whereas if during this process any number gets repeated, the cycle will run infinitely and such numbers are called unhappy numbers.

For example − 13 is a happy number because,

1^2 + 3^2 = 10 and,
1^2 + 0^2 = 1

On the other hand, 36 is an unhappy number.

We are required to write a function that uses recursion to determine whether or not a number is a happy number.

So, let’s write this function out. The key to this function is that we will have to keep a record of the numbers that have already appeared, if the same number makes another appearance, we return false else if the squared digits add up to 1, we return true.

We will use an object to keep track of the already appeared number, we could’ve also used set or Map, but a simple object will do it for us as well.

The code for doing so will be −

Example

const squareSumRecursively = (n, res = 0) => {
   if(n){
      return squareSumRecursively(Math.floor(n/10), res+Math.pow((n%10),2));
   };
   return res;
};
const isHappy = (num, map = {}) => {
   if(num !== 1){
      if(map[num]){
         return false;
      }
      map[num] = 1;
      return isHappy(squareSumRecursively(num), map);
   };
   return true;
}
console.log(isHappy(36));
console.log(isHappy(13));
console.log(isHappy(36));
console.log(isHappy(23));

Output

The output in the console will be −

false
true
false
true