How do I print the return value of a function?

If I understand what you are trying to achieve correctly, here is a working version of your code.

   function product(num){
     let result = 1;
     strNum = num.toString()
     strNumArr = strNum.split("")
     
     if(strNum.length === 1){
       return num;
     }
     
     
     for(let i = 0; i< strNum.length; i++){
       result *= parseInt(strNumArr[i])  
     }
     return result;
   }
  
 console.log(product(39)); // should return 27
 console.log(product(5)); // should return 5
 console.log(product(234)); // should return 24

You should return the result after you are done with the loop.

By the way the same can be achieved with a one liner. For example

function product(num) { 
    return Array.from(String(num).split('')).reduce((c,p) => p * c, 1)
}

Replace product(result) to return product(result). This way if the function calls itself it returns the value generated in the nested function call.

let count = 0;
   
   
   function product(num){
     let result = 1;
     strNum = num.toString()
     strNumArr = strNum.split("")
     
     if(strNum.length === 1){
       return count;
     }
     
     
     for(let i = 0; i< strNum.length; i++){
       result *= parseInt(strNumArr[i])  
     }
     count++;
     //console.log(count)
     return product(result)
   }
  
 let bool = product(39);
 console.log(product(39));