Is the reversed number a prime number in JavaScript

We are required to write a JavaScript function that takes in a number and returns true if the reverse of that number is a prime number, false otherwise.

Problem Understanding

To solve this problem, we need two helper functions:

• A function to reverse the digits of a number
• A function to check if a number is prime

Solution Implementation

Here's the complete solution with helper functions:

const num = 13;

const findReverse = (num) => {
   return +num
   .toString()
   .split('')
   .reverse()
   .join('');
};

const isPrime = (num) => {
   if (num <= 1) return false;
   if (num <= 3) return true;
   if (num % 2 === 0 || num % 3 === 0) return false;
   
   let sqrtnum = Math.floor(Math.sqrt(num));
   let prime = num !== 1;
   for(let i = 2; i < sqrtnum + 1; i++){
      if(num % i === 0){
         prime = false;
         break;
      };
   };
   return prime;
}

const isReversePrime = num => isPrime(findReverse(num));

console.log(`Original number: ${num}`);
console.log(`Reversed number: ${findReverse(num)}`);
console.log(`Is reversed number prime? ${isReversePrime(num)}`);

Original number: 13
Reversed number: 31
Is reversed number prime? true

Testing with Multiple Examples

Let's test the function with different numbers to verify it works correctly:

const testNumbers = [13, 17, 19, 23, 12, 45];

testNumbers.forEach(num => {
   const reversed = findReverse(num);
   const result = isReversePrime(num);
   console.log(`${num} ? ${reversed} ? Prime: ${result}`);
});

13 ? 31 ? Prime: true
17 ? 71 ? Prime: true
19 ? 91 ? Prime: false
23 ? 32 ? Prime: false
12 ? 21 ? Prime: false
45 ? 54 ? Prime: false

How It Works

1. findReverse(): Converts number to string, splits into array, reverses, joins back, and converts to number
2. isPrime(): Checks if a number is prime by testing divisibility from 2 to ?n
3. isReversePrime(): Combines both functions to check if the reversed number is prime

Conclusion

This solution efficiently reverses a number and checks if the result is prime. The prime checking algorithm optimizes by only testing up to the square root of the number.