Reversing the prime length words - JavaScript

We are required to write a JavaScript function that takes in a string that contains words joined by whitespaces. Our function should create a new string that has all the words from the original string, but words whose length is a prime number should be reversed (i.e., words with length 2, 3, 5, 7, 11, etc.).

What are Prime Numbers?

A prime number is a natural number greater than 1 that has no positive divisors other than 1 and itself. For example: 2, 3, 5, 7, 11, 13, etc.

Algorithm

Our approach involves three main steps:

• Create a helper function to check if a number is prime
• Create a helper function to reverse a string
• Split the input string into words, check each word's length, and reverse if prime

Example

Following is the complete code:

const str = 'His father is an engineer by profession';

// Helper function to check if a number is prime
const isPrime = n => {
    if (n === 1) {
        return false;
    } else if (n === 2) {
        return true;
    } else {
        for (let x = 2; x < n; x++) {
            if (n % x === 0) {
                return false;
            }
        }
        return true;
    }
};

// Helper function to reverse a string
const reverseString = str => str.split('').reverse().join('');

// Main function to reverse words with prime length
const reversePrime = str => {
    return str.split(' ').reduce((acc, val) => {
        const { length } = val;
        if (isPrime(length)) {
            acc += reverseString(val) + ' ';
        } else {
            acc += val + ' ';
        }
        return acc;
    }, '');
};

console.log(reversePrime(str));

Output

siH father si na engineer yb profession 

How It Works

Let's trace through the example string 'His father is an engineer by profession':

• 'His' (length 3) - 3 is prime ? reversed to 'siH'
• 'father' (length 6) - 6 is not prime ? stays 'father'
• 'is' (length 2) - 2 is prime ? reversed to 'si'
• 'an' (length 2) - 2 is prime ? reversed to 'na'
• 'engineer' (length 8) - 8 is not prime ? stays 'engineer'
• 'by' (length 2) - 2 is prime ? reversed to 'yb'
• 'profession' (length 10) - 10 is not prime ? stays 'profession'

Optimized Version

Here's a more concise version using array methods:

const reversePrimeWords = str => {
    const isPrime = n => n > 1 && ![...Array(n).keys()].slice(2).some(i => n % i === 0);
    
    return str.split(' ')
        .map(word => isPrime(word.length) ? word.split('').reverse().join('') : word)
        .join(' ');
};

const testStr = 'His father is an engineer by profession';
console.log(reversePrimeWords(testStr));

siH father si na engineer yb profession

Conclusion

This solution efficiently identifies words with prime-length characters and reverses them while keeping other words intact. The key is combining prime number detection with string manipulation techniques.