Generating Random Prime Number in JavaScript

We are required to write a JavaScript function that takes in two numbers specifying a range. Our function should return a random prime number that falls in that range.

Algorithm Overview

The solution uses the Sieve of Eratosthenes algorithm to find all prime numbers in the range, then randomly selects one. This approach ensures efficiency for finding multiple primes.

Example

The code for this will be ?

const range = [100, 1000];

const getPrimes = (min, max) => {
    const result = Array(max + 1)
        .fill(0)
        .map((_, i) => i);
    
    for (let i = 2; i  {
    return Math.floor(Math.random() * (max - min + 1) + min);
};

const getRandomPrime = ([min, max]) => {
    const primes = getPrimes(min, max);
    return primes[getRandomNum(0, primes.length - 1)];
};

console.log(getRandomPrime(range));

Output

The output in the console will be ?

311

Output is likely to differ in each run since we're selecting a random prime from the range.

How It Works

The getPrimes() function implements the Sieve of Eratosthenes:

1. Creates an array from 0 to max
2. Marks multiples of each prime as composite (deletes them)
3. Returns remaining numbers (primes) in the specified range

The getRandomPrime() function then picks a random index from the primes array.

Testing with Different Ranges

// Test with smaller range
console.log("Prime between 10-50:", getRandomPrime([10, 50]));

// Test with larger range
console.log("Prime between 500-1000:", getRandomPrime([500, 1000]));

// Multiple calls to show randomness
console.log("Multiple calls:");
for (let i = 0; i 

Prime between 10-50: 37
Prime between 500-1000: 743
Multiple calls:
109
137
167
191
149


Conclusion

This solution efficiently generates random prime numbers using the Sieve of Eratosthenes algorithm. The approach works well for moderate ranges and ensures truly random selection from all available primes.