Number prime test in JavaScript by creating a custom function?

A prime number is a natural number greater than 1 that has no positive divisors other than 1 and itself. In JavaScript, we can create a custom function to check if a number is prime.

Basic Prime Number Function

Here's a simple approach to check if a number is prime:

function checkNumberIsPrime(number) {
    if (number <= 1) return false;
    if (number <= 3) return true;
    if (number % 2 === 0 || number % 3 === 0) return false;
    
    for (let i = 5; i * i <= number; i += 6) {
        if (number % i === 0 || number % (i + 2) === 0) {
            return false;
        }
    }
    return true;
}

// Test with different numbers
console.log("2 is prime:", checkNumberIsPrime(2));
console.log("11 is prime:", checkNumberIsPrime(11));
console.log("15 is prime:", checkNumberIsPrime(15));
console.log("17 is prime:", checkNumberIsPrime(17));

2 is prime: true
11 is prime: true
15 is prime: false
17 is prime: true

Simple Prime Check Function

For beginners, here's a more straightforward implementation:

function isPrime(num) {
    if (num < 2) return false;
    
    for (let i = 2; i <= Math.sqrt(num); i++) {
        if (num % i === 0) {
            return false;
        }
    }
    return true;
}

// Test the function
let testNumbers = [2, 3, 4, 5, 9, 11, 13, 15, 17, 20];

testNumbers.forEach(num => {
    console.log(`${num} is ${isPrime(num) ? 'prime' : 'not prime'}`);
});

2 is prime
3 is prime
4 is not prime
5 is prime
9 is not prime
11 is prime
13 is prime
15 is not prime
17 is prime
20 is not prime

How It Works

The prime checking algorithm works by:

1. Edge cases: Numbers less than 2 are not prime
2. Division test: Check if the number is divisible by any integer from 2 to ?n
3. Optimization: We only need to check up to the square root because factors come in pairs

Complete Example with Validation

function isPrimeNumber(number) {
    // Handle edge cases
    if (typeof number !== 'number' || !Number.isInteger(number)) {
        return false;
    }
    
    if (number < 2) return false;
    if (number === 2) return true;
    if (number % 2 === 0) return false;
    
    // Check odd divisors up to sqrt(number)
    for (let i = 3; i <= Math.sqrt(number); i += 2) {
        if (number % i === 0) {
            return false;
        }
    }
    
    return true;
}

// Example usage
function testPrimeNumbers() {
    const numbers = [1, 2, 7, 10, 11, 25, 29, 100, 101];
    
    numbers.forEach(num => {
        const result = isPrimeNumber(num);
        console.log(`${num}: ${result ? 'Prime' : 'Not Prime'}`);
    });
}

testPrimeNumbers();

1: Not Prime
2: Prime
7: Prime
10: Not Prime
11: Prime
25: Not Prime
29: Prime
100: Not Prime
101: Prime

Performance Comparison

Conclusion

The square root method provides an efficient way to check prime numbers in JavaScript. For most applications, checking divisors up to ?n offers the best balance of simplicity and performance.