4 ways in JavaScript to find if a number is prime or not

What is a prime number:

A number is called prime if that number is divisible by 1 and the number itself. For example, 2, 3, 5, 7, etc. are prime numbers. In this post, I will show you how to check if a number is prime or not in JavaScript with examples.

Method 1: By using a for loop:

This is the simplest way to find a prime number. We will write one loop that will iterate from 2 to number/2. On each iteration, we will check if the current value of the loop variable can divide the given number or not. If it can divide, it is not a prime number. If we don’t find any value in the loop that can divide the number, it is a prime number.

function isPrime(num) {
  if (num < 2 || isNaN(num) || !isFinite(num)) return false;

  for (let i = 2; i <= num / 2; i++) {
    if (num % i == 0) {
      return false;
    }
  }
  return true;
}

console.log(isPrime(2));
console.log(isPrime(53));

Download it on GitHub

Explanation:

• The isPrime method is used to check if a number is prime or not. It takes one number num as its parameter and returns one boolean value based on whether the parameter is a prime number or not.
• If the value of num is less than 2 or if it is not a number or if it is not a finite value it returns false.
• The for loop iterates from 2 to num/2 and if any number can divide num, it returns false.
• At the end of the method, it returns true i.e. no number is found in the loop that can divide the given number.

Method 2: By iterating up to the square root of the number:

With this method, the loop will iterate up to the square root of the number. We can find all the divisors of a number if we iterate up to the square root of that number. With this approach, we can reduce the iteration number of the loop.

function isPrime(num) {
  if (num < 2 || isNaN(num) || !isFinite(num)) return false;

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

console.log(isPrime(2));
console.log(isPrime(53));

Download it on GitHub

We are using the Math.sqrt method to find the square root of the number num. It will print similar results.

Method 3: By iterating over the odd numbers:

With this approach, we will first check if the number is divisible by 2 or not. If it is not divisible by 2, we will iterate over the odd numbers from 3 to the square root of the number.

function isPrime(num) {
  if (num < 2 || isNaN(num) || !isFinite(num)) return false;
  if (num % 2 == 0) return num == 2;

  for (let i = 3; i <= Math.sqrt(num); i += 2) {
    if (num % i == 0) {
      return false;
    }
  }
  return true;
}

console.log(isPrime(2));
console.log(isPrime(53));

Download it on GitHub

• It checks if the value of num is divisible by 2 or not. If yes, it returns true only if num is equal to 2.
• The for loop iterates from 3 and on each step, it increments the value of i by 2 i.e. it iterates over the odd numbers.

It will print similar results.

Method 4: By jumping between the numbers:

We can represent any prime number as 6i ± 1, where i is any integer value. Only 2 and 3 are excluded from this formula. This approach will first check if the number is divisible by 2 or 3. If it is not, it will start the loop from i = 5, and on each iteration, it will increment its value by 6. If i or i + 2 can divide the number, it won’t be a prime number.

function isPrime(num) {
  if (num < 2 || isNaN(num) || !isFinite(num)) return false;
  if (num % 2 == 0) return num == 2;
  if (num % 3 == 0) return num == 3;

  for (let i = 5; i <= Math.sqrt(num); i += 6) {
    if (num % i == 0 || num % (i + 2) == 0) {
      return false;
    }
  }
  return true;
}

console.log(isPrime(2));
console.log(isPrime(53));

Download it on GitHub

It will print the same result.

Finding all prime numbers in between 1 to 100:

We can use any of the above methods with a loop to find out all prime numbers between 1 to 100 or in any range:

function isPrime(num) {
  if (num <= 1) return false;
  if (num == 2) return true;

  for (let i = 2; i <= num / 2; i++) {
    if (num % i == 0) {
      return false;
    }
  }
  return true;
}

let result = [];

for (let j = 1; j <= 100; j++) {
  if (isPrime(j)) {
    result.push(j);
  }
}

console.log(result);

Download it on GitHub

Here, we are calling the isPrime method to check if a number is prime or not. The for loop runs from j = 1 to j = 100 and appends all prime numbers to the array result.

It will print the below output:

[
  2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71,
  73, 79, 83, 89, 97,
];

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