JavaScript: Check a number is prime or not

JavaScript Function: Exercise-8 with Solution

Write a JavaScript function that accepts a number as a parameter and checks whether it is prime or not using recursion.

Note : A prime number (or a prime) is a natural number greater than 1 that has no positive divisors other than 1 and itself.

Visual Presentation:

Sample Solution-1:

JavaScript Code:

// Define a function named test_prime that checks whether a given number n is a prime number
function test_prime(n) {

  // Check if the number is equal to 1, which is not a prime number
  if (n === 1) {
    return false;
  }
  // Check if the number is equal to 2, which is a prime number
  else if (n === 2) {
    return true;
  } else {
    // Iterate from 2 to n-1 to check for factors of n
    for (var x = 2; x < n; x++) {
      // If n is divisible by x without a remainder, it is not a prime number
      if (n % x === 0) {
        return false;
      }
    }
    // If no factors are found, the number is a prime number
    return true;  
  }
}

// Log the result of calling test_prime with the input number 37 to the console
console.log(test_prime(37));

Output:

true

Flowchart:

Live Demo:

See the Pen javascript-function-exercise-8 by w3resource (@w3resource) on CodePen.

Sample Solution-2:

JavaScript Code:

// Define a function named test_prime that checks whether a given number num is a prime number
function test_prime(num) {
  // Check if the number is less than or equal to 1, as numbers less than or equal to 1 are not prime
  if (num <= 1) {
    return false;
  }

  // Iterate from 2 to the square root of num to check for factors
  for (let i = 2; i <= Math.sqrt(num); i++) {
    // If num is divisible by i without a remainder, it is not a prime number
    if (num % i === 0) {
      return false;
    }
  }

  // If no factors are found, the number is a prime number
  return true;
}

// Log the result of calling test_prime with the input number 37 to the console
console.log(test_prime(37));

// Log the result of calling test_prime with the input number 36 to the console
console.log(test_prime(36)); 

Output:

true
false

Explanation:
The above function first checks if the input number is less than or equal to 1, which is not a prime number, and immediately returns false if it is. It then loops through all numbers from 2 to the square root of the input number using a for loop. It uses the modulus operator to check if each number is divisible by that number. If the input number is divisible by any number other than 1 and itself, it is not a prime number and the function returns false. Upon completion of the loop without finding a divisor, the function returns true, indicating that the input number is prime.

Flowchart:

Live Demo:

See the Pen javascript-function-exercise-8-1 by w3resource (@w3resource) on CodePen.

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