# Code recipe: JavaScript program to find prime number(s) from an array

Posted on May 18, 2021

A guide on writing a JavaScript program to find prime number(s) from an array with code examples

A prime number is a number that’s only divisible by two numbers: one and itself. Some examples of prime numbers are 2, 3, 5, 7, 11, and 13.

This tutorial will help you to write a JavaScript program that can find prime number(s) from an array. But first, let’s write a function to find out if a number is a prime number.

## Writing a function to check for a prime number

To find prime numbers using a JavaScript program, you can use a combination of the for loop and the conditional if..else statement.

First, create a function that accepts a number to check whether it’s a prime number or not. Let’s call this function checkPrime():

function checkPrime(number) {}

Then, create an if block to check if the number value equals to 1 or lower. Return false if it is because 0, 1, and negative numbers are not prime numbers.

function checkPrime(number) {
if (number <= 1) {
return false;
}
}

Next, create an else block and initiate the for loop inside it. The for loop will be initialized from the number 2 and will continue to iterate until it’s equal to the number value:

function checkPrime(number) {
if (number <= 1) {
return false;
} else {
for (let i = 2; i < number; i++) {
// TODO: write the code to check for prime numbers
}
}
}

Inside the for loop, create another if block that checks whether the number value is divisible by the value of i and return false if it is. This is because the number must not be divisible by any other number except one and itself:

function checkPrime(number) {
if (number <= 1) {
return false;
} else {
for (let i = 2; i < number; i++) {
if (number % i == 0) {
return false;
}
}
}
}

The return keyword will stop the function execution and send the value false to the caller. You can store this value in a variable as follows:

const isPrime = checkPrime(9);
console.log(isPrime); // false

Finally, return true just below the for block that you’ve created to let the caller know that the number is a prime number:

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

This is because if the for loop finished without returning false, it means the number value can’t be divided by any number between 1 and itself. In other words, it’s a prime number.

This is why you should return true to let the function caller know that the number is a prime number.

Here’s the complete source code for you to inspect:

function checkPrime(number) {
if (number <= 1) {
return false;
} else {
for (let i = 2; i < number; i++) {
if (number % i == 0) {
return false;
}
}
return true;
}
}

Your checkPrime() function is now done. You can test the code by calling the function multiple times with different numbers:

let isPrime;

isPrime = checkPrime(3);
console.log(isPrime); // true
isPrime = checkPrime(12);
console.log(isPrime); // false
isPrime = checkPrime(50);
console.log(isPrime); // false
isPrime = checkPrime(23);
console.log(isPrime); // true
isPrime = checkPrime(0);
console.log(isPrime); // false

## Finding prime number(s) from an array of numbers

Now that you have a working checkPrime() function, you can check if a number is a prime or not from an array of numbers as follows:

let arr = [3, 12, 50, 23, 0];

To check the numbers inside the arr array above, you need to iterate through the array using the forEach() method, then call the checkPrime() function for each element passed from the callback function as in the code below:

arr.forEach(function (element) {
const isPrime = checkPrime(element);
if (isPrime) {
console.log(`\${element} is a prime number`);
} else {
console.log(`\${element} is NOT a prime number`);
}
});

And that’s how you can find prime numbers from an array of numbers. Feel free to use the code as you require it 😉

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