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

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

Posted on May 18, 2021

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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