 # How to get or check prime numbers in JavaScript?

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December 19, 2022

If you want to get the prime numbers up to a given number then here is a function that generates prime numbers in JavaScript:

```function getPrimeNumbers(n) {
// create an array of booleans to represent whether each number is prime or not
let isPrime = new Array(n + 1).fill(true);

// set 0 and 1 to not prime
isPrime = isPrime = false;

// loop through all the numbers up to n
for (let i = 2; i <= n; i++) {
// if the number is prime, mark all its multiples as not prime
if (isPrime[i]) {
for (let j = 2 * i; j <= n; j += i) {
isPrime[j] = false;
}
}
}

// create an array to store the prime numbers
let primes = [];

// loop through the isPrime array and add the prime numbers to the primes array
for (let i = 0; i <= n; i++) {
if (isPrime[i]) {
primes.push(i);
}
}

return primes;
}
```

To use the function, you can call it with a number `n` as an argument, like this:

```let primeNumbers = getPrimeNumbers(20);

// [2, 3, 5, 7, 11, 13, 17, 19]
```

This will generate an array of all the prime numbers up to n. In this case, the output will be an array containing prime numbers from 2 to 20.

## How to Check Prime Number in JavaScript?

You can also check if a number is prime in JavaScript, you can use the following function:

```function isPrime(n) {
// check if n is less than 2
if (n < 2) {
return false;
}

// loop through the numbers from 2 to the square root of n
for (let i = 2; i <= Math.sqrt(n); i++) {
// if n is divisible by i, it is not prime
if (n % i === 0) {
return false;
}
}

// if the loop finishes, n is prime
return true;
}
```

Now call the function` isPrime(number)` to test a number if it is a prime or not. To use the above function, you can call it with a number as an argument, like this:

```console.log(isPrime(2)); // true
console.log(isPrime(3)); // true
console.log(isPrime(4)); // false
console.log(isPrime(19)); // true
```

This function uses a loop to check if the number is divisible by any other number between 2 and the square root of the number. If the number is not divisible by any of these numbers, it is considered prime.