Generate Array of First N Prime Numbers in JavaScript

We are required to write a JavaScript function that takes in a number n and returns an array that contains first n prime numbers. We know that prime numbers are those numbers that are only divisible by 1 and themselves like 2, 3, 19, 37, 73 etc.

Let's understand the problem with an example ?

Input: n = 6; 
Output: prime_numbers = [ 2, 3, 5, 7, 11, 13 ]

Using Iteration

We will first write a function that checks whether a given number is prime or not and then run a loop till the given number n to generate n prime numbers.

Example

The JavaScript program to generate first prime numbers is ?

const isPrime = (n) => {
   for(let i = 2; i <= n/2; i++){
      if(n % i === 0){
         return false;
      }
   };
   return true;
};
const generatePrime = num => {
   const arr = [];
   let i = 2;
   while(arr.length < num){
      if(isPrime(i)){
         arr.push(i);
      };
      i = i === 2 ? i+1 : i+2;
   };
   return arr;
};
console.log("First 6 prime numbers are: ");
console.log(generatePrime(6));
console.log("First 16 prime numbers are: ");
console.log(generatePrime(16));

The output in the console will be ?

First 6 prime numbers are: 
[ 2, 3, 5, 7, 11, 13 ]
First 16 prime numbers are: 
[
   2,  3,  5,  7, 11, 13,
  17, 19, 23, 29, 31, 37,
  41, 43, 47, 53
]

Using Sieve of Eratosthenes Algorithm

This algorithm requires the following output ?

• Initialize a boolean array of size 10000 with TRUE values.
• Then, iterate through each number starting from 2.
• If a number is still marked as TRUE, add it to the array of primes, and all its multiples are marked as FALSE in boolean array.
• Continue this process until the array of primes contains n prime numbers.

Example

Let's see the practical implementation ?

function generatePrime(n) {
   const limit = 10000; 
   const arr = [];
   const newArray = new Array(limit).fill(true);
   
   for (let i = 2; i < limit; i++) {
      if (newArray[i]) {
         arr.push(i);
         for (let j = i * i; j < limit; j += i) {
            newArray[j] = false;
         }
      }
      if (arr.length === n) {
         break;
      }
   }
   return arr.slice(0, n);
}
console.log(generatePrime(10));

The output in the console will be ?

[
   2,  3,  5,  7, 11,
  13, 17, 19, 23, 29
]