30-seconds-of-code/snippets/primes.md

primes

Generates primes up to a given number, using the Sieve of Eratosthenes.

Generate an array from 2 to the given number. Use Array.filter() to filter out the values divisible by any number from 2 to the square root of the provided number.

const primes = num => {
  let arr =  Array.from({length:num-1}).map((x,i)=> i+2), 
    sqroot  = Math.floor(Math.sqrt(num)),
    numsTillSqroot  = Array.from({length:sqroot-1}).map((x,i)=> i+2);
  numsTillSqroot.forEach(x => arr = arr.filter(y => ((y%x)!==0)||(y==x)));
  return arr; 
}
// primes(10) -> [2,3,5,7]