Given an array nums, return a new array containing only the values that are prime, in their original order. Checking a single number for primality is its own exercise; here the point is decompositionDecompositionSplitting a problem into smaller named pieces — e.g. "is this prime?" and "keep the ones that pass".. Pulling the primality test out into isPrime(n) leaves the main function with one simple job, and gives the test a place where it is easy to improve: the helper stops at √n, while the inline version checks every candidate up to n − 1.