Sieve of Eratosthenes

primes.js

function * generateMultiplesOf (number) {
    for (let factor = 2;; factor++) {
        yield number * factor
    }
}

function * generatePrimes (maxNumber = 1000) {
    const firstPrime = 2
    const nonPrimeNumbers = new Set()

    for (let nextPrime = firstPrime; nextPrime < maxNumber; nextPrime++) {
        if (nonPrimeNumbers.has(nextPrime)) {
            continue
        }

        const multiples = generateMultiplesOf(nextPrime)

        for (let nextMultiple of multiples) {
            if (nextMultiple > maxNumber) {
                break
            }

            nonPrimeNumbers.add(nextMultiple)
        }

        yield nextPrime
    }
}

function floorPrime (number) {
    const primes = generatePrimes(number)

    return [...primes].slice(-1)[0]    
}

const primes = generatePrimes()

console.log(primes.next().value)
console.log(primes.next().value)
console.log(primes.next().value)

console.log(floorPrime(32432))