A prime iterator in JavaScript

Question

This is an implementation of a Sieve of Eratosthenes as [Symbol.iterator] in JavaScript.

Comments welcome, my JS is a bit rusty. Making a class of this is mainly the reason to have fun with the iterator. I will build a generator of it next.

UPDATE: inverting the sieve flags removes the need to initialize the array with true

console.log([...new Sieve(30)].join(","));
// prints 2,3,5,7,11,13,17,19,23,29

class Sieve {
    constructor(size) {
        this.size = size;
    }

    [Symbol.iterator]() {
        let primes = Array(this.size);
        let nextPrime = 1;
        return {
            next() {
                // find the next prime, marked by falsy in prime[]
                do {
                    if(++nextPrime >= primes.length) return {done :true};
                } while(primes[nextPrime])

                // remove the multiples of the prime in the sieve
                for(let idx=2; idx*nextPrime < primes.length; idx++) {
                    primes[idx*nextPrime] = true;
                }

                return { value: nextPrime };
            },
            [Symbol.iterator]: this
        }
    }
}