Validation

Question

Someone please explain this code. It find the list of prime numbers from 0 to max. I don't understand what !store[i] means and how if statement validates it since store is an empty array and the value of store[i] when i is an integer should be undefined because the array is empty. Secondly I don't understand the use of the bitwise operator << is the second loop. Thank you.

function primeFactorsTo(max)
{
var store = [], i, j, primes = [];
for (i = 2; i <= max; ++i)
{
if (!store[i])
{
primes.push(i);
for (j = i << 1; j <= max; j += i)
{
store[j] = true;
}
}
}
return primes;
}

console.log(primeFactorsTo(5)); // 2,3,5
console.log(primeFactorsTo(15)); // 2,3,5,7,11,13

Answer 1

!store[i]
initially store[0] it is undefined but in loop store[j] =true; adding boolean true
When store[i] is false, and !store[i] becomes true and if executes...
I<<1 is left shift operation..
Look at here..
https://www.sololearn.com/learn/4087/?ref=app