In this solution, I used some check with indexOf method in order to avoid some multiplications with reduce() method.
Logic was: why bother with multiplications: if we have zeroamong multiplicators, product will be zero anyway. Not sure if it was necessary??
I removed from code below 1000-digit number for readability 
It can be found in challenge step.
function largestProductinaSeries(n) {
/* let thousandDigits = here should be 1000-element array ;*/
let theLargestProduct = 0;
for (let i = 0; i <= thousandDigits.length - n; i++) {
const testedChunk = thousandDigits.slice(i, i + n);
//way to optimize: if zero is present in testedChunk, can 'leap over' some loopsteps
//depending on indexOf(i)
if (testedChunk.indexOf(0) === -1) {
//need to declare testedProduct in order not to use reduce twice
const testedProduct = testedChunk.reduce(
(previous, current) => previous * current, 1
);
if (testedProduct > theLargestProduct) {
theLargestProduct = testedProduct;
}
}
}
return theLargestProduct;
}
largestProductinaSeries(13);