Użyj indeksu tablicy w ramach pętli
const logArrayElements = (element, index, array) => {
console.log('a[' + index + '] = ' + element);
};
// Notice that index 2 is skipped, since there is no item at
// that position in the array...
[2, 5,, 9].forEach(logArrayElements);
// logs:
// a[0] = 2
// a[1] = 5
// a[3] = 9
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