reduce with index code example

Example 1: .reduce mdn

arr.reduce(callback( accumulator, currentValue[, index[, array]] )[, initialValue])

Example 2: reduce javascript

//note idx and sourceArray are optional
const sum = array.reduce((accumulator, element[, idx[, sourceArray]]) => {
	//arbitrary example of why idx might be needed
	return accumulator + idx * 2 + element 
}, 0);