let flattened = [[0, 1], [2, 3], [4, 5]].reduce( function(accumulator, currentValue) { return accumulator.concat(currentValue) }, [] ) // flattened is [0, 1, 2, 3, 4, 5]
//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);