Solution to Sub-task: Reduce

// Reduce
function reducer (grouped_obj) {
    for (x in grouped_obj){
        var arr1 = grouped_obj[x][0];
        var arr2 = grouped_obj[x][1];
        // take intersection of the two and assign to property name x
        grouped_obj[x] = arr1.filter(i => arr2.indexOf(i) !== -1 
            && x.charAt(0) !== i
            && x.charAt(1) !== i
        )
    }
    return grouped_obj;
    // return an object {(man,friend): [Friends]}
}
var grouped_obj = { 
    'AB': [ [ 'B', 'C', 'D' ], [ 'A', 'C', 'D', 'E' ] ],
    'AC': [ [ 'B', 'C', 'D' ], [ 'A', 'B', 'D', 'E' ] ],
    'AD': [ [ 'B', 'C', 'D' ], [ 'A', 'B', 'C', 'E' ] ],
    'BC': [ [ 'A', 'C', 'D', 'E' ], [ 'A', 'B', 'D', 'E' ] ],
    'BD': [ [ 'A', 'C', 'D', 'E' ], [ 'A', 'B', 'C', 'E' ] ],
    'BE': [ [ 'A', 'C', 'D', 'E' ], [ 'B', 'C', 'D' ] ],
    'CD': [ [ 'A', 'B', 'D', 'E' ], [ 'A', 'B', 'C', 'E' ] ],
    'CE': [ [ 'A', 'B', 'D', 'E' ], [ 'B', 'C', 'D' ] ],
    'DE': [ [ 'A', 'B', 'C', 'E' ], [ 'B', 'C', 'D' ] ] 
}
console.log(reducer(grouped_obj));