[
[ "a", 30.0, 27.969999999999999, 0.0 ],
[ "a", 20.0, 20.66, 0.0 ],
[ "b", 160.0, 127.08, 5.0 ],
[ "b", 30.0, 2.0, 1.0 ],
[ "c", 1.0, 0.0, 0.0 ],
[ "a", 10.0, 10.859999999999999, 0.0 ],
[ "b", 8.0, 5.3200000000000003, 1.0 ],
[ "b", 20.0, 9.6500000000000004, 0.0 ],
[ "a", 20.0, 11.300000000000001, 1.0 ],
[ "d", 60.0, 56.609999999999999, 6.0 ],
[ "d", 60.0, 57.340000000000003, 4.0 ],
[ "c", 90.0, 72.420000000000002, 8.0 ],
[ "a", 80.0, 7.9000000000000004, 1.0 ]
]
得到如下一个数组
[
[ "a", 160.0, 78.689999999999998, 2.0 ],
[ "b", 218.0, 144.05000000000001, 7.0 ],
[ "c", 91.0, 72.420000000000002, 8.0 ],
[ "d", 120.0, 113.95, 10.0 ]
]

如何把这样的一个数组，根据数组的"a","b"相同，其它的元素相加，返回一个新的数组

let arr = [ ['a',123,456,789], ['a',123,456,789], ['b',123,456,789], ['a',123,456,789], ['c',123,456,789], ['d',123,456,789] ] ;
for(let i=0;i<arr.length;i++){
for(let j=i+1 ; j<arr.length;j++){
if(arr[i][0] == arr[j][0]){
for(let k in arr[i]){
if(k !=0){ arr[i][k] = arr[i][k]+arr[j] [k]; }
}
arr.splice(j,1)
}
}
}