Grouping and summing within an array of objects

Question

I'm trying to find the sum of each type in my array of roomList... Tried out reduce() but it is only returning the last value. Is there a way to achieve this using reduce()... I'm open to other approaches also.

let roomList=[
        {value:1,type:2},{value:2,type:2},{value:3,type:2},{value:4,type:3}, 
        {value:5,type:4},{value:6,type:4},{value:7,type:6},{value:8,type:6}, 
        {value:9,type:1},{value:10,type:8},{value:11,type:8},{value:12,type:8}
];
roomList.reduce((acc,obj)=>{
   acc[obj.type]=obj.value;
   return acc;
},{})

I'm expecting a result like this {1:9,2:6,3:4,4:11...}

Mathyn · Accepted Answer · 2020-07-10 12:58:48Z

2

You only get the last value because you overwrite and not add. Change your reduce code to this:

roomList.reduce((acc,obj)=>{
    acc[obj.type] = (acc[obj.type] || 0) + obj.value;
    return acc;
},{})

To clarify this line:

acc[obj.type] = (acc[obj.type] || 0) + obj.value;

We cannot simply do this:

acc[obj.type] += obj.value;

Because acc[obj.type] might not be initialized and therefore return an undefined value. Instead we do acc[obj.type] || 0 to retrieve the value and if it is falsy use a value of 0.

answered Jul 10, 2020 at 12:58

Mathyn

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Sven.hig · Accepted Answer · 2020-07-10 13:09:48Z

1

You could use map to accumulate the values

let roomList=[ {value:1,type:2},{value:2,type:2},{value:3,type:2},{value:4,type:3}, {value:5,type:4},{value:6,type:4},{value:7,type:6},{value:8,type:6}, {value:9,type:1},{value:10,type:8},{value:11,type:8},{value:12,type:8} ];
g={}
roomList.map(o=>{
  !g[o.type]? g[o.type]=o.value : g[o.type]= g[o.type]+o.value
})
console.log(g)

answered Jul 10, 2020 at 13:09

Sven.hig

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