Finding multiple objects in an array and adding to count value for that object

Question

I have been toying with the best way to go about doing this and have found some options but not one that preserves the format I am trying to keep for the array/object.

The overview is I have an array that gets random objects pushed into it and there can be duplicates of the same object, but I want to change it so there is only 1 of each object and instead, having a count property for each object.

An example of what I am working with and what I am aiming to have.

arr = [
  { Name: Item1, Value: 20 },
  { Name: Item2, Value: 20 },
  { Name: Item1, Value: 20 }
];

result = [
  { Name: Item1, Value: 20, Count: 2 },
  { Name: Item2, Value: 20, Count: 1 }
];

As a side note, I am wondering if it better to do this after the array is filled or if it is better to do this while pushing the objects into the array?

Answer 1

If items are continuously being added, you can maintain an "index" (by Name ) of objects you've already added to the array so that whenever a new object is added, you can update the count if it's already present or push it to the array if it's not:

 var arr = []; var index = {}; function addItem(o) { if (o.Name in index) { index[o.Name].Count += 1; } else { index[o.Name] = o; o.Count = 1; arr.push(o); } } addItem({ Name: 'Item1', Value: 20 }); addItem({ Name: 'Item2', Value: 20 }); addItem({ Name: 'Item1', Value: 20 }); console.log(arr); 

The benefit of this approach is that you don't have to recompute counts from scratch (which is an O(n) operation) every time you want to get the result array.

Answer 2

Assuming the value of each item is the same, you can do a simple forEach loop. For each element, check to see if the object exists in 'result'. If so, increment the count, otherwise add it to the result array.

let result = [];

arr.forEach(item => {
  let resObj = result.find(resObj => resObj.Name === item.Name);
  resObj ? resObj.Count++ : result.push({'Name':item.Name, 'Value': item.Value, 'Count': 1});
});

console.log(result);

In this case, you don't want to use arr.map, since we're not updating the original array.

Answer 3

You could take a hash table and count same items.

 var array = [{ Name: 'Item1', Value: 20 }, { Name: 'Item2', Value: 20 }, { Name: 'Item1', Value: 20 }], result = Object.values(array.reduce((r, { Name, Value }) => { r[Name] = r[Name] || { Name, Value, Count: 0 }; r[Name].Count++; return r; }, {})); console.log(result); 

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Answer 4

You can loop through the array and maintain 1 temporary object with "Name" as a key, If same name is found increment "count" by 1

 let array = [{ Name: 'Item1', Value: 20 }, { Name: 'Item2', Value: 20 }, { Name: 'Item1', Value: 20 }] let tempResult = {} for (let d of array) { tempResult[d.Name] = { count: 1, ...d, ...(tempResult[d.Name] && { count: tempResult[d.Name].count + 1 }) } } let result = Object.values(tempResult) console.log(result);