Find elements in array of objects that have same properties and create a new array of objects

Question

I have an array of objects like this:

arr = [
  { a: "dog", b: 8, c: "male", d: "big", e: 100 },
  { a: "dog1", b: 5, c: "female", d: "big", e: 200},
  { a: "dog2", b: 18, c: "female", d: "big", e: 350},
  { a: "dog2", b: 18, c: "female", d: "big", e: 350},
  { a: "dog", b: 3, c: "male", d: "big", e: 100 },
  { a: "dog", b: 8, c: "male", d: "big", e: 100 },
];

I want to create a new array containing only the property a and the sum of e of those objects that have the same values for b, c and d, so that I'll have and array like this:

arr2 = [
  { a: "dog", e: 300 },
  { a: "dog2", e: 700},
];

I've already tried to use a map function inside a map function looking for the elements that have the same properties b, c and d, then pushing the result in the arr2, but it pushes every time all the objects of my initial arr.

Probably looking for reduce

Brian Thompson
– Brian Thompson

2021-08-10 14:46:34 +00:00
Commented Aug 10, 2021 at 14:46 — Brian Thompson
– Brian Thompson, Commented Aug 10, 2021 at 14:46

Terry Lennox · Accepted Answer · 2021-08-10 14:53:45Z

4

You could use Array.reduce to create a map of the array elements, using b,c and d combined as the key.

Once we have this map, we can use Object.values() to get an array as the desired result.

arr = [
  { a: 'dog', b: 8, c: 'male', d: 'big', e: 100 },
  { a: 'dog1', b: 5, c: 'female', d: 'big', e: 200},
  { a: 'dog2', b: 18, c: 'female', d: 'big', e: 350},
  { a: 'dog2', b: 18, c: 'female', d: 'big', e: 350},
  { a: 'dog', b: 3, c: 'male', d: 'big', e: 100 },
  { a: 'dog', b: 8, c: 'male', d: 'big', e: 100 },
];

const result = Object.values(arr.reduce((acc, cur) => {
    // Create a key from b,c,d..
    const key = cur.b + cur.c + cur.d
    acc[key] = acc[key] || { a: cur.a, e: 0 };
    acc[key].e += cur.e;
    return acc;
}, {}))

console.log('Result:', result);

answered Aug 10, 2021 at 14:53

Terry Lennox

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Comments

Yevhen Horbunkov · Accepted Answer · 2021-08-10 20:27:22Z

Universal solution for arbitrary array and keys to summarize

If you're looking for vanilla JS solution (no lodash), you may come up with something, adjustable to arbitrary group key property and property to summarize with Array.prototype.reduce():

build the Map, having desired property as a key;
increment Map items, based on the matching key
get Map.prototype.values() of that Map into array

const src = [
  { a: "dog", b: 8, c: "male", d: "big", e: 100 },
  { a: "dog1", b: 5, c: "female", d: "big", e: 200},
  { a: "dog2", b: 18, c: "female", d: "big", e: 350},
  { a: "dog2", b: 18, c: "female", d: "big", e: 350},
  { a: "dog", b: 3, c: "male", d: "big", e: 100 },
  { a: "dog", b: 8, c: "male", d: "big", e: 100 },
]

const summarizeValueByKey = (arr, key, val) => 
  [...arr
    .reduce((acc,{[key]: keyProp, [val]: valProp}) => {
      const group = acc.get(keyProp)
      group
        ? group[val] += valProp
        : acc.set(keyProp, {[key]: keyProp, [val]: valProp})
      return acc
    }, new Map)
    .values()]
    
console.log(summarizeValueByKey(src, 'a', 'e'))

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