How do you merge an array of objects based on equal property values?

Question

I have the following issue. I have an array of objects and I would like to add the values where the date is equal.

This is the starting array:

0: {date: "07-04-2020", value: 10}
1: {date: "10-04-2020", value: 20}
2: {date: "07-04-2020", value: 30}
3: {date: "14-04-2020", value: 60}

It should result in the following:

0: {date: "07-04-2020", value: 40}
1: {date: "10-04-2020", value: 20}
3: {date: "14-04-2020", value: 60}

I tried some higher order functions, such as filter, map and reduce but I am clueless.

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`var foo = [{date: "07-04-2020", value: 10}, {date: "10-04-2020", value: 20}, {date: "07-04-2020", value: 30}, {date: "14-04-2020", value: 60}];` `var bar = {};` `foo.forEach(i => { if(bar[i.date]) { bar[i.date] += i.value} else {bar[i.date] = i.value} })` - then map them back to your original structure — developer, Apr 14 '20 at 14:40
map back with `Object.keys(bar).map(k => { return {date: k, value: bar[k]}})` — developer, Apr 14 '20 at 14:47

score 1 · Accepted Answer · answered Apr 14 '20 at 14:41

You could use Array.reduce:

[...].reduce((acc, next) => {
  // Check if an item with the given date exists
  const existingItem = acc.find(item => item.date === next.date);
  // If not, add the new one to the array
  if (!existingItem) {
    return [...acc, next];
  }
  // If there's one already, mutate the value property and return everything
  existingItem.value += next.value;
  return acc;
}, []);

score 0 · Answer 2 · answered Apr 14 '20 at 14:55

Will this work for you?

let myArr = [{
    date: "07-04-2020",
    value: 10
  },
  {
    date: "10-04-2020",
    value: 20
  },
  {
    date: "07-04-2020",
    value: 30
  },
  {
    date: "14-04-2020",
    value: 60
  },
]

function sum(arr) {
  let result = [];
  let temp = {};

  arr.forEach((row) => {
    temp[row.date] = temp[row.date] ? temp[row.date] + row.value : row.value;
  });

  Object.entries(temp).forEach((dateValue) => {
    result.push({
      date: dateValue[0],
      value: dateValue[1]
    });
  });

  return result;
}
console.log(sum(myArr));

How do you merge an array of objects based on equal property values?

2 Answers2