Merge objects of the same key name in arrays

customMergeObjectsWithSameKeys.js

var customMergeObjectsWithSameKeys = function (arr, key, mergeFunction) {
  var _arr = []
  arr.forEach(item => {
    var index = _arr.findIndex(_item => _item[key] === item[key])
    if (index === -1) {
      _arr.push(item)
    } else {
      _arr[index] = mergeFunction(_arr[index], item)
    }
  })
  return _arr
}

// Example
customMergeObjectsWithSameKeys(data, 'date', (item, _item) => ({
      ...item,
      cost: Number(item.cost) + Number(_item.cost),
      duration: Number(item.duration) + Number(_item.duration)
}))