How to merge two objects and make sure when there’s duplicated keys, take one of the objects’ values first?

I have two objects created with Lodash’s groupBy like this:

const prevGroupedData = {
  '01/04/2000': [{data: 1}, {data: 2}, {data: 3}],
  '02/04/2000': [{data: 1}, {data: 2}]
}

const GroupedData = {
  '02/04/2000': [{data: 3}, {data: 4}],
  '03/04/2000': [{data: 1}]
}

So when I merge these two objects, I wanna make sure that it is the prevGroupedData that is merged first, that means something like:

const result = {
  '01/04/2000': [{data: 1}, {data: 2}, {data: 3}],
  '02/04/2000': [{data: 1}, {data: 2}, {data: 3}, {data: 4}], 
  '03/04/2000': [{data: 1}]
}

But not:

const result = {
  '01/04/2000': [{data: 1}, {data: 2}, {data: 3}],
  '02/04/2000': [{data: 3}, {data: 4}, {data: 1}, {data: 2}], 
  '03/04/2000': [{data: 1}]
}

I have tried something like lodash’s merge but it gives the second result:

let result = _.merge(prevGroupedData, GroupedData);
//gives second result

So how do I make sure that it is the prevGroupedData that has higher priority when there’s duplicated keys?