FlatMap vs Reduce for extracting data from a nested object

I have a dataset as such, where there may be multiple testResults array elements, along with there being 1 or many response array elements; and I an wanting to extract all of the potential completionDate values into their own array.

const testResults = [
  {
    "value": {
      "data": {
        "response": [
          {
            "completionDate": "2024-10-10T17:44:00",
          },
          {
            "completionDate": "2024-10-11T17:44:00",
          }
        ]
      }
    }
  },
  ...
];

I’m curious about the most effective/efficient way in which this could be done that is also readable by the rest of my team.

I have plenty of experience with using map and flatMap for things like this but have recently being using reduce more and wonder if one is more advantageous over the other, and if there’s any other method I have not considered.

flatMap

const timestamps = testResults.flatMap(result => 
  result.value.data.response.map(check => check?.completionDate)
);

reduce

const timestamps = testResults.reduce((acc, res) => {
  res.value.data.response.forEach(check => {
    acc.push(check.completionDate);
  });
  return acc;
}, []);

I did put use console.time around each of them and found that

map: 0.103ms

reduce: 0.067ms

Does this really indicate that the reduce is that much more efficient?

You need to sign in to view this answers