How do I sort a JSON by Date?

Question

I'm trying to loop through a JSON and sort it by the date so I can see the latest date to the oldest date, and then write it to the file.

Here is my code

var reader = JSON.parse(fs.readFileSync('txt.json', 'utf8'));
function sortByDate(a, b) {
    return new Date(a.lastUpdated).toJSON() - new Date(b.lastUpdated).toJSON();
}

reader.sort(sortByDate)

JSON Data Example

{
    "Data": {
        "Contents": [
            {
                "Key": [
                    "HelloTest"
                ],
                "lastUpdated": [
                    "2019-10-25T10:30:50.558Z"
                ]
            },
            {
                "Key": [
                    "TestHello"
                ],
                "lastUpdated": [
                    "2019-03-26T10:30:50.558Z"
                ]
            }
        ]
    }
}

Answer 1

Here are a couple of errors I found in your code:

• Your function name has a typo, it should be sortByDate and not sortbyDate.
• You need top sort the inner json.Data.Contents array, not the outer json object.
• You need to reference the first element of your lastUpdated arrays using lastUpdated[0].
• Finally, you do not need to call toJSON() on the date objects in your sorting function, simply convert to date and return the difference.

Also your inner data fields are arrays, which seems strange for a Key and a lastUpdated value.

If you keep your fields as arrays, here is a working example showing how to sort the inner Data.Contents array by date:

const jsonString = `{
  "Data": {
    "Contents": [{
        "Key": ["HelloTest"],
        "lastUpdated": ["2019-10-25T10:30:50.558Z"]
      }, {
        "Key": ["TestHello"],
        "lastUpdated": ["2019-03-26T10:30:50.558Z"]
      }]
  }
}`;

function sortByDate(a, b) {
    return new Date(a.lastUpdated[0]) - new Date(b.lastUpdated[0]);
}

const json = JSON.parse(jsonString);
const defaultValue = { Data: { Contents: [] } };
const sortedContents = [...(json || defaultValue).Data.Contents].sort(sortByDate);
const output = { ...json, Data: { Contents: sortedContents } };

console.log(output);

If you change your fields to scalars, which I suggest, here is another example:

const jsonString = `{
  "Data": {
    "Contents": [{
        "Key": "HelloTest",
        "lastUpdated": "2019-10-25T10:30:50.558Z"
      }, {
        "Key": "TestHello",
        "lastUpdated": "2019-03-26T10:30:50.558Z"
      }]
  }
}`;

function sortByDate(a, b) {
    return new Date(a.lastUpdated) - new Date(b.lastUpdated);
}

const json = JSON.parse(jsonString);
const defaultValue = { Data: { Contents: [] } };
const sortedContents = [...(json || defaultValue).Data.Contents].sort(sortByDate);
const output = { ...json, Data: { Contents: sortedContents } };

console.log(output);