Make tags serializable for disk caching

[vjpr]

Is your feature request related to a problem? Please describe.

I want to cache tags to disk. That is, write exiftool.read results to JSON file on disk.

The results of exiftool.read involve DateTime objects though which makes deserialization difficult.

Describe the solution you'd like

I'd like for be able to call ExifTool.fromJSON(json), to get the original structure back. And maybe a toJSON also.

Describe alternatives you've considered

function exifDateTimeFromJson(dt) {
  return new ExifDateTime(
    dt.year,
    dt.month,
    dt.day,
    dt.hour,
    dt.minute,
    dt.second,
    dt.millisecond,
    dt.offset === UnsetZoneOffsetMinutes ? undefined : dt.offset,
    dt.rawValue,
    dt.zone?.name === UnsetZone.name ? undefined : dt.zoneName,
  )
}

[mceachen]

I don't need this, but I think it's a reasonable request. If you want to take a crack at this, I'd be happy to review your PR!

Your API suggestion (ExifTool.toJSON(tags: Tags): string and ExifTool.fromJSON(json: string): Tags) sound good to me.

I've added .toJSON methods to the ExifDateTime, ExifDate, and ExifTime classes, which means that JSON.stringify() "just works".

To reconstitute from JSON, use the new exported parseJSON method:

import { exiftool, parseJSON } from "exiftool-vendored"

const tags: Tags = await exiftool.readTags("/path/to/file.jpg")
const str: string = JSON.stringify(tags)

// parseJSON doesn't validate the input, so we don't assert that it's a Tags
// instance, but you can cast it (unsafely...)

const tags2: Tags = parseJSON(str) as Tags