Is there a way to get both FP and UTC functionality?

[daveyarwood]

I see that there is a date-fns/fp submodule and a date-fns/utc package, but is there a way to combine them?

[daveyarwood]

I found this (old?) documentation which mentioned UTC being in development, and included instructions for how to import the package to use either FP, UTC, or both:

// The main submodule:
import addDays from 'date-fns/addDays'

// FP variation:
import addDays from 'date-fns/fp/addDays'

// UTC variation:
import addDays from 'date-fns/utc/addDays'

// Both FP and UTC:
import addDays from 'date-fns/fp/utc/addDays'

// With tree-shaking enabled:
import { addDays, format } from 'date-fns/fp'

But that seems like outdated information at this point, as the latest docs no longer mention UTC in that section.

After installing the latest date-fns, date-fns-tz, and @date-fns/utc packages, I tried requiring the combined FP/UTC package above, but it didn't work:

> const fp = require('date-fns/fp')
undefined
> const utc = require('@date-fns/utc')
undefined
> const fpAndUtc = require('@date-fns/fp/utc')
Uncaught Error: Cannot find module '@date-fns/fp/utc'
Require stack:
- <repl>
    at Module._resolveFilename (node:internal/modules/cjs/loader:1048:15)
    at Module._load (node:internal/modules/cjs/loader:901:27)
    at Module.require (node:internal/modules/cjs/loader:1115:19)
    at require (node:internal/modules/helpers:130:18) {
  code: 'MODULE_NOT_FOUND',
  requireStack: [ '<repl>' ]
}
> const fpAndUtcNoAtSymbol = require('date-fns/fp/utc')
Uncaught:
Error [ERR_PACKAGE_PATH_NOT_EXPORTED]: Package subpath './fp/utc' is not defined by "exports" in /Users/daveyarwood/code/spark/api/node_modules/date-fns/package.json
    at __node_internal_captureLargerStackTrace (node:internal/errors:497:5)
    at new NodeError (node:internal/errors:406:5)
    at exportsNotFound (node:internal/modules/esm/resolve:268:10)
    at packageExportsResolve (node:internal/modules/esm/resolve:598:9)
    at resolveExports (node:internal/modules/cjs/loader:547:36)
    at Module._findPath (node:internal/modules/cjs/loader:621:31)
    at Module._resolveFilename (node:internal/modules/cjs/loader:1034:27)
    at Module._load (node:internal/modules/cjs/loader:901:27)
    at Module.require (node:internal/modules/cjs/loader:1115:19)
    at require (node:internal/modules/helpers:130:18) {
  code: 'ERR_PACKAGE_PATH_NOT_EXPORTED'
}

Was support for this removed?

[fturmel]

These old docs are no good. I think at some point in v2 there were undocumented UTC variations for some date-fns functions, but since v3 the way to go is to use the UTCDate package you originally mentioned.

So, it should work - with some caveats though:

• The third party library date-fns-tz is not compatible with date-fns v3 yet, betas are out
  
• UTCDate is in the package @date-fns/utc and needs to be installed as a separate dependency

Building off the example in the docs for FP use:

import { UTCDate } from '@date-fns/utc';
import { addYears, format } from 'date-fns/fp';
import { flow } from 'lodash-es';

const addFiveYears = addYears(5);
const formatToString = format('yyyy-MM-dd');

const dates = [new UTCDate(2017, 0, 1), new UTCDate(2017, 1, 11), new UTCDate(2017, 6, 2)];

const results = dates.map(flow(addFiveYears, formatToString));
console.log(results); // [ '2022-01-01', '2022-02-11', '2022-07-02' ]

[daveyarwood]

startOfDay doesn't seem to work correctly with UTCDate instances:

> const { UTCDate } = require('@date-fns/utc')
undefined
> const { startOfDay } = require('date-fns/fp/index.js')
undefined
> new UTCDate("2024-03-29")
UTCDate 2024-03-29T00:00:00.000Z
> startOfDay(new UTCDate("2024-03-29"))
2024-03-28T04:00:00.000Z

I expected that to return 2024-03-28T00:00:00Z, but it offset it by 4 hours because I'm in the Eastern time zone.

[fturmel]

You need to use UTCDate along with date-fns v3 for things to work properly (sorry if I wasn't clear). Looking at your logs I'm pretty sure it's v2 and the startOfDay() function returns a Date instead of UTCDate.

I double checked and it does work for me with v3.

import { UTCDate } from '@date-fns/utc';
import { startOfDay } from 'date-fns/fp';

const date = new UTCDate('2024-03-29');
console.log(date); // UTCDate 2024-03-29T00:00:00.000Z
console.log(startOfDay(date)); // UTCDate 2024-03-29T00:00:00.000Z

[daveyarwood]

Thanks for the clarification! After updating the date-fns/fp package, it's working as expected 🎉