FP Guide

date-fns v2.x provides functional programming (FP) friendly functions, like those in lodash, that support currying.

Table of Contents

• Usage

• Using Function Composition

Usage

FP functions are provided via 'date-fns/fp' submodule.

Functions with options (format, parse, etc.) have two FP counterparts: one that has the options object as its first argument and one that hasn't. The name of the former has WithOptions added to the end of its name.

In date-fns' FP functions, the order of arguments is reversed.

import { addYears, formatWithOptions } from 'date-fns/fp'
import { eo } from 'date-fns/locale'
import toUpper from 'lodash/fp/toUpper' // 'date-fns/fp' is compatible with 'lodash/fp'!

// If FP function has not received enough arguments, it returns another function
const addFiveYears = addYears(5)

// Several arguments can be curried at once
const dateToString = formatWithOptions({ locale: eo }, 'd MMMM yyyy')

const dates = [
  new Date(2017, 0 /* Jan */, 1),
  new Date(2017, 1 /* Feb */, 11),
  new Date(2017, 6 /* Jul */, 2)
]

const formattedDates = dates.map(addFiveYears).map(dateToString).map(toUpper)
//=> ['1 JANUARO 2022', '11 FEBRUARO 2022', '2 JULIO 2022']

Using Function Composition

The main advantage of FP functions is support of functional-style function composing.

In the example above, you can compose addFiveYears, dateToString and toUpper into a single function:

const formattedDates = dates.map((date) => toUpper(dateToString(addFiveYears(date))))

Or you can use compose function provided by lodash to do the same in more idiomatic way:

import compose from 'lodash/fp/compose'

const formattedDates = dates.map(compose(toUpper, dateToString, addFiveYears))

Or if you prefer natural direction of composing (as opposed to the computationally correct order), you can use lodash' flow instead:

import flow from 'lodash/fp/flow'

const formattedDates = dates.map(flow(addFiveYears, dateToString, toUpper))