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