73 lines
2.3 KiB
Markdown
73 lines
2.3 KiB
Markdown
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# FP Guide
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**date-fns** v2.x provides [functional programming](https://en.wikipedia.org/wiki/Functional_programming) (FP)
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friendly functions, like those in [lodash](https://github.com/lodash/lodash/wiki/FP-Guide),
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that support [currying](https://en.wikipedia.org/wiki/Currying).
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## Table of Contents
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- [Usage](#usage)
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- [Using Function Composition](#using-function-composition)
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## Usage
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FP functions are provided via `'date-fns/fp'` submodule.
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Functions with options (`format`, `parse`, etc.) have two FP counterparts:
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one that has the options object as its first argument and one that hasn't.
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The name of the former has `WithOptions` added to the end of its name.
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In **date-fns'** FP functions, the order of arguments is reversed.
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```javascript
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import { addYears, formatWithOptions } from "date-fns/fp";
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import { eo } from "date-fns/locale";
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import toUpper from "lodash/fp/toUpper"; // 'date-fns/fp' is compatible with 'lodash/fp'!
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// If FP function has not received enough arguments, it returns another function
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const addFiveYears = addYears(5);
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// Several arguments can be curried at once
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const dateToString = formatWithOptions({ locale: eo }, "d MMMM yyyy");
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const dates = [
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new Date(2017, 0 /* Jan */, 1),
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new Date(2017, 1 /* Feb */, 11),
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new Date(2017, 6 /* Jul */, 2),
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];
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const formattedDates = dates.map(addFiveYears).map(dateToString).map(toUpper);
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//=> ['1 JANUARO 2022', '11 FEBRUARO 2022', '2 JULIO 2022']
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```
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## Using Function Composition
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The main advantage of FP functions is support of functional-style
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[function composing](https://medium.com/making-internets/why-using-chain-is-a-mistake-9bc1f80d51ba).
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In the example above, you can compose `addFiveYears`, `dateToString` and `toUpper` into a single function:
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```javascript
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const formattedDates = dates.map((date) =>
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toUpper(dateToString(addFiveYears(date))),
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);
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```
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Or you can use `compose` function provided by [lodash](https://lodash.com) to do the same in more idiomatic way:
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```javascript
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import { compose } from "lodash/fp/compose";
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const formattedDates = dates.map(compose(toUpper, dateToString, addFiveYears));
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```
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Or if you prefer natural direction of composing (as opposed to the computationally correct order),
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you can use lodash' `flow` instead:
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```javascript
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import flow from "lodash/fp/flow";
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const formattedDates = dates.map(flow(addFiveYears, dateToString, toUpper));
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```
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