Overview

fnts is an abbreviation for something like a "functional TypeScript". You could see a similar concept in libraries like fp-ts or ramda, each endorsing their own implementation of the idea.

Here, the philosophy is to give the small set of functions, the possession of which makes it easier to operate the code in a functional style.

The minimal amount of abstractions not present in the TypeScript itself, or being hard to implement and use, (like HKT, typeclasses, overwhelming amount of iterators and transducers) aims to reduce the learning curve of this seemingly "different" style of programming.

Tooling

The library basically provides the tools to handily operate the following concepts: monads, currying, composition, guarding, handling side effects and control flows. Nothing more, nothing less.

Monads

Out of the variety of monads fnts chooses two presumably most suitable ones: maybe and either. Each have their own constructors and operators (as opposed to classes and methods based approach seen commonly).

import maybe from 'fnts/maybe';
import { foldMap } from 'fnts/maybe/operators';

foldMap(
  maybe([1, 2, 3].find((n) => n > 2)),
  (n) => n === 3
); // true

import either from 'fnts/either';
import { bifoldMap } from 'fnts/either/operators';

bifoldMap(
  await either(
    () => fetch('https://github.com')
  ),
  (error) => console.error(error),
  (data) => data
)

import eitherSync from 'fnts/either';
import { bifoldMap } from 'fnts/either/operators';

bifoldMap(
  eitherSync(
   () => JSON.parse(localStorage.getItem('context'))
  ),
  (error) => console.error(error),
  (data) => data
);

Composition

Composition in fnts is represented through the compose and pipe functions. Both are implemented without the overloads, which causes the types to be rather imposed than inferred.

import compose from 'fnts/compose';

const isTwoDigits = compose(
  compose(
    (b: boolean) => b ? 'true' : 'false',
    (s: string) => s.length === 2 // argument type imposed from the next function
  ),
  (n: number) => `${n}`,
); // will accept only a number as argument

isTwoDigits(5) === 'false';
isTwoDigits(14) === 'true';

import pipe from 'fnts/pipe';

const isTwoDigits = pipe(
  pipe(
    (n: number) => `${n}`,
    (s: string) => s.length === 2
  ),
  (b: boolean) => b ? 'true' : 'false',
);

isTwoDigits(5) === 'false';
isTwoDigits(14) === 'true';

Currying

The curry function is here to help with auto-currying of variadic or fixed amount of arguments.

import curry from 'fnts/curry';

const sumOfThree = curry(
  (a: number, b: number, c: number): number => {
    return a + b + c;
  }
);

sumOfThree(1, 2, 3) === 
sumOfThree(1, 2)(3) === 
sumOfThree(1)(2, 3) === 
sumOfThree(1)(2)(3);

Application

apply function calls the functions provided to it on the same set of arguments and returns the tuple with results.

import apply from 'fnts/apply';

apply(
    (a: number, b: number) => a + b,
    (a: number, b: number) => a - b,
    (a: number, b: number) => a / b,
    (a: number, b: number) => a * b,
)(3, 2); // [5, 1, 1.5, 6]

Arguments Permutation

For non-commutative operations or functions that can be applied in the compositional context it is handy to be able to automatically permutate (switch places of) their arguments. For some functions in fnts this is already implemented:

import { fmap } from 'fnts/maybe/operators';

const mapToNumber = (maybe: Maybe<string>): Maybe<number> => fmap(maybe, (value) => Number(value));

Here, mapToNumber declaration is equivalent to:

import { fmap } from 'fnts/maybe/operators';

const mapToNumber = fmap<string, number>(Number);

Side effects

For handling side effects there are a couple of functions, the underlying concept of which is to not interfere with the main execution flow:

import inject from 'fnts/inject';

const computeAndLog = inject(
  compute,
  (...args) => console.log('Computing with args: ', args)
);

computeAndLog(1, 2, 3);

Guarding

In computing there's a pattern called "guard". Specifically, Haskell has a dedicated syntax for that, which fnts also implemented in a more JavaScripty way:

import guard from 'fnts/guard';

guard<(x: number) => number>(
  [(x) => x < 5, (x) => x + 1],
  [(x) => x === 5, (x) => x - 1],
  () => 1
)(5) // 4

Lenses

There's a pattern in functional programming called "lenses". fnts brings two necessary functions for this pattern to work, although in a simplified manner, – get and set.

import get from 'fnts/lens/get';
import set from 'fnts/lens/set';

const object = {
  a: {
    b: {
      c: 1
    }
  }
};

const value = get(object, 'a.b.c'); // 1
const objectCopy = set(object, 'a.b.c', 4);

Alternatively, lens function which combines both APIs together is available:

import lens from 'fnts/lens';

const object = {
  a: {
    b: [
      { c: 0 },
      { c: 1 },
      { c: 2 },
      { c: 3 },
    ]
  }
};

const objectLens = lens(object);

const value = objectLens('a.b.1.c'); // 1
const objectCopy = objectLens('a.b.1.c', 4);

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For a full overview of the available tools consult with the API reference.