Functional Programming Notes

compose.js

const compose = (...fns) => (...args) => fns.reduceRight((acc, fn) => [fn(...acc)], args)[0];

// sub :: (a, b) -> a
const sub = curry((a, b) => b - a);

const mathPipeline = compose(increment, sub(2), add(4));
mathPipeline(10);// 13

curry.js

/**
 * This programmatically breaks a function from this
 * (a, b, c) => doSomething
 * to this:
 * (a) => (b) => (c) => doSomething || (a, b) => (c) => doSomething
 * This is one reason why JS is quite a functional language and very reflective
 * @param {Function} fn - A function with two or more parameters that you want broken down into a curry
 */
const curry = (fn) => {
  const arity = fn.length;
  
  return function $curry(...args) {
    if (args.length < arity) {
      return $curry.bind(null, ...args);
    }
    
    return fn.apply(null, args);
  };
};

const add = curry((a, b) => a + b);

const increment = add(1);
increment(10);// 11

const filter = curry((fn, items) => items.filter(fn));
const noLetterAFilter = filter((item) => !/a/ig.test(item));
noLetterAFilter(["hello", "world", "apple"]);// ["hello", "world"]