Two most important functions for functional js

quickStart.js

/**
 * Here are two essential functions for functional style in JavaScript.
 * Unfortunately, functions are not curried by default in JS.
 * Also, there is no function composition operator in JS.
 * Thus, we have to implement these on our own.
 **/
const comp = (...fns) => (...args) =>
  fns.reduceRight((res, fn) => fn(...[].concat(res)), args);

const curryN = (num, fn) => (...args) =>
  args.length < num
    ? Function.prototype.bind.apply(fn, [this, ...args])
    : fn(...args);