functional-js.js

// helper functions
const compose = (...fns) => x => fns.reduceRight((res, fn) => fn(res), x);

const tap = f => x => { f(x); return x; };
const trace = label => tap(console.log.bind(console, label + ':'));

// container helpers
const isFunction = fn => fn && Object.prototype.toString.call(fn) === '[object Function]';
const isAsync = fn => fn && Object.prototype.toString.call(fn) === '[object AsyncFunction]';
const isPromise = p => p && Object.prototype.toString.call(p) === '[object Promise]';

class Container {
  constructor(public value) {
    //this.value = fn;
    if (!isFunction(this.value) && !isAsync(this.value)) {
      throw new TypeError(`Container expects a function, not a ${typeof this.value}.`);
    };
  }
  run() {
    return this.value();
  }
  map(fn) {
    if (!isFunction(fn) && !isAsync(fn)) {
      throw new TypeError(`The map method expects a function, not a ${typeof fn}.`);
    };
    return new Container(
      () => isPromise(this.value()) ?
        this.value().then(fn) : fn(this.value())
    )
  }
};


// code 
const divideBy100 = num => num / 100;
const roundTo2dp = num => num.toFixed(2);
const addDollarSign = str => '$' + String(str);
const addSeparators = str => {
  // add commas before the decimal point
  str = str.replace(/(?<!\.\d+)\B(?=(\d{3})+\b)/g, `,`);
  // add commas after the decimal point
  str = str.replace(/(?<=\.(\d{3})+)\B/g, `,`);
  return str;
};

const centsToDollars = compose(
  addSeparators,
  addDollarSign,
  trace('before add dollar sign'),
  roundTo2dp,
  divideBy100,
);

console.log(centsToDollars(100000), '100000')