An example of functional programming in JavaScript, featuring compose, pipe, tap and trace functions

fp.js

/**
 * Performs right-to-left composition, combining multiple functions into a single function.
 * @function compose
 * @param {...Function} args
 * @returns {Function}
 */
const compose = (...fns) => x => fns.reduceRight((output, fn) => fn(output), x, fns);

/**
 * Performs left-to-right composition, combining multiple functions into a single function. Sometimes called `sequence`. Right-to-left composition is typically known as `compose`.
 * @function pipe
 * @param {...Function} args
 * @returns {Function}
 */
const pipe = (...fns) => x => fns.reduce((output, fn) => fn(output), x);

/** Runs the given function with the supplied object, then returns the object.
 * @function tap
 * @param {Function} f
 * @returns {*}
 */
const tap = f => x => {
  f(x);
  return x;
};

/** Logs the given label and a provided object to the console, the returns the object.
 * @function trace
 * @param {String} label
 * @returns {Function}
 */
const trace = label => tap(console.log.bind(console, label + ':' || ''));

/**
 * Divides a number by 100.
 * @function divideBy100
 * @param {Number} num
 * @returns {Number} 
 */
const divideBy100 = num => num / 100;

/**
 * Returns a number to 2 decimal places.
 * @function roundTo2dp
 * @param {Number} num
 * @returns {String}
 */
const roundTo2dp = num => num.toFixed(2);

/**
 * Adds a dollar sign in front of a number or string.
 * @function addDollarSign
 * @param {Number|String} str
 * @returns {String}
 */
const addDollarSign = str => '$' + String(str);

/**
 * Add commas every three digits.
 * @function addSeparators
 * @param {Number|String} str
 * @returns {String}
 */
const addSeparators = str => {
  // Insert commas before the decimal point
  str = str.replace(/(?<!\.\d+)\B(?=(\d{3})+\b)/g, `,`);
  // Insert commas after the decimal point
  str = str.replace(/(?<=\.(\d{3})+)\B/g, `,`);
  return str;
}

/**
 * Convert cents to dollars.
 * @function centsToDollars
 * @param {Number} num - The value in cents.
 * @returns {String} - The value in dollars, with a currency symbol and separators.
 */
const centsToDollars = compose(
  trace('addSeparators'),
  addSeparators,
  trace('addDollarSign'),
  addDollarSign,
  trace('roundTo2dp'),
  roundTo2dp,
  trace('divideBy100'),
  divideBy100,
  trace('argument'),
);

centsToDollars(100000000);