Key Value Parser & Stringify Example: QueryString- & Hash-Style String Parsing & Encoding Using `Functional Programming` in JavaScript!

KeyValueTools.js

/** 
 * Key Value Helpers 
 * Features QueryString & Hash Helper Utils 
 * API for `QS` & `Hash` inspired by the `JSON` browser API. 
 * 
 * Implements `parse` & `stringify`
 *
 * JSON.stringify({ foo: 'bar' })
 * -> '{"foo":"bar"}'
 * QS.stringify({ foo: 'bar' })
 * -> 'foo=bar'
 * 
 * The 4 following lines return (essentially) the same object:
 * JSON.parse('{"foo":"bar","bar":"baz"}')
 * QS.parse('?foo=bar&bar=baz')
 * QS.parse('foo=bar&bar=baz')
 * Hash.parse('#foo=bar&bar=baz')
 * // ^^^ All Return:
 * -> { foo: 'bar', bar: 'baz' }
 *
 * @author Dan Levy <dan@danlevy.net> 
 */
// Functional Programming Example in JavaScript!!!
export const QS = {parse, stringify}; //<-- primary API export, (think JSON api)
export const Hash = {parse, stringify}; //<-- Also works for hash key=value strings 

export function parse(s) {
  const keyValify   = kv => kv.split(/=/, 2); // < split limit=2 to stop data being truncated
  const packObject  = (obj = {}, [key, val]) => {
    obj[dec(key)] = dec(val);
    return obj;
  }
  
  return typeof s === 'string' && s
  .replace(/^[\?#]?/, '') // trim any unwanted ?/# prefix
  .split(/&/)         // make array of keyval chunks
  .map(keyValify)     // split keyval str on "=" char (so, we have array of tuples)
  //Finally, packObject uses some new ES2015/ES6 awesomeness:
  // 1. Destructuring in params: [key, val]
  // 2. Param defaults 
  // 3. Arrow functions (as named vars for overall readability)
  .reduce(packObject);
}

export function stringify(obj) {
  return typeof obj === 'object' && Object
  .keys(obj)
  .reduce((qs, key) => qs.concat(`${enc(key)}=${enc(obj[key])}`), [])
  .join('&') || '';
}

/* Disclaimer: while this is NOT the fastest, IMHO it is the prettiest to humans in both code, flame graph & stacktrace forms. */

const enc = encodeURIComponent, // <-- aliases
      dec = decodeURIComponent;