JSON_parser_recursive.js

  const fs = require('fs')
  const path = require('path')
  const example = fs.readFileSync(path.join(__dirname,'./example.json')).toString()
  var numRegex = /^[-+]?(\d+(\.\d*)?|\.\d+)([e][+-]?\d+)?/

  var nullParser = function (input) {
    input = spaceParser(input)
    return input.slice(0, 4) === 'null' ? [null, input.slice(4)] : null
  }

  var booleanParser = function (input) {
    input = spaceParser(input)
    var match = /(^true|^false)/.exec(input)
    return match !== null ? [match[0], input.slice(match[0].length)] : null
  }

  var commaParser = function (input) {
    input = spaceParser(input)
    return input[0] === ',' ? [',', input.slice(1)] : null
  }

  var spaceParser = function (input) {
    return input.match(/\s*/) ? input.replace(/\s*/, '') : input
  }

  var stringParser = function (input) {
    input = spaceParser(input)
    if (input[0] === '"') {
      input = input.slice(1)
      var endIndex = input.indexOf('"')
      if (input.indexOf('\\') > 0 && input.indexOf('\\') < endIndex) input = input.replace('\\', '')
      return [input.slice(0, endIndex), input.slice(endIndex + 1, input.length)]
    }
    return null
  }

  /* Array Validity Checker */
  var checkValidArray = function (res) {
    var input = spaceParser(res[1])
    var hasComma = commaParser(input)
    if (input === '') return null  // to check for input of type [12 , 13 , or [12 , 13
    if (hasComma) {
      if (!parser(spaceParser(hasComma[1]))) return null  // to check for input of type [12 , 14 , ]
    }
    if (res[0] !== ',' && !hasComma && input[0] !== ']') return null // to check for input of type [12 , 13 14]
    else return input
  }

  /* Helper function for arrayParser */
  var helperArrayParser = function (input, outArr) {
    if (input.length > 0 && input[0] !== ']') {
      var res = parser(spaceParser(input))
      if (res === null) return null
      if (res[0] !== ',') outArr.push(res[0])
      input = spaceParser(res[1])
      if (!checkValidArray(res)) return null
      return helperArrayParser(input, outArr)
    }
    return [spaceParser(input), outArr]
  }

  var arrayParser = function (input) {
    input = spaceParser(input)
    if (input[0] === '[') {
      input = spaceParser(input.slice(1))
      if (commaParser(input)) return null   // checked validity for [,]
      var outArr = []
      var res = helperArrayParser(input, outArr)
      if (res === null) return null
      else [input, outArr] = res
      if (input[0] !== ']') return null       // checked validity for [
      return [outArr, input.slice(1)]
    }
    return null
  }

  var numberParser = function (input) {
    input = spaceParser(input)
    var res
    if (res = numRegex.exec(input)) {
      var resLen = res.index + res[0].length
      var num = parseFloat(res[0])
      return [num, input.slice(resLen)]
    }
    return null
  }

  var keyParser = function (input) {
    return stringParser(spaceParser(input))
  }

  var valueParser = function (input) {
    return parser(spaceParser(input))
  }

  var colonParser = function (input) {
    return input[0] === ':' ? [':', input.slice(1)] : null
  }

  var checkValidObject = function (input) {
    if (input[0] === ',') {
      input = input.slice(1)
      if (keyParser(input) === null) return null   // to check Validity of {"k1" : 23 , }
    } else if (input[0] !== ',') {
      if (keyParser(input) !== null && input[0] !== '}') return null // to check Validity of {"k1":23 "K2"}
    }
    return input
  }

  /* Helper function for objectParser */
  var helperObjectParser = function (input, outArrObj) {
    if (input.length > 0 && input[0] !== '}') {
      var key = keyParser(input)
      if (key === null) return null
      input = colonParser(spaceParser(key[1]))
      if (!input) return null
      var value = valueParser(spaceParser(input[1]))
      if (value === null) return null
      input = spaceParser(value[1])
      outArrObj[key[0]] = value[0]
      input = checkValidObject(input)
      if (!input) return null
      return helperObjectParser(input, outArrObj)
    }
    return [input, outArrObj]
  }

  var objectParser = function (input) {
    input = spaceParser(input)
    var outArrObj = {}
    if (input[0] === '{') {
      input = spaceParser(input.slice(1))
      var res = helperObjectParser(input, outArrObj)
      if (!res) return null
      else [input, outArrObj] = res
      if (input[0] !== '}') return null    // checked validity for {
      if (outArrObj !== undefined) return [outArrObj, input.slice(1)]
    }
    return null
  }

  function parser (input) {
    var res
    if (res = nullParser(input))
      return res
    if (res = booleanParser(input))
      return res
    if (res = stringParser(input))
      return res
    if (res= arrayParser(input))
      return res
    if (res = commaParser(input))
      return res
    if (res = numberParser(input))
      return res
    if (res = objectParser(input))
      return res
    if (input !== '')
      return null
  }

  var parsedOutput = parser(example)
  parsedOutput !== null ? (spaceParser(parsedOutput[1]) === '' ? console.log(JSON.stringify(parsedOutput[0], null, 2)) : console.log('Invalid JSON')) : console.log('Invalid JSON')