parser.js

const fs = require('fs')

function valueParser (input) {
  let returned = null
  let value
  const parsers = [nullParser, booleanParser, numberParser, stringParser, arrayParser, objectParser]

  for (let parserFunction of parsers) {
    returned = parserFunction(input)
    if (returned != null) {
      [value, input] = returned
      break
    }
  }
  if (returned === null) {
    throw new TypeError()
  } else {
    return [value, input]
  }
}

function nullParser (input) {
  let result = /^null/.exec(input)
  if (result === null) {
    return null
  } else {
    return [null, input.slice(4)]
  }
}

function booleanParser (input) {
  let result = /^(true|false)/.exec(input)
  if (result === null) {
    return null
  } else {
    return [result[1] === 'true', input.slice(result[0].length)]
  }
}

function processNumber (numberString) {
  let sign = 1
  let number = 0
  let length = 0

  if (/-/.test(numberString)) {
    sign = -1
  } numberString = /\d+/.exec(numberString)[0]
  for (let digit of numberString) {
    digit = ['0', '1', '2', '3', '4', '5', '6', '7', '8', '9'].indexOf(digit)
    number = number * 10 + digit
    length++
  } return [sign, number, length]
}

function numberParser (input) {
  let sign = 1
  let exponentSign = 1
  let number = 0
  let decimal = 0
  let exponent = 0
  let length = 0

  let result = /^(-?(0|\d+))([.]\d+)?([Ee][+-]?\d+)?/.exec(input)
  if (result === null) {
    return null
  } else {
    [sign, number, length] = processNumber(result[2])
    if (result[3] !== undefined) {
      [exponentSign, decimal, length] = processNumber(result[3])
      number += decimal / Math.pow(10, length)
    }
    if (result[4] !== undefined) {
      [exponentSign, exponent, length] = processNumber(result[4])
      number *= Math.pow(10, exponentSign * exponent)
    }
    number *= sign
    return [number, input.slice(result[0].length)]
  }
}

function stringParser (input) {
  let result = /^"(([^"\\]|\\["/\\bfnrt]|\\u[a-fA-F0-9]{4})*)"/.exec(input)
  if (result === null) {
    return null
  }
  return [result[1], input.slice(result[0].length)]
}

function arrayParser (input) {
  let permittingEnd = true
  let requiringEnd = false
  let processString = input
  let result = []
  let value

  if (/^\s*\[/.test(input)) {
    processString = processString.replace(/\s*\[\s*/, '')
    while (/^\s*]/.test(processString) === false) {
      if (requiringEnd) {
        throw new TypeError()
      }
      [value, processString] = valueParser(processString)
      result.push(value)
      if (/^\s*,/.test(processString)) {
        processString = processString.replace(/\s*,\s*/, '')
        permittingEnd = false
        requiringEnd = false
      } else {
        permittingEnd = true
        requiringEnd = true
      }
    } if (permittingEnd === false) {
      throw new TypeError()
    } return [result, processString.replace(/^\s*]\s*/, '')]
  } else {
    return null
  }
}

function objectParser (input) {
  let permittingEnd = true
  let requiringEnd = false
  let processString = input
  let result = {}
  let key = ''
  let value

  if (/^\s*{/.test(input)) {
    processString = processString.replace(/\s*{\s*/, '')
    while (/^\s*}/.test(processString) === false) {
      if (requiringEnd) {
        throw new TypeError()
      }
      [key, processString] = stringParser(processString)
      if (/^\s*:/.test(processString)) {
        processString = processString.replace(/\s*:\s*/, '')
      } else {
        throw new TypeError()
      }
      [value, processString] = valueParser(processString)
      result[key] = value
      if (/^\s*,/.test(processString)) {
        processString = processString.replace(/\s*,\s*/, '')
        permittingEnd = false
        requiringEnd = false
      } else {
        requiringEnd = true
        permittingEnd = true
      }
    } if (permittingEnd === false) {
      throw new TypeError()
    } return [result, processString.replace(/^\s*}\s*/, '')]
  } else {
    return null
  }
}

let jsonString = fs.readFileSync('./json.txt', 'utf8')
console.log(valueParser(jsonString))