Run results for: ITFB. Reinventing JSON parser

Run details:
User agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/78.0.3904.87 Safari/537.36
Browser: Chrome 78
Operating system: Windows
Device Platform: Desktop
Date tested: 4 years ago
Test name	Executions per second
eval	1499.2 Ops/sec
ES4 (old JS, Siebel eScript)	1581.9 Ops/sec
ES6 (new JS)	1689.1 Ops/sec
Script Preparation code:
var jsObj = {
  boolY: true,
  boolX: false,
  str: 'string',
  num: 123,
  obj: {},
  arr: [{ name: 'Alex', type: 'manager' }, { name: 'Bob', type: 'ceo' }],
  undef: undefined,
  null: null,
};

var json = JSON.stringify(jsObj, null, 2);

console.log({ json: json });

function parseJSON_eval(json) {
  return eval(`(${json})`);
}

function parseJSON_ES4(json) {
  // Higher-order function to be used for detecting type
  var firstAndLastChars = function firstAndLastChars(first, last) {
    return function(str) {
      return str[0] === first && str[str.length - 1] === last;
    };
  };

  var isArray = firstAndLastChars('[', ']');
  var isObj = firstAndLastChars('{', '}');
  var hasDoubleQuotes = firstAndLastChars('"', '"');
  var hasSingleQuotes = firstAndLastChars("'", "'");

  var isNumber = function isNumber(str) {
    return +str + '' === str;
  };

  var isString = function isString(str) {
    str = str.trim();
    return (hasSingleQuotes(str) || hasDoubleQuotes(str)) && str[str.length - 2] !== '\\';
  };

  var removeFirstAndLastChar = function removeFirstAndLastChar(str) {
    str = str.trim();
    return str.substring(1).slice(0, str.length - 2) || '';
  }; // Higher-order function to be used for splitting string

  var splitByChar = function splitByChar(base_char) {
    return function(str) {
      var result = [];
      var double_string_open = false;
      var single_string_open = false;
      var array_open = false;
      var object_open = false;
      var array_bracket_count = 0;
      var object_bracket_count = 0;
      var curr_str = '';
      var prev_ch = '';

      for (var i = 0; i < str.length; i += 1) {
        var ch = str[i];

        if (ch === '"') {
          double_string_open = !double_string_open;
        }

        if (ch === "'") {
          single_string_open = !single_string_open;
        }

        if (ch === '[') {
          array_bracket_count += 1;
          array_open = true;
        }

        if (ch === ']') {
          array_bracket_count -= 1;

          if (array_bracket_count === 0) {
            array_open = false;
          }
        }

        if (ch === '{') {
          object_bracket_count += 1;
          object_open = true;
        }

        if (ch === '}') {
          object_bracket_count -= 1;

          if (object_bracket_count === 0) {
            object_open = false;
          }
        }

        if (ch === base_char && !double_string_open && !single_string_open && !array_open && !object_open) {
          if (curr_str !== '') result.push(curr_str.trim());
          curr_str = '';
          prev_ch = '';
        } else {
          curr_str += ch;
          prev_ch = ch;
        }
      }

      if (curr_str !== '') result.push(curr_str.trim());
      return result;
    };
  };

  var separeateStringByCommas = splitByChar(',');
  var separeateStringByColons = splitByChar(':');

  var parseJSONString = function parseJSONString(str, parent) {
    str = str.trim();
    if (isArray(str)) return separeateStringByCommas(removeFirstAndLastChar(str)).map(parseJSONString);

    if (isObj(str)) {
      var obj = {};

      var _obj = separeateStringByCommas(removeFirstAndLastChar(str)); // _obj is an array of strings with 'key: value'

      _obj.forEach(function(val, i) {
        var key_val_pair = separeateStringByColons(val); // split into key, value

        if (key_val_pair.length === 2) {
          obj[parseJSONString(key_val_pair[0])] = parseJSONString(key_val_pair[1]);
        }
      });

      return obj;
    }

    if (isString(str)) return removeFirstAndLastChar(str).replace(/([\\]{1})([\\\"]{1})/g, '$2');
    if (isNumber(str)) return +str;
    if (str === 'null') return null;
    if (str === 'false') return false;
    if (str === 'true') return true;
    if (str === 'undefined') return undefined;
    throw new SyntaxError('Unexpected end of input');
  };

  return parseJSONString(json);
}

function parseJSON_ES6(json) {
  // Higher-order function to be used for detecting type
  const firstAndLastChars = function(first, last) {
    return str => str[0] === first && str[str.length - 1] === last;
  };
  const isArray = firstAndLastChars('[', ']');
  const isObj = firstAndLastChars('{', '}');
  const hasDoubleQuotes = firstAndLastChars('"', '"');
  const hasSingleQuotes = firstAndLastChars("'", "'");
  const isNumber = str => +str + '' === str;
  const isString = function(str) {
    str = str.trim();
    return (hasSingleQuotes(str) || hasDoubleQuotes(str)) && str[str.length - 2] !== '\\';
  };
  const removeFirstAndLastChar = function(str) {
    str = str.trim();
    return str.substring(1).slice(0, str.length - 2) || '';
  };
  // Higher-order function to be used for splitting string
  const splitByChar = function(base_char) {
    return function(str) {
      var result = [];
      var double_string_open = false;
      var single_string_open = false;
      var array_open = false;
      var object_open = false;
      var array_bracket_count = 0;
      var object_bracket_count = 0;
      var curr_str = '';
      var prev_ch = '';
      for (var i = 0; i < str.length; i += 1) {
        var ch = str[i];
        if (ch === '"') {
          double_string_open = !double_string_open;
        }
        if (ch === "'") {
          single_string_open = !single_string_open;
        }
        if (ch === '[') {
          array_bracket_count += 1;
          array_open = true;
        }
        if (ch === ']') {
          array_bracket_count -= 1;
          if (array_bracket_count === 0) {
            array_open = false;
          }
        }
        if (ch === '{') {
          object_bracket_count += 1;
          object_open = true;
        }
        if (ch === '}') {
          object_bracket_count -= 1;
          if (object_bracket_count === 0) {
            object_open = false;
          }
        }
        if (ch === base_char && !double_string_open && !single_string_open && !array_open && !object_open) {
          if (curr_str !== '') result.push(curr_str.trim());
          curr_str = '';
          prev_ch = '';
        } else {
          curr_str += ch;
          prev_ch = ch;
        }
      }
      if (curr_str !== '') result.push(curr_str.trim());
      return result;
    };
  };
  const separeateStringByCommas = splitByChar(',');
  const separeateStringByColons = splitByChar(':');
  const parseJSONString = function(str, parent) {
    str = str.trim();
    if (isArray(str)) return separeateStringByCommas(removeFirstAndLastChar(str)).map(parseJSONString);
    if (isObj(str)) {
      var obj = {};
      var _obj = separeateStringByCommas(removeFirstAndLastChar(str));
      // _obj is an array of strings with 'key: value'
      _obj.forEach(function(val, i) {
        var key_val_pair = separeateStringByColons(val); // split into key, value
        if (key_val_pair.length === 2) {
          obj[parseJSONString(key_val_pair[0])] = parseJSONString(key_val_pair[1]);
        }
      });
      return obj;
    }
    if (isString(str)) return removeFirstAndLastChar(str).replace(/([\\]{1})([\\\"]{1})/g, '$2');
    if (isNumber(str)) return +str;
    if (str === 'null') return null;
    if (str === 'false') return false;
    if (str === 'true') return true;
    if (str === 'undefined') return undefined;
    throw new SyntaxError('Unexpected end of input');
  };
  return parseJSONString(json);
}
Tests:
eval
parseJSON_eval(json)
ES4 (old JS, Siebel eScript)
parseJSON_ES4(json)
ES6 (new JS)
parseJSON_ES6(json)
Run results for: ITFB. Reinventing JSON parser

Writing our own json parser for internal purposes only

eval

ES4 (old JS, Siebel eScript)

ES6 (new JS)
