JSON parser written in JavaScript

json_parser.js

module.exports = parse = (function() {
  function isDigit(c) {
    return c >= '0' && c <= '9';
  }
  function Parser(source) {
    this.cursor = 0;
    this.lineno = 1;
    this.tokens = [];
    this.lookahead = 0;
    this.tokindex = 0;
    this.source = String(source);
  }
  Parser.prototype = {
    skip: function() {
      for (;;) {
        var c = this.source[this.cursor];
        if (c === ' ' || c === '\t') {
          this.cursor++;
        } else if (c === '\n') {
          this.cursor++;
          this.lineno++;
        } else {
          break;
        }
      }
    },
    get: function(checkEOF) {
      this.assertEOF();
      this.skip();
      if (this.cursor === this.source.length) {
        return;
      }
      var c = this.source[this.cursor], r;
      switch (c) {
      case '{':
        r = this.scanObject(); break;
      case '"':
        r = this.scanString(); break;
      case '[':
        r = this.scanArray(); break;
      case '-': case '0': case '1': case '2': case '3': case '4': case '5':
      case '6': case '7': case '8': case '9':
        r = this.scanNumber(); break;
      case 't':
        this.getc();
        this.assertToken('r');
        this.assertToken('u');
        this.assertToken('e');
        r = true; break;
      case 'f':
        this.getc();
        this.assertToken('a');
        this.assertToken('l');
        this.assertToken('s');
        this.assertToken('e');
        r = false; break;
      case 'n':
        this.getc();
        this.assertToken('u');
        this.assertToken('l');
        this.assertToken('l');
        r = null; break;
      default:
        throw Error('Unexpected character: ' + c);
      }
      this.skip();
      if (checkEOF && !this.eof()) {
        throw Error('Unexpected character: ' + this.source[this.cursor]);
      }
      return r;
    },
    getc: function() {
      this.assertEOF();
      return this.source[this.cursor++];
    },
    peekc: function() {
      this.assertEOF();
      return this.source[this.cursor];
    },
    eof: function() {
      return this.cursor >= this.source.length;
    },
    assertToken: function(token) {
      this.assertEOF();
      var c = this.source[this.cursor];
      if (c !== token) {
        throw Error('Unexpected character: ' + c);
      }
      this.cursor++;
    },
    assertEOF: function() {
      if (this.eof()) {
        throw Error('Unexpected end of input');
      }
    },
    scanObject: function() {
      var obj = {}, c;
      this.assertToken('{');
      for (;;) {
        this.skip();
        c = this.peekc();
        if (c === '}') {
          this.cursor++;
          return obj;
        }
        var key = this.scanString();
        this.skip();
        this.assertToken(':');
        var val = this.get();
        obj[key] = val;
        this.skip();
        c = this.source[this.cursor];
        if (c === '}') {
          this.cursor++;
          return obj;
        }
        this.assertToken(',');
        this.skip();
        c = this.source[this.cursor];
        if (c === '}') {
          throw Error('Unexpected character: }');
        }
      }
    },
    scanArray: function() {
      var arr = [], c;
      this.assertToken('[');
      for (;;) {
        this.skip();
        c = this.source[this.cursor];
        if (c === ']') {
          this.cursor++;
          return arr;
        }
        arr.push(this.get());
        this.skip();
        c = this.source[this.cursor];
        if (c === ']') {
          this.cursor++;
          return arr;
        }
        this.assertToken(',');
        this.skip();
        c = this.source[this.cursor];
        if (c === ']') {
          throw Error('Unexpected character: ]');
        }
      }
    },
    scanNumber: function() {
      var num = '', c;
      c = this.getc();
      if (c === '-') {
        num += c;
        c = this.getc();
      }
      if (!isDigit(c)) {
        throw Error('Unexpected character: ' + c);
      }
      num += c;
      if (c === '0') {
        if (isDigit(this.peekc())) {
          throw Error('Unexpected character: ' + this.peekc());
        }
      } else {
        while (isDigit(this.peekc())) {
          num += this.getc();
        }
      }
      if (this.peekc() === '.') {
        num += '.';
        this.getc();
        while (isDigit(this.peekc())) {
          num += this.getc();
        }
        if (num.charAt(num.length - 1) === '.') {
          throw Error('Unexpected end of number');
        }
      }
      c = this.peekc();
      if (c === 'e' || c === 'E') {
        num += 'e';
        this.getc();
        c = this.peekc();
        if (c === '-' || c === '+') {
          num += c;
          this.getc();
        }
        while (isDigit(this.peekc())) {
          num += this.getc();
        }
        if (num.charAt(num.length - 1) === 'e') {
          throw Error('Unexpected end of number');
        }
      }
      return +num;
    },
    scanString: function() {
      this.assertToken('"');
      var str = '';
      var esc = false;
      for (;;) {
        this.assertEOF();
        var c = this.source[this.cursor++];
        if (esc) {
          esc = false;
          switch (c) {
          case '"': str += '"'; break;
          case '\\': str += '\\'; break;
          case '\/': str += '/'; break;
          case 'b': str += '\b'; break;
          case 'f': str += '\f'; break;
          case 'n': str += '\n'; break;
          case 'r': str += '\r'; break;
          case 't': str += '\t'; break;
          case 'u':
            var n = this.getc() + this.getc() + this.getc() + this.getc();
            if (!/^[0-9abcdef]{4}$/i.test(n)) {
              throw Error('Invalid unicode character');
            }
            str += String.fromCharCode(parseInt(n, 16));
            break;
          default:
            throw Error('Invalid escape character: ' + c);
          }
        } else if (c === '"') {
          return str;
        } else if (!esc && c === '\\') {
          esc = true;
        } else {
          str += c;
        }
      }
    }
  };
  return function(source) {
    return new Parser(source).get(true);
  };
})();