Parsing source maps

gistfile1.txt

var base64Alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=";

function decodeVLQ64(s) {
  var i = 0;
  var result = [];
  var n = 0, shift = 0;
  while (i < s.length) {
    var j = base64Alphabet.indexOf(s[i++]);
    if (j < 0) throw new Error('Bad base64 symbol');
    var more = !!(j & 0x20);
    n = n | ((j & 0x1F) << shift);
    if (!more) {
      var sign = (n & 1);
      n = sign ? -(n >>> 1) : (n >>> 1);
      result.push(n);
      n = 0;
      shift = 0;
    } else {
      shift += 5;
      if (shift > 27) throw new Error('Overflow');
    }
  }
  if (shift) {
    throw new Error('Incomplete encoding');
  }
  return result;
}

function parseMappings(mappings) {
  var fields = {c: undefined, s: undefined, sl: undefined, sc: undefined, n: undefined};
  var lines = mappings.split(";").map(function (line) {
    if (line === "") return null;
    fields.c = undefined;
    var groups = line.split(",").map(function (group) {
      var parsed = decodeVLQ64(group);
      var result = {
        c: (fields.c = (fields.c === undefined ? parsed[0] : parsed[0] + fields.c))
      };
      if (parsed.length > 1) {
        result.s = (fields.s = (fields.s === undefined ? parsed[1] : parsed[1] + fields.s));
        result.sl = (fields.sl = (fields.sl === undefined ? parsed[2] : parsed[2] + fields.sl));
        result.sc = (fields.sc = (fields.sc === undefined ? parsed[3] : parsed[3] + fields.sc));
      }      
      if (parsed.length > 4) {
        result.n = (fields.n = (fields.n === undefined ? parsed[4] : parsed[4] + fields.n));        
      }
      return result;
    });
    return groups;
  });
  return lines;
}