Decoding function for Google's polyline encoded strings

decode.js

/*
This is more complicated than it would normally be, because A) it delivers
output in pairs, and B) those pairs are all offsets from the previous pair.
If you're just delivering a stream of values, this is about 1/2 as long.
*/
function decode(str) {
  // last known coordinate for adding offsets
  var from = [0, 0];
  var value = 0;
  var shift = 0;
  // lat/long pair and internal index
  var pair = [];
  var latLong = 0;
  // final output
  var output = [];
  for (var char of str) {
    // get the actual numerical value of the character
    var code = char.charCodeAt(0) - 63;
    // the data in the chunk is the rightmost 5 bits, the leftmost bit is a continuation flag
    var byte = code & 31;
    // add this to the value
    value |= byte << shift;
    shift += 5;
    // if the leftmost bit was set, we're done with the assembled value
    if (code < 32) {
      // if it was negative, the rightmost bit will be set
      // so shift and get the twos complement
      if (value & 1) {
        value = ~(value >> 1);
      } else {
        value = value >> 1;
      }
      // reduce by the scaling factor and add to the last known coordinate pair
      value *= .00001;
      pair[latLong] = from[latLong] + value;
      value = 0;
      shift = 0;
      latLong++;
      // once we're done with the pair, push it and reset
      if (latLong == 2) {
        latLong = 0;
        from = pair;
        output.push(pair);
        pair = [];
      }
    }
  }
  return output;
}

Reverses the format (used in some Maps products) found here into lat/long pairs: https://developers.google.com/maps/documentation/utilities/polylinealgorithm Useful for scraping these into actual GIS coordinates.