Just a refactor of graphhopper's decodePath (https://github.com/graphhopper/directions-api-js-client/blob/master/src/GHUtil.js#L27)

gistfile1.txt

public decode(encoded, index){
      let b, result=0, shift=0
      do {
        b = encoded.charCodeAt(index++) - 63;
        result |= (b & 0x1f) << shift;
        shift += 5;
      } while (b >= 0x20);
      return [result,index]
    }
    public decodePath(encoded, is3D){
        let len = encoded.length;
        let index = 0;
        let array = [];
        let lat = 0;
        let lng = 0;
        let ele = 0;

        while (index < len) {
            let result = 0;
            [result,index] = this.decode(encoded,index)
            let deltaLat = ((result & 1) ? ~(result >> 1) : (result >> 1));
            lat += deltaLat;

            [result,index] = this.decode(encoded,index)

            let deltaLon = ((result & 1) ? ~(result >> 1) : (result >> 1));
            lng += deltaLon;

            if (is3D) {
                // elevation
              [result,index] = this.decode(encoded,index)
                let deltaEle = ((result & 1) ? ~(result >> 1) : (result >> 1));
                ele += deltaEle;
                array.push([lng * 1e-5, lat * 1e-5, ele / 100]);
            } else
                array.push([lng * 1e-5, lat * 1e-5]);
        }
        return array;
    }