Google Maps Polyline Encoding to GeoJSON coordinates for Mapbox integration (Credit to @ivansams)

mapbox-polyline-geojson.js

// Obtained from https://github.com/ivansams at https://jsfiddle.net/ivansams/tw7qLvh4/2/
// Only the function that converts polyline to geojson coordinates for Mapbox integration
// Was previously called GetGeoJSON, but this function simply returns a flat array of polylines in [lng, lat] format

function getCoordsFromPolyline(str, precision) {
    var precision = precision || 5; // Precision as 5 yields 00.000  
    var index = 0,
        lat = 0,
        lng = 0,
        coordinates = [],
        shift = 0,
        result = 0,
        byte = null,
        latitude_change,
        longitude_change,
        factor = Math.pow(10, precision || 5);

    // Coordinates have variable length when encoded, so just keep
    // track of whether we've hit the end of the string. In each
    // loop iteration, a single coordinate is decoded.
    while (index < str.length) {

        // Reset shift, result, and byte
        byte = null;
        shift = 0;
        result = 0;

        do {
            byte = str.charCodeAt(index++) - 63;
            result |= (byte & 0x1f) << shift;
            shift += 5;
        } while (byte >= 0x20);

        latitude_change = ((result & 1) ? ~(result >> 1) : (result >> 1));

        shift = result = 0;

        do {
            byte = str.charCodeAt(index++) - 63;
            result |= (byte & 0x1f) << shift;
            shift += 5;
        } while (byte >= 0x20);

        longitude_change = ((result & 1) ? ~(result >> 1) : (result >> 1));

        lat += latitude_change;
        lng += longitude_change;

        coordinates.push([lng / factor, lat / factor]);
    }

    return coordinates;
};