Offsets creates a lineString offset from the original one

getOffsetOutline.js

function getOffsetOutline(lineString, thickness, side) {
    // thickness in km

    var direction = 1;
    if (side === 'left') {
        direction = -1;
    } else if (side === 'right') {
        direction = 1;
    } else {
        console.log('Please specify direction as \'left\' or \'right\'');
    }

    // Start
    var uppers = [];
    var newCoordinates = [];
    var r = thickness / 2;
    var coordinates = lineString.geometry.coordinates;
    var initialBearing = turf.bearing(turf.point(coordinates[1]), turf.point(coordinates[0]));
    var firstUpper = turf.destination(turf.point(coordinates[0]), r, initialBearing - 90 * direction);

    newCoordinates.push(firstUpper.geometry.coordinates);

    // Mid
    for (var i = 1; i < coordinates.length - 1; i++) {
        var prev = coordinates[i - 1];
        var curr = coordinates[i];
        var next = coordinates[i + 1];
        var bearingFromPrev = turf.bearing(turf.point(curr), turf.point(prev));
        var bearingToNext = turf.bearing(turf.point(next), turf.point(curr));

        var bearingMid = getAverageBearing(bearingFromPrev, bearingToNext);

        console.log(bearingFromPrev, bearingToNext, bearingMid);
        var upper = turf.destination(turf.point(curr), r, bearingMid - 90 * direction);
        newCoordinates.push(upper.geometry.coordinates);
    }

    // End
    var lastBearing = turf.bearing(turf.point(coordinates[coordinates.length - 2]), turf.point(coordinates[coordinates.length - 1]));
    var lastUpper = turf.destination(turf.point(coordinates[coordinates.length - 1]), r, lastBearing + 90 * direction);

    newCoordinates.push(lastUpper.geometry.coordinates);
    return turf.lineString(newCoordinates);
}