function sum(a) {
    var s = 0;
    for (var i = 0; i < a.length; i++) s += a[i];
    return s;
}

function degToRad(a) {
    return Math.PI / 180 * a;
}

function meanAngleDeg(a) {
    return 180 / Math.PI * Math.atan2(
        sum(a.map(degToRad).map(Math.sin)) / a.length,
        sum(a.map(degToRad).map(Math.cos)) / a.length
    );
}

var a = [350, 10], b = [90, 180, 270, 360],  c = [10, 20, 30];
console.log(meanAngleDeg(a));
console.log(meanAngleDeg(b));
console.log(meanAngleDeg(c));