How to find a geographic midpoint in js

midpoint

function setLatLng(dataset) {
	var lat = dataset.lat
	var lng = dataset.lng
	return new L.LatLng(lat, lng)
}

function latLngRadians(dataset) {
	return _.map(dataset, function(item) {
		var latRad = item.lat*(Math.PI/180)
		var lngRad = item.lng*(Math.PI/180)
		return [latRad, lngRad]
	})
}

function radToCart(radians) {
	var cartesian = []
	_.each(radians, function(coordinate) {
		var cartesianLat = Math.cos(coordinate[0]) * Math.cos(coordinate[1]);
		var cartesianLng = Math.cos(coordinate[0]) * Math.sin(coordinate[1]);
		var sineLat = Math.sin(coordinate[0]);
		cartesian.push({"x":cartesianLat, "y":cartesianLng, "z":sineLat})
	})
	return cartesian
}
function geoMidpoint(coordinates) {
	var carts = radToCart(coordinates);
	var allX = _.map(carts, function(cart) {
		return cart.x
	})
	var meanX = allX.reduce(function(p, c, i, a) {
		return (p + c)/a.length
	})
	var allY = _.map(carts, function(cart) {
		return cart.y
	})
	var meanY = allY.reduce(function(p, c, i, a) {
		return (p + c)/a.length
	})
	var allZ = _.map(carts, function(cart) {
		return cart.z
	})
	var meanZ = allZ.reduce(function(p, c, i, a) {
		return (p + c)/a.length
	})
	var midpoint = [meanX, meanY, meanZ]
	var lon = Math.atan2(meanY, meanX)
	var hyp = Math.sqrt(meanX * meanX + meanY * meanY)
	var lat = Math.atan2(meanZ, hyp)
	lat = lat*(180/Math.PI)
	lon = lon*(180/Math.PI)
	midpoint = [lat, lon]
	return midpoint
}