splitting polyline paths across the international dateline in leaflet (sorry that it uses a mix of jQuery and lo-dash for helpers and is in coffeescript)

gistfile1.coffee

Array.prototype.splitAtEvery = (callback) ->
	sections = []
	arrayClone = this.slice(0)
	$.each(arrayClone, (idx, item) =>
		sectionsLength = 0
		_.each(sections, (section) => 
			sectionsLength += section.length;
		)
		if callback(this, idx, item) == true
			sections.push(arrayClone.slice(0, idx+1-sectionsLength))
			arrayClone = arrayClone.slice(idx+1-sectionsLength, arrayClone.length)
		return
	)
	sections.push(arrayClone)
	return sections
    
drawPolylines = (latLons, leafletMap) ->
	normalizeIndexes = []
	$.each(latLons, (i, latLon) =>				
		last = latLons[i-1]
		if last?
			if Math.abs(latLon.lng - last.lng) > 180 					
				normalizeIndexes.push(i)
	)
	
	normalizeIndexesRef = normalizeIndexes.slice(0)
	$.each(normalizeIndexes, (i, normalizeIndex) =>
		flipDirection = if latLons[normalizeIndexesRef[i]].lng < 0 then -1 else 1
		latLons.splice(normalizeIndexesRef[i], 0, L.latLng(latLons[normalizeIndexesRef[i]].lat, flipDirection * -179.99999))
		latLons.splice(normalizeIndexesRef[i]+1, 0, L.latLng(latLons[normalizeIndexesRef[i]].lat, flipDirection * 179.99999))	
		normalizeIndexesRef = _.map(normalizeIndexesRef, (idx) => idx+2)			
	)

	sections = latLons.splitAtEvery((array, i, val) -> 
		return (Math.abs(val.lng) == 179.99999) && \
				(Math.abs(array[i+1].lng) == 179.99999)
	)

	$.each(sections, (i, section) ->
		polyline = L.polyline(section, {color:'#0099cc'}).addTo(leafletMap);
	)
	
	return latLons	

Does it work now ?

The whiteboard is a bit misleading as the gist does not attempt to calculate the correct latitude at the antimeridian intersection, which leads to those unusual horizontal lines and incorrect slopes in the action shot. Adding it in is pretty simple though. Just calculate the latitude in the step before the splice:

latLons.splice(normalizeIndexesRef[i], 0, L.latLng(latLons[normalizeIndexesRef[i]].lat, flipDirection * -179.99999))

I opted to use the simple linear equation of y = mx+b to determine the latitude (y) at the antimeridian (x=180) using the slope (points i and i-1), though the SE link mentioned above goes into a more accurate spherical determination.