Allow overriding of GeoJSON output

[inkstak]

Background:

I reported in #6586 a problem about accuracy in L.Util.formatNum rounding method.
Following my pull request, a performance issue has been reported in #6668, and my changes have been reverted.

Problem:

I agree with the arguments advanced in this last thread.
In the end, I especially have a problem with the inaccuracy of the toGeoJson function.

The solution for me was to overwrite the GeoJSON.latLngToCoords but it's not so easy because Leaflet uses internal API.
If I overwrite this function, it won't be used by GeoJSON.latLngsToCoords.
If I also overwrite this function, it won't be used by every toGeoJSON declared for each feature.
Et cetera. At the end, I have to copy so many functions.

Solution:

I just replaced latLngToCoords with GeoJSON.latLngToCoords

EDIT : the first PR text was submitted by error.

[inkstak]

This request intends to facilitate overwriting of toGeoJSON output.

An other approach to my main problem of accuracy may be that toGeoJSON skip numbers formatting when not precision (or a specific value) is provided. This is the approach I used in my monkey-patch for now (see the spec attached).

[johnd0e]

Suggested change

	var marker = new L.Marker([-1.4837191022531273, 43.49222084042808]),
	layerGroup = new L.LayerGroup([marker]);
	expect(layerGroup.toGeoJSON()).to.eql({
	type: 'FeatureCollection',
	features: [{
	type: 'Feature',
	properties: {},
	geometry: {
	type: 'Point',
	coordinates: [43.492221, -1.483719]
	}
	}]
	});
	L.GeoJSON.latLngToCoords = function (latlng, precision) {
	if (typeof precision === 'number') {
	return latlng.alt !== undefined ?
	[L.Util.formatNum(latlng.lng, precision), L.Util.formatNum(latlng.lat, precision), L.Util.formatNum(latlng.alt, precision)] :
	[L.Util.formatNum(latlng.lng, precision), L.Util.formatNum(latlng.lat, precision)];
	} else {
	return latlng.alt !== undefined ? [latlng.lng, latlng.lat, latlng.alt] : [latlng.lng, latlng.lat];
	}
	};
	expect(layerGroup.toGeoJSON()).to.eql({
	type: 'FeatureCollection',
	features: [{
	type: 'Feature',
	properties: {},
	geometry: {
	type: 'Point',
	coordinates: [43.49222084042808, -1.4837191022531273]
	}
	}]
	});
	var arrLatLng = [-1.4837191022531273, 43.49222084042808];
	var arrLngLat = [arrLatLng[1], arrLatLng[0]];
	var latLng = L.latLng(arrLatLng);
	var latLngToCoords = L.GeoJSON.latLngToCoords; // keep ref to original func
	// override with custom function, to prevent latlngs formatting when precision is not specified
	// (to avoid round-off errors)
	L.GeoJSON.latLngToCoords = function (latlng, precision) {
	if (typeof precision === 'number') {
	return latLngToCoords.apply(this, arguments);
	}
	return latlng.alt !== undefined ? [latlng.lng, latlng.lat, latlng.alt] : [latlng.lng, latlng.lat];
	};
	expect(L.GeoJSON.latLngToCoords(latLng, 6)).to.eql([43.492221, -1.483719]);
	expect(L.GeoJSON.latLngToCoords(latLng)).to.eql(arrLngLat);
	var marker = new L.Marker(latLng);
	expect(marker.toGeoJSON()).to.eql({
	type: 'Feature',
	properties: {},
	geometry: {
	type: 'Point',
	coordinates: arrLngLat
	}
	});
	L.GeoJSON.latLngToCoords = latLngToCoords;

[johnd0e]

I understand the issue, but I do not like proposed solution, as it is too specific.

Currently most of internals routines are not overridable. Even those that look like overridable.

E.g., the most straight-forward way to solve your issue could be overriding just single function: L.Util.formatNum, we just need to remove line 259 (which is excessive anyway), look here:

Leaflet/src/layer/GeoJSON.js

Lines 258 to 261 in 1ae785b

	export function latLngToCoords(latlng, precision) {
	precision = typeof precision === 'number' ? precision : 6;
	return latlng.alt !== undefined ?
	[Util.formatNum(latlng.lng, precision), Util.formatNum(latlng.lat, precision), Util.formatNum(latlng.alt, precision)] :

Unfortunately, if you look into built leaflet-src.js, you see that that wouldn't work:

function latLngToCoords(latlng, precision) {
	precision = typeof precision === 'number' ? precision : 6;
	return latlng.alt !== undefined ?
		[formatNum(latlng.lng, precision), formatNum(latlng.lat, precision), formatNum(latlng.alt, precision)] :

Thus allowing overriding of some arbitrary function looks like inconsistent decision.

[johnd0e]

I would better think about alternative approaches:

1. Refactor to allow overriding of most internal routines
2. Or solve your issue in some explicit way, e.g. what if we change function in this way:

// @function latLngToCoords(latlng: LatLng, precision?: Number or Function): Array
// precision:
// - if given value is `number` than apply `L.Util.formatNum` function to latlng values
// - if it is `function` than apply it to to format latlng values
function latLngToCoords(latlng, precision) {

3. Alternatively: make overridable copy of formatNum in L.GeoJSON namespace.
4. ???

What do think?

[johnd0e]

I have an idea: enhance L.Util.formatNum to allow special false value of digits:

	if (digits === false) { return num; } // no formatting

[inkstak]

Hi @johnd0e, thank you for looking into this old problem.

I like your idea, and the possibility to skip precision with toGeoJSON(false)
With this solution, this PR is no longer needed

[inkstak]

Closed in favor of #7100