esps3857.fromEPSG4326() is returning NaN for Y when Math.tan() returns negative value

In the output file for proj.js I noticed that the function only converts the longitude.

_ol_proj_.toLonLat = function(coordinate, opt_projection) {
  var lonLat = _ol_proj_.transform(coordinate, opt_projection !== undefined ? opt_projection : 'EPSG:3857', 'EPSG:4326');
  var lon = lonLat[0];
  if (lon < -180 || lon > 180) {
    lonLat[0] = _ol_math_.modulo(lon + 180, 360) - 180;
  }
  return lonLat;
};

I am not 100% sure why the latitude is being untouched

I was able to finish this function by added a few lines.

_ol_proj_.toLonLat = function(coordinate, opt_projection) {
  var lonLat = _ol_proj_.transform(coordinate, opt_projection !== undefined ? opt_projection : 'EPSG:3857', 'EPSG:4326');
  var lon = lonLat[0];
  var lat = lonLat[1];
  if (lon < -180 || lon > 180) {
    lonLat[0] = _ol_math_.modulo(lon + 180, 360) - 180;
  }
  if (lat < -90 || lat > 90) {
    lonLat[1] = _ol_math_.modulo(lat + 90, 360) - 90;
  }
  return lonLat;
};

Things were working properly after that.

[ahocevar]

Latitudes are expected to be in the range of -85.06 to 85.06. You are probably passing projected coordinates instead of longitudes and latitudes to ol/proj/fromLonLat or epst3857/fromEPSG4326.

[ahocevar]

This is expected for coordinates outside the projection’s validity extent. For EPSG:3857, this is latitude -85.06 to 85.06 (see https://epsg.io/3857).

The change you suggested above would not change the latitude if it was NaN already.

Have you checked the input values you pass as xy to toLonLat()?