proj4/lib/common/tsfnz.js

Proj4js is a JavaScript library to transform point coordinates from one coordinate system to another, including datum transformations.

var HALF_PI = Math.PI/2;

module.exports = function(eccent, phi, sinphi) {
  var con = eccent * sinphi;
  var com = 0.5 * eccent;
  con = Math.pow(((1 - con) / (1 + con)), com);
  return (Math.tan(0.5 * (HALF_PI - phi)) / con);
};