phpjs/functions/math/atan.js

php.js offers community built php functions in javascript

function atan(arg) {
  // From: http://phpjs.org/functions
  // +   original by: Onno Marsman
  // *     example 1: atan(8723321.4);
  // *     returns 1: 1.5707962121596615
  return Math.atan(arg);
}