phpjs/functions/math/atan.js

php.js offers community built php functions in javascript

function atan(arg) {
  //  discuss at: http://phpjs.org/functions/atan/
  // original by: Onno Marsman
  //   example 1: atan(8723321.4);
  //   returns 1: 1.5707962121596615

  return Math.atan(arg);
}