phpjs/functions/math/log.js

php.js offers community built php functions in javascript

function log(arg, base) {
  // From: http://phpjs.org/functions
  // +   original by: Onno Marsman
  // +   improved by: Brett Zamir (http://brett-zamir.me)
  // *     example 1: log(8723321.4, 7);
  // *     returns 1: 8.212871815082147
  return (typeof base === 'undefined') ?
      Math.log(arg) :
      Math.log(arg) / Math.log(base);
}