Find the logarithm of a given number using the given base.

log(x, opt_base).js

var log;
(function(CACHE_LOG_INVERSE_BASE) {
  log = function(x, opt_base) {
    opt_base = opt_base || Math.E;
    return Math.log(x)
      * (
        CACHE_LOG_INVERSE_BASE[opt_base] = CACHE_LOG_INVERSE_BASE.hasOwnProperty(opt_base)
          ? CACHE_LOG_INVERSE_BASE[opt_base]
          : (5 / Math.log(Math.pow(opt_base, 5)))
      );
  };
})({});