code: function getCaller (offset) {
    /* eslint-disable handle-callback-err */
    if (typeof offset !== 'number') offset = 1
    var old = Error.prepareStackTrace
    var err = new Error()
    Error.prepareStackTrace = function (err, stack) {
      return stack
    }
    var fname = err.stack[1 + offset].getFileName()
    Error.prepareStackTrace = old
    return fname
  }
docs: Internal: gets the filename of the caller function. The `offset` defines how
  many hops away it's expected to be from in the stack trace.
  
  See: http://stackoverflow.com/questions/16697791/nodejs-get-filename-of-caller-function