code: def test_with_nan(self):
      # basic asof test
      rng = date_range("1/1/2000", "1/2/2000", freq="4h")
      s = Series(np.arange(len(rng)), index=rng)
      r = s.resample("2h").mean()
  
      result = r.asof(r.index)
      expected = Series(
          [0, 0, 1, 1, 2, 2, 3, 3, 4, 4, 5, 5, 6.0],
          index=date_range("1/1/2000", "1/2/2000", freq="2h"),
      )
      tm.assert_series_equal(result, expected)
  
      r.iloc[3:5] = np.nan
      result = r.asof(r.index)
      expected = Series(
          [0, 0, 1, 1, 1, 1, 3, 3, 4, 4, 5, 5, 6.0],
          index=date_range("1/1/2000", "1/2/2000", freq="2h"),
      )
      tm.assert_series_equal(result, expected)
  
      r.iloc[-3:] = np.nan
      result = r.asof(r.index)
      expected = Series(
          [0, 0, 1, 1, 1, 1, 3, 3, 4, 4, 4, 4, 4.0],
          index=date_range("1/1/2000", "1/2/2000", freq="2h"),
      )
      tm.assert_series_equal(result, expected)
  
file_path: pandas/pandas/tests/series/methods/test_asof.py
imports: import numpy
  import pytest
  from pandas._libs.tslibs import IncompatibleFrequency
  from pandas import DatetimeIndex, PeriodIndex, Series, Timestamp, date_range, isna, notna, offsets, period_range
  import pandas._testing
label_desc: Use image node_id 4 for calling the TestSeriesAsof obj's underlying member method code with example usage: obj.test_with_nan() without return types