How we can manipulating specific Column Only

[wapt49]

Hi,

I am very new in danfojs. Can someone help me how I can manipulating specific column?
For example I want

1. remove NaN on Column B Only
2. Column B = Column B - Column A

let data = [[1, NaN, 3], [4, 5, 6], [20, 30, NaN], [4, 5, 6]]
let cols = ["A", "B", "C"]
let df = new dfd.DataFrame(data, { columns: cols })

Expected Result:
[4,1,6], [20,10,NaN], [4,1,6]

Any help will be appreciated