Map values using a function, Record, Series, or ES6 Map — mirrors pandas.Series.map.
Apply a function (value, index, pos) => U to every element.
Look up each value (stringified) in a plain JS object. Missing keys produce null.
Look up each value by label in another Series. Missing labels produce null.
Use a native Map<Scalar, U> for non-string keys (numbers, booleans, null, etc.).
Pass { naAction: "ignore" } to preserve existing null/NaN values without looking them up.