I have a nested Immutable.Map of Immutable.Map type defined as follows:

export type Data = Immutable.Map<string, Immutable.Map<string, string>>;

const nested : Data = Immutable.Map();

Data in the nested should look like this:

{
  'a': {
    'a1': 'value1',
    'a2': 'value2',
  },
  'b': {
    'b1': 'value1',
    'b2': 'value2',
  },
...
}

How should I insert/update a value for the nested variable (assume I have input like {'a', 'a1', 'value1'})? I know I can do checks and create the second immutable map if it's null, but is there an easier way to do that?