A map is simply a collection of keys with values. What you have is a map with the key categories, and the value {1: 'a', 2: 'b'}. The value does not automatically become an Immutable map just because you place it inside another map.

Now, the getIn() function will only see other Immutable.js maps. What you have in there is a regular ol' javascript object. If you just want to get the 1 value, you can just do:

var obj = Immutable.Map({categories: {1: 'a', 2: 'b'}});

var output = obj.getIn([categories])[1];

alert(output);

<script src=https://cdnjs.cloudflare.com/ajax/libs/immutable/3.8.1/immutable.min.js></script>

If you want the categories collection to also be a map, you'll have to define that explicitly. Also, as Amit pointed out, you'll need to use strings with the getIn() function.

var obj = Immutable.Map({

	categories: Immutable.Map({

		1: 'a',

		2: 'b'

	})

});

var output = obj.getIn(['categories', '1']);

alert(output);

<script src=https://cdnjs.cloudflare.com/ajax/libs/immutable/3.8.1/immutable.min.js></script>

The above code will return 'a'.

It would probably be more convenient for you to do something like below, if you have many nested maps.

var obj = Immutable.fromJS({

	categories: {

		1: 'a',

		2: 'b'

	}

});

var output = obj.getIn(['categories', '1']);

alert(output);

<script src=https://cdnjs.cloudflare.com/ajax/libs/immutable/3.8.1/immutable.min.js></script>

The fromJs() function will convert any nested objects into maps or lists automatically for you.