Shortly:

map.getIn([a, b, c]) is a shortcut to map.get(a).get(b).get(c)

In details:

You have probably got into one of the fromJS traps. Calling :

const map = Immutable.fromJS({a: {b: {c: banana}}});

creates a Map with only key a, which's value is also a Map with only key b, which's value is also a Map with only key c and value banana.

With other words fromJS goes into the deep of the object provided, and defines Map for each Object and a List for each Array

With this example, calling map.getIn([a, b, c]) is kind of a shortcut to map.get(a).get(b).get(c)

But if you define map as a Map:

const map = new Immutable.Map({a: {b: {c: banana}}});

it creates a Map, with only key a, which's value is plain object {b: {c: banana}}, and calling a map.get(a).get(b).get(c) will throw you something like get is not a function, since map.get(a) will return {b: ...} plain object.

Same reasons are why map.getIn([a, b, c]) will not work as you might expect.