Another scenario for passing data to Immutable.js constructors is for passing other Immutable.js collections. For example, let's say that you have a map instance, just like the one that we created in the preceding section. You can then do the following:

const firstMap = Map({ a: 1, b: 2, c: 3 });
console.log('myMap', myMap.get('a'));
// -> myMap 1

As expected, you get a new map instance in the firstMap constant. Now let's use this first instance as the input for creating another map:

const myMap = Map(firstMap);
console.log('firstMap === myMap', firstMap === myMap);
// -> firstMap === myMap true

Wait, if Immutable.js maps are immutable, how can firstMap be the same reference as myMap? This is a trick that Immutable.js uses to avoid having to create another instance of the exact same collection. By doing this, you're not actually violating any kind of immutability constraints. The collection that is passed to Map() can't change, so creating a copy of it is wasteful.

This can be useful if you're creating a function that accepts a collection as an argument:

const myFunc = map => Map(map).toJS();

console.log('myFunc(object)', myFunc({ a: 1, b: 2, c: 3 }));
// -> myFunc(object) { a: 1, b: 2, c: 3 }
console.log('myFunc(map)', myFunc(myMap));
// -> myFunc(map) { a: 1, b: 2, c: 3 }