The real power of the fromJS() function is its ability to turn complex JavaScript objects into complex immutable collections:

const myMap = fromJS({
  a: {
    b: ['c', 'd'],
    e: {
      f: 'g',
      h: 'i'
    }
  }
});
console.log(
  'myMap nested list',
  myMap.getIn(['a', 'b']).toJS()
);
// -> myMap nested list [ 'c', 'd' ]

console.log('myMap nested value', myMap.getIn(['a', 'b', 1]));
// -> myMap nested value d

The fromJS() function will recursively transform native JavaScript structures into their immutable counterparts; this means for both lists and maps. One use case for this function is when the initial data that your application uses is based on JSON data, and you need a way to translate it into Immutable.js collections. Without fromJS(), this would be deceptively difficult.