Mastering Immutable.js

Flattening nested maps

Another common structure is deeply-nested maps. Let's imagine that you want to filter keys, including nested keys, by flattening the nested map structure:

const myMap = Map.of(  'first', 1,  'second', Map.of(    'third', 3,    'fourth', 4,    'fifth', Map.of(      'sixth', Map.of(        'seventh', 7      )    )  ));console.log('myMap', myMap.toJS());// -> { first: 1,// ->   second: {// ->     third: 3,// ->     fourth: 4,// ->     fifth: {// ->       sixth: { seventh: 7 } } } }myMap  .toSeq()  .flatten()  .filter((v, k) => k.startsWith('f'))  .forEach((v, k) => console.log(k, v));  // -> first 1  // -> fourth 4

Here, we are able to find all nested values whose key started with 'f'. One of the keys—'fifth'—starts with 'f', but it doesn't appear in the results. When you flatten ...