In this chapter, we looked at mapping and reducing Immutable.js collections, starting with producing simple values. We then looked at mapping lists of maps to new lists of maps, adding or removing keys along the way. Reducing collections means iterating over them and producing a value. If you can filter or map the collection instead, this should be the preferred approach.
Lazy evaluation is central to transforming Immutable.js collections before passing them to side-effects. We looked at how map() fits into lazy workflows and how it forms a common pattern for your applications in conjunction with filter() and reduce().
In the next chapter, we'll look at some more advanced techniques for mapping and reducing collections.