Collection methods return new data

Let's say that you have an Immutable.js collection: coll1. Then, you call a method to insert a new value into the collection: push('newValue'). This is what a persistent change looks like: coll1 persists and push() returns a new collection that includes the new value.

Not all collection methods are mutative. For example, if you want to filter a collection so that only items that pass a given criteria are returned, you would call the filter() method. This would result in a new collection, just like adding a new item would result in a new collection. The difference is that filter() isn't a persistent change—it's part of a sequence transformation.

Think of a sequence transformation as an Instagram filter. You have the original collection: the picture. Then, you have a filter() transformation, which takes the original picture data and rearranges some of the pixel data. You want to be able to see the results of the filter, but you don't want to change the original. The filter is just a view of the original.