I'll give some brief context first for why it could be useful, then explain what DeepWritable does. AWS Amplify JS has a library called DataStore that takes GraphQL schema and generates ActiveRecord-like model classes (https://github.com/dabit3/amplify-datastore-example/blob/mas...). Then you can create, query, update, and delete these models in your web app client. The generated classes have are immutable and have read-only fields by default. This means none of the model objects will change under you before they've actually been persisted, which is a good guarantee to have IMO.

To make updates, you create a new model object with the same ID and whatever changes you need, and persist it back. DataStore uses an immer-like mechanism where each model object has a copyOf method, to which you pass a function that takes a mutable version of the model and makes whatever mutations. Then the copyOf method returns a copy of the model object that has all the new field values, leaving the original object the same. The DeepWritable type expresses the "mutable version" of the model by making all fields writable, so for example the TypeScript compiler won't yell at you for writing to fields on the mutable version of the model. It also does this recursively for any fields whose type is an object that isn't a primitive.