composable-indexes are implemented in TypeScript, and focuses on type safety. So, understanding what the types mean helps a lot in understanding how to use the library.

Here are the main types:

Collection

class Collection<T> {}
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Nothing interesting here. A Collection holds values of type T. It does not have much assumptions about T. The most important warning here is that most of the functionality expect T to not change outside of the Collection methods. So, it is recommended to use the Readonly<T> type here.

Index

abstract class Index<In, Out> {}
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An Index represents a class of values that is kept up to date with the changes happening in the underlying collection. At minimum it requires two type parameters:

• In: The type of the values that the index is going to be applied on.
• Out: The type of the collection that this index belongs to.

The subclasses normally has methods that is used to query the index. For example, the BTreeIndex inherits from Index, and exposes a range method.

UnregisteredIndex

class UnregisteredIndex<In, Out, Ix extends Index<In, Out>> {}
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An UnregisteredIndex is a description of an Index that is not yet "registered" to a collection. You can pass an UnregisteredIndex to the Collection's registerIndex method to get your Index.

UnregisteredIndex is also the place where you compose the indexes. Functions like premap and group work on UnregisteredIndexes.

AggregateIndex

abstract class AggregateIndex<In, Value> extends Index<In, never> {}
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An AggregateIndex is a special kind of Index that is used to calculate "aggregate values" from a collection. They don't return values from the collection, so you don't see the Out type paremeter, but they always return a single Value.

They can be constructed and composed more easily than ordinary Index'es.