Module 00 Why

Most applications require a state to be maintained. This can be a simple cache for a webserver, a complex data structure for a game, or a database for a command line application.

There are many options for storing state. composable-index is one of them, and it is useful when:

• Your data fits in memory, and do not need to persist it (or have another mechanism for persisting it).
• You want a thin layer to represent your data and queries as plain JavaScript objects.
• You want to be able to query your data in multiple ways, without having to maintain multiple data structures or resort to iteration.
• (Bonus) You want to be able to query/aggregate your data in a customisable and performant way.

With composable-index, you can:

• Create and manipulate a collection of arbitrary values with plain JS (or TS) functions.
• Create "indexes" that can be used to query the collection with different criterias.
• Create "aggregations" that can be used to calculate values from the collection.

Those index and aggregations:

• Implemented with performant data structures.
• Stay in sync with the collection automatically.
• Can compose with combinators to create more complex indexes.

Example:

import { Collection } from "composable-indexes";
import { group, premap, sumIndex, uniqueHashIndex } from "composable-indexes"

type Player = {
  name: string;
  score: number;
  alliance: string;
};

const collection = new Collection<Readonly<Player>>();

// Add indexes using ordinary TypeScript functions
const ixPlayersByName =
  collection.registerIndex(
    premap(
      (p) => p.name,
      uniqueHashIndex()
    )
  );

const ixTotalScoresPerAlliance =
  collection.registerIndex(
    group(
      (p) => p.alliance,
      premap(
        (p) => p.score,
        sumIndex()
      )
    )
  );

// Manipulate the collection
collection.add({ name: "Thor", score: 0, alliance: "Norse" });
collection.add({ name: "Cybele", score: 0, alliance: "Anatolia" });
collection.adjust(
    ixPlayersByName.get.eq("Cybele")!.id,
    (p) => ({ ...p, score: p.score + 10 })
);

// Query the collection
console.log(ixTotalScoresPerAlliance.get("Anatolia")!.get.value)
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