@karmaniverous/mock-db

MockDb is a tiny, test‑oriented helper that simulates a small subset of Amazon DynamoDB behaviors over an in‑memory array of JSON objects. Its goal is to make it easy to unit/integration test code that expects “Dynamo‑like” reads without needing to provision a real database.

What you get:

• querySync and query — scan across all items or query a partition (hash key)
• Optional sorting, filtering, limits, and pagination (pageKey)
• Sync and async flavors (async returns with a normally‑distributed delay)

What you do NOT get:

• A database. This is a small helper for tests, not a general data store.
• The full DynamoDB API. Only a small, ergonomic subset of behavior.

MockDb uses @karmaniverous/entity-tools for type modeling and sorting.

Installation

You’ll typically install this as a dev dependency:

npm i -D @karmaniverous/mock-db

Node: >= 18 recommended (ESM module). The package ships both ESM and CJS outputs.

Quick start

import type { Entity } from '@karmaniverous/mock-db'; // convenience re-export
import { MockDb, type QueryOptions } from '@karmaniverous/mock-db';

interface User extends Entity {
  partition: string; // hash key
  id: number;
  name: string;
}

const users: User[] = [
  { partition: 'a', id: 4, name: 'Alice' },
  { partition: 'b', id: 3, name: 'Bob' },
  { partition: 'a', id: 2, name: 'Charlie' },
  { partition: 'a', id: 1, name: 'Dave' },
];

const db = new MockDb(users); // default async mean=100ms, std=20ms

// 1) Synchronous “scan” across all items with filter + sort
const scan = db.querySync({
  filter: ({ id }) => id > 2,
  sortOrder: [{ property: 'id' }],
});
// => { count: 2, items: [...], pageKey: undefined }

// 2) Asynchronous, paged, sorted “query” within a partition (hash key)
const opts: QueryOptions<User> = {
  hashKey: 'partition',
  hashValue: 'a',
  indexComponents: ['partition', 'id'],
  limit: 2,
  sortOrder: [{ property: 'id' }],
};

let page = await db.query(opts, 100);
// => first two items plus pageKey for next page

page = await db.query({ ...opts, pageKey: page.pageKey }, 100);
// => next page (remaining items)

CommonJS example:

const { MockDb } = require('@karmaniverous/mock-db');

API overview

class MockDb<E extends Entity, T extends TranscodeRegistry = DefaultTranscodeRegistry>

Replicates a limited set of DynamoDB scan/query behaviors over a local array.

Constructor

• new MockDb(data: E[], delayMean = 100, delayStd = 20)
  • delayMean/delayStd (ms) control async delay for query(...). querySync is always synchronous.

Methods

• querySync(options?: QueryOptions<E, T>): QueryReturn<E, T>
• query(options?: QueryOptions<E, T>, delayMean?: number, delayStd?: number): Promise<QueryReturn<E, T>>

type QueryOptions<E, T>

Options for query/querySync (T is a TranscodeRegistry):

• hashKey?: TranscodableProperties<E, T>
  When provided with hashValue, restricts results to the “partition” (like DynamoDB query).
• hashValue?: T[keyof T]
• sortOrder?: SortOrder
  Sorting keys and directions; powered by entity‑tools’ sort.
• filter?: (item: E) => unknown
  Predicate to include items.
• limit?: number
  Maximum number of items to return.
• pageKey?: E | Partial<Pick<E, TranscodableProperties<E, T>>>
  If set, results begin after this item (pagination).
• indexComponents?: TranscodableProperties<E, T>[]
  When limit is reached, the returned pageKey contains only these properties; otherwise it will be the entire last item.

type QueryReturn<E, T>

Result from query/querySync:

• count: number
  Count of items returned in this page (not the entire dataset).
• items: E[]
  The data items in this page.
• pageKey?: E | Pick<E, TranscodableProperties<E, T>>
  Present when limit was reached; pass back as options.pageKey to get the next page.

Behavior notes

• Scan vs Query:
  • If hashKey + hashValue are omitted, behavior is a “scan” across all items.
  • If both are present, behavior is a “query” restricted to that partition.
• Sorting: Applied before pagination/filtering (like Dynamo’s sorted result sets).
• Filtering: A plain predicate function; no Dynamo expression syntax.
• Pagination: Provide limit; when the page is full, the last item’s indexComponents (or the entire item) is returned as pageKey. Pass it back to get the next page.
• Async timing: query(...) simulates network latency with a normal distribution around delayMean (std delayStd). querySync(...) performs the same logic without delay.

Re‑exported convenience types

To avoid a direct dependency on @karmaniverous/entity‑tools in your imports, the following types are re‑exported:

import type {
  Entity,
  SortOrder,
  TranscodeRegistry,
  DefaultTranscodeRegistry,
} from '@karmaniverous/mock-db';

You can still import them from @karmaniverous/entity‑tools if you prefer.

Project scripts (local)

• Test: npm test (Vitest; coverage via V8)
• Lint/format: npm run lint (ESLint) • npm run lint:fix (ESLint + Prettier)
• Typecheck: npm run typecheck (tsc, no emit)
• Build: npm run build (Rollup – ESM/CJS + dts)
• Docs: npm run docs (TypeDoc)

Typedocs

See the online API Documentation for the complete, generated API reference.

Why not a real database?

This library is meant for fast, deterministic test runs where mocking a small subset of Dynamo behaviors is sufficient (hash key filtering, sorting, pagination). If you need to exercise full DynamoDB semantics, consider running a local emulator or integration tests against a real service.

---

Built for you with ❤️ on Bali! Find more tools & templates on my GitHub Profile.