# JavaScript SDK

The **JavaScript SDK** provides programmatic access to EventSourcingDB **from JavaScript applications**. It offers a convenient interface for **writing**, **reading**, and **observing events** without having to interact with the HTTP API directly.

> **Official SDK**
>
> This SDK is **officially developed** and maintained by **[the native web GmbH](https://www.thenativeweb.io)**, the **creators of EventSourcingDB**. It is **aligned with the latest platform features** and **receives regular updates**.

## Installation

To **install the SDK**, run:

```shell
npm install eventsourcingdb
```

You can also find this SDK on **[npm](https://www.npmjs.com/package/eventsourcingdb)**.

## Capabilities

This SDK intentionally focuses on **application-level use cases**. It provides **core functionality** for **writing**, **reading**, and **observing events**, and deliberately **excludes operational concerns** such as backup, restore, or health monitoring.

For a detailed explanation of the specific requirements behind each capability, see the **[Compliance Criteria](/docs/eventsourcingdb/sdks/compliance-criteria)**.

> **Compatibility**
>
> JavaScript SDK 1.3–1.4 runs with **EventSourcingDB 1.1** and later.

> **Essentials**
>
> <span style="color:lime;">✓</span> Initialize the Client<br />
> <span style="color:lime;">✓</span> Ping the Server<br />
> <span style="color:lime;">✓</span> Verify API Token

> **Writing Events**
>
> <span style="color:lime;">✓</span> Write Single Events<br />
> <span style="color:lime;">✓</span> Write Multiple Events<br />
> <span style="color:lime;">✓</span> Use `isSubjectPristine`<br />
> <span style="color:lime;">✓</span> Use `isSubjectOnEventId`<br />
> *(Added in 1.1)* <span style="color:lime;">✓</span> Use `isEventQlQueryTrue`<br />
> <span style="color:lime;">✓</span> Include OpenTelemetry Trace Context

> **Reading Events**
>
> <span style="color:lime;">✓</span> Read Events by Subject<br />
> <span style="color:lime;">✓</span> Read Recursively<br />
> <span style="color:lime;">✓</span> Read Chronological and Anti-Chronological<br />
> <span style="color:lime;">✓</span> Use Boundaries<br />
> <span style="color:lime;">✓</span> Use `fromLatestEvent`

> **Using EventQL**
>
> <span style="color:lime;">✓</span> Execute EventQL Queries

> **Observing Events**
>
> <span style="color:lime;">✓</span> Observe Events By Subject<br />
> <span style="color:lime;">✓</span> Observe Recursively<br />
> <span style="color:lime;">✓</span> Use Boundaries<br />
> <span style="color:lime;">✓</span> Use `fromLatestEvent`

> **Metadata and Discovery**
>
> <span style="color:lime;">✓</span> Register Event Schemas<br />
> <span style="color:lime;">✓</span> List Subjects<br />
> <span style="color:lime;">✓</span> List Event Types<br />
> *(Added in 1.2)* <span style="color:lime;">✓</span> List a Specific Event Type<br />
> *(Added in 1.3)* <span style="color:lime;">✓</span> Verify Event Hashes<br />
> *(Added in 1.3)* <span style="color:lime;">✓</span> Verify Event Signatures

> **Testcontainers Support**
>
> <span style="color:lime;">✓</span> Start and Stop EventSourcingDB in Tests<br />
> <span style="color:lime;">✓</span> Use Custom Configuration (e.g., API Token, Port, …)

## Getting Started

Import the `Client` class and create an instance by providing the URL of your EventSourcingDB instance and the API token to use:

```typescript
import { Client } from 'eventsourcingdb';

const url = new URL('http://localhost:3000');
const apiToken = 'secret';

const client = new Client(url, apiToken);
```

Then call the `ping` function to check whether the instance is reachable. If it is not, the function will throw an error:

```typescript
await client.ping();
```

*Note that `ping` does not require authentication, so the call may succeed even if the API token is invalid.*

If you want to verify the API token, call `verifyApiToken`. If the token is invalid, the function will throw an error:

```typescript
await client.verifyApiToken();
```

## Writing Events

Call the `writeEvents` function and hand over an array with one or more events. You do not have to provide all event fields – some are automatically added by the server.

Specify `source`, `subject`, `type`, and `data` according to the [CloudEvents](/docs/eventsourcingdb/cloud-events) format.

The function returns the written events, including the fields added by the server:

```typescript
const writtenEvents = await client.writeEvents([
  {
    source: 'https://library.eventsourcingdb.io',
    subject: '/books/42',
    type: 'io.eventsourcingdb.library.book-acquired',
    data: {
      title: '2001 – A Space Odyssey',
      author: 'Arthur C. Clarke',
      isbn: '978-0756906788'
    }
  }
]);
```

### Using the `isSubjectPristine` precondition

If you only want to write events in case a subject (such as `/books/42`) does not yet have any events, import the `isSubjectPristine` function and pass it as the second argument as an array of preconditions:

```typescript
import { isSubjectPristine } from 'eventsourcingdb';

const writtenEvents = await client.writeEvents([
  // events
], [
  isSubjectPristine('/books/42')
]);
```

### Using the `isSubjectOnEventId` precondition

If you only want to write events in case the last event of a subject (such as `/books/42`) has a specific ID (e.g., `0`), import the `isSubjectOnEventId` function and pass it as an array of preconditions in the second argument:

```typescript
import { isSubjectOnEventId } from 'eventsourcingdb';

const writtenEvents = await client.writeEvents([
  // events
], [
  isSubjectOnEventId('/books/42', '0')
]);
```

*Note that according to the CloudEvents standard, event IDs must be of type string.*

### Using the `isEventQlQueryTrue` precondition *(Changed in 1.2)*

If you want to write events depending on an EventQL query, import the `isEventQlQueryTrue` function and pass it as an array of preconditions in the second argument:

```typescript
import { isEventQlQueryTrue } from 'eventsourcingdb';

const writtenEvents = await client.writeEvents([
  // events
], [
  isEventQlQueryTrue(`FROM e IN events WHERE e.type == 'io.eventsourcingdb.library.book-borrowed' PROJECT INTO COUNT() < 10`)
]);
```

*Note that the query must return a single row with a single value, which is interpreted as a boolean.*

## Reading Events

To read all events of a subject, call the `readEvents` function with the subject as the first argument and an options object as the second argument. Set the `recursive` option to `false`. This ensures that only events of the given subject are returned, not events of nested subjects.

The function returns an asynchronous iterator, which you can use e.g. inside a `for await` loop:

```typescript
for await (const event of client.readEvents('/books/42', {
  recursive: false
})) {
  // ...
}
```

### Reading From Subjects Recursively

If you want to read not only all the events of a subject, but also the events of all nested subjects, set the `recursive` option to `true`:

```typescript
for await (const event of client.readEvents('/books/42', {
  recursive: true
})) {
  // ...
}
```

This also allows you to read *all* events ever written. To do so, provide `/` as the subject and set `recursive` to `true`, since all subjects are nested under the root subject.

### Reading in Anti-Chronological Order

By default, events are read in chronological order. To read in anti-chronological order, provide the `order` option and set it to `antichronological`:

```typescript
for await (const event of client.readEvents('/books/42', {
  recursive: false,
  order: 'antichronological'
})) {
  // ...
}
```

*Note that you can also specify `chronological` to explicitly enforce the default order.*

### Specifying Bounds

Sometimes you do not want to read all events, but only a range of events. For that, you can specify the `lowerBound` and `upperBound` options – either one of them or even both at the same time.

Specify the ID and whether to include or exclude it, for both the lower and upper bound:

```typescript
for await (const event of client.readEvents('/books/42', {
  recursive: false,
  lowerBound: { id: '100', type: 'inclusive' },
  upperBound: { id: '200', type: 'exclusive' }
})) {
  // ...
}
```

### Starting From the Latest Event of a Given Type

To read starting from the latest event of a given type, provide the `fromLatestEvent` option and specify the subject, the type, and how to proceed if no such event exists.

Possible options are `read-nothing`, which skips reading entirely, or `read-everything`, which effectively behaves as if `fromLatestEvent` was not specified:

```typescript
for await (const event of client.readEvents('/books/42', {
  recursive: false,
  fromLatestEvent: {
    subject: '/books/42',
    type: 'io.eventsourcingdb.library.book-borrowed',
    ifEventIsMissing: 'read-everything'
  }
})) {
  // ...
}
```

*Note that `fromLatestEvent` and `lowerBound` can not be provided at the same time.*

### Aborting Reading

If you need to abort reading use `break` or `return` within the `for await` loop. However, this only works if there is currently an iteration going on.

To abort reading independently of that, hand over an abort signal as third argument when calling `readEvents`, and abort the appropriate `AbortController`:

```typescript
const controller = new AbortController();

for await (const event of client.readEvents('/books/42', {
  recursive: false
}, controller.signal)) {
  // ...
}

// Somewhere else, abort the controller, which will cause
// reading to end.
controller.abort();
```

## Running EventQL Queries

To run an EventQL query, call the `runEventQlQuery` function and provide the query as argument. The function returns an asynchronous iterator, which you can use e.g. inside a `for await` loop:

```typescript
for await (const row of client.runEventQlQuery(`
  FROM e IN events
  PROJECT INTO e
`)) {
  // ...
}
```

*Note that each row returned by the iterator matches the projection specified in your query.*

### Aborting a Query

If you need to abort a query use `break` or `return` within the `for await` loop. However, this only works if there is currently an iteration going on.

To abort the query independently of that, hand over an abort signal as second argument when calling `runEventQlQuery`, and abort the appropriate AbortController:

```typescript
const controller = new AbortController();

for await (const row of client.runEventQlQuery(`
  FROM e IN events
  PROJECT INTO e
`, controller.signal)) {
  // ...
}

// Somewhere else, abort the controller, which will cause
// the query to end.
controller.abort();
```

## Observing Events

To observe all events of a subject, call the `observeEvents` function with the subject as the first argument and an options object as the second argument. Set the `recursive` option to `false`. This ensures that only events of the given subject are returned, not events of nested subjects.

The function returns an asynchronous iterator, which you can use e.g. inside a `for await` loop:

```typescript
for await (const event of client.observeEvents('/books/42', {
  recursive: false
})) {
  // ...
}
```

### Observing From Subjects Recursively

If you want to observe not only all the events of a subject, but also the events of all nested subjects, set the `recursive` option to `true`:

```typescript
for await (const event of client.observeEvents('/books/42', {
  recursive: true
})) {
  // ...
}
```

This also allows you to observe *all* events ever written. To do so, provide `/` as the subject and set `recursive` to `true`, since all subjects are nested under the root subject.

### Specifying Bounds

Sometimes you do not want to observe all events, but only a range of events. For that, you can specify the `lowerBound` option.

Specify the ID and whether to include or exclude it:

```typescript
for await (const event of client.observeEvents('/books/42', {
  recursive: false,
  lowerBound: { id: '100', type: 'inclusive' }
})) {
  // ...
}
```

### Starting From the Latest Event of a Given Type

To observe starting from the latest event of a given type, provide the `fromLatestEvent` option and specify the subject, the type, and how to proceed if no such event exists.

Possible options are `wait-for-event`, which waits for an event of the given type to happen, or `read-everything`, which effectively behaves as if `fromLatestEvent` was not specified:

```typescript
for await (const event of client.observeEvents('/books/42', {
  recursive: false,
  fromLatestEvent: {
    subject: '/books/42',
    type: 'io.eventsourcingdb.library.book-borrowed',
    ifEventIsMissing: 'read-everything'
  }
})) {
  // ...
}
```

*Note that `fromLatestEvent` and `lowerBound` can not be provided at the same time.*

### Aborting Observing

If you need to abort observing use `break` or `return` within the `for await` loop. However, this only works if there is currently an iteration going on.

To abort observing independently of that, hand over an abort signal as third argument when calling `observeEvents`, and abort the appropriate `AbortController`:

```typescript
const controller = new AbortController();

for await (const event of client.observeEvents('/books/42', {
  recursive: false
}, controller.signal)) {
  // ...
}

// Somewhere else, abort the controller, which will cause
// observing to end.
controller.abort();
```

## Registering an Event Schema

To register an event schema, call the `registerEventSchema` function and hand over an event type and the desired schema:

```typescript
await client.registerEventSchema('io.eventsourcingdb.library.book-acquired', {
  type: 'object',
  properties: {
    title: { type: 'string' },
    author: { type: 'string' },
    isbn: { type: 'string' }
  },
  required: [
    'title',
    'author',
    'isbn'
  ],
  additionalProperties: false
});
```

## Listing Subjects

To list all subjects, call the `readSubjects` function with `/` as the base subject. The function returns an asynchronous iterator, which you can use e.g. inside a `for await` loop:

```typescript
for await (const subject of client.readSubjects('/')) {
  // ...
}
```

If you only want to list subjects within a specific branch, provide the desired base subject instead:

```typescript
for await (const subject of client.readSubjects('/books')) {
  // ...
}
```

### Aborting Listing

If you need to abort listing use `break` or `return` within the `for await` loop. However, this only works if there is currently an iteration going on.

To abort listing independently of that, hand over an abort signal as second argument when calling `readSubjects`, and abort the appropriate `AbortController`:

```typescript
const controller = new AbortController();

for await (const subject of client.readSubjects(
  '/', controller.signal)
) {
  // ...
}

// Somewhere else, abort the controller, which will cause
// reading to end.
controller.abort();
```

## Listing Event Types

To list all event types, call the `readEventTypes` function. The function returns an asynchronous iterator, which you can use e.g. inside a `for await` loop:

```typescript
for await (const eventType of client.readEventTypes()) {
  // ...
}
```

### Aborting Listing

If you need to abort listing use `break` or `return` within the `for await` loop. However, this only works if there is currently an iteration going on.

To abort listing independently of that, hand over an abort signal as argument when calling `readEventTypes`, and abort the appropriate `AbortController`:

```typescript
const controller = new AbortController();

for await (const eventType of client.readEventTypes()) {
  // ...
}

// Somewhere else, abort the controller, which will cause
// reading to end.
controller.abort();
```

## Listing a Specific Event Type *(Added in 1.2)*

To list a specific event type, call the `readEventType` function with the event type as an argument. The function returns the detailed event type, which includes the schema:

```typescript
eventType = await client.readEventType("io.eventsourcingdb.library.book-acquired");
```

## Verifying an Event's Hash *(Added in 1.3)*

To verify the integrity of an event, call the `verifyHash` function on the event instance. This recomputes the event's hash locally and compares it to the hash stored in the event. If the hashes differ, the function returns an error:

```typescript
event.verifyHash();
```

*Note that this only verifies the hash. If you also want to verify the signature, you can skip this step and call `verifySignature` directly, which performs a hash verification internally.*

## Verifying an Event's Signature *(Added in 1.3)*

To verify the authenticity of an event, call the `verifySignature` function on the event instance. This requires the public key that matches the private key used for signing on the server.

The function first verifies the event's hash, and then checks the signature. If any verification step fails, it returns an error:

```typescript
const verificationKey = // an ed25519 public key

event.verifySignature(verificationKey);
```

## Using Testcontainers

Import the `Container` class, create an instance, call the `start` function to run a test container, get a client, run your test code, and finally call the `stop` function to stop the test container:

```typescript
import { Container } from 'eventsourcingdb';

const container = new Container();
await container.start();

const client = container.getClient();

// ...

await container.stop();
```

To check if the test container is running, call the `isRunning` function:

```typescript
const isRunning = container.isRunning();
```

### Configuring the Container Instance *(Changed in 1.3)*

By default, `Container` uses the `latest` tag of the official EventSourcingDB Docker image. To change that, call the `withImageTag` function:

```typescript
const container = new Container()
  .withImageTag('1.0.0');
```

Similarly, you can configure the port to use and the API token. Call the `withPort` or the `withApiToken` function respectively:

```typescript
const container = new Container()
  .withPort(4000)
  .withApiToken('secret');
```

If you want to sign events, call the `withSigningKey` function. This generates a new signing and verification key pair inside the container:

```typescript
const container = new Container()
  .withSigningKey();
```

You can retrieve the private key (for signing) and the public key (for verifying signatures) once the container has been started:

```typescript
const signingKey = container.getSigningKey();
const verificationKey = container.getVerificationKey();
```

The `signingKey` is the private key EventSourcingDB signs events with. The `verificationKey` can be passed to `verifySignature` when verifying events read from the database.

### Configuring the Client Manually

In case you need to set up the client yourself, use the following functions to get details on the container:

- `getHost()` returns the host name
- `getMappedPort()` returns the port
- `getBaseUrl()` returns the full URL of the container
- `getApiToken()` returns the API token

## Further Resources

- **Repository**: **[thenativeweb/eventsourcingdb-client-javascript](https://github.com/thenativeweb/eventsourcingdb-client-javascript)**
- **Registry**: **[npm](https://www.npmjs.com/package/eventsourcingdb)**
- **License (MIT)**: **[License file](https://github.com/thenativeweb/eventsourcingdb-client-javascript/blob/main/LICENSE.md)**
- **Issue tracker**: **[GitHub Issues](https://github.com/thenativeweb/eventsourcingdb-client-javascript/issues)**