---
name: aws-sdk-js-v3-usage
description: |
  AWS SDK for JavaScript v3 development patterns. Use when writing JavaScript or TypeScript code that uses AWS services via @aws-sdk/* packages (aws-sdk-js-v3), or when asked about schemas, runtime validation, serialization, or code generation in the context of the JS/TS AWS SDK.
---

> Do not use emojis in any code, comments, or output when this skill is active.

# AWS SDK for JavaScript v3

## Package Structure

- `@aws-sdk/client-*` — one per service, generated by [smithy-typescript](https://github.com/awslabs/smithy-typescript); one-to-one with AWS services and operations
- `@aws-sdk/lib-*` — higher-level helpers (e.g. `lib-dynamodb`, `lib-storage`)
- `@aws-sdk/*` (no prefix) — utility packages (mostly internal; don't import deep paths)

Always import from the package root:

```js
import { S3Client } from "@aws-sdk/client-s3"; // correct
// NOT: import { S3Client } from "@aws-sdk/client-s3/dist-cjs/S3Client"
```

## Two Client Styles

**Bare-bones** (preferred — smaller bundle):

```js
import { S3Client, GetObjectCommand } from "@aws-sdk/client-s3";
const client = new S3Client({ region: "us-east-1" });
const output = await client.send(new GetObjectCommand({ Bucket: "b", Key: "k" }));
```

**Aggregated** (v2-style but NOT v2, larger bundle):

```js
import { S3 } from "@aws-sdk/client-s3";
const client = new S3({ region: "us-east-1" });
const output = await client.getObject({ Bucket: "b", Key: "k" });
```

## Client Configuration

No global config in v3 — pass config to each client. `region` is always required; set it explicitly or via `AWS_REGION` env var.

```js
const config = { region: "us-east-1", maxAttempts: 5 };
const s3 = new S3Client(config);
const dynamo = new DynamoDBClient(config);
```

**Do not read or mutate `client.config` after instantiation** — it is a resolved form (e.g. `region` becomes an async function). See `references/effective-practices.md`.

For HTTP handler (`NodeHttpHandler` from `@smithy/node-http-handler`), retry strategy, endpoint details, logging, FIPS, dual-stack, protocol selection, and S3-specific options → see `references/clients.md`.

## Credentials

All providers from `@aws-sdk/credential-providers`. Credentials are lazy and cached per client until ~5 min before expiry.

```js
// Default chain (env → ini → IMDS/ECS) — use in most Node.js apps
const client = new S3Client({ credentials: fromNodeProviderChain() });

// Assume role (NOTE: fromTemporaryCredentials is correct for STS AssumeRole)
const client = new S3Client({
  credentials: fromTemporaryCredentials({ params: { RoleArn: "arn:aws:iam::123456789012:role/MyRole" } }),
});

// Named profile
const client = new S3Client({ profile: "my-profile" });
```

Share credentials and socket pool across multi-region clients:

```js
const east = new S3Client({ region: "us-east-1" });
const { credentials, requestHandler } = east.config;
const west = new S3Client({ region: "us-west-2", credentials, requestHandler });
```

For all providers (Cognito, SSO, web identity, custom chains, STS region priority) → see `references/credentials.md`.

## Streams (e.g. S3 GetObject Body)

**Always read or discard streaming responses** — unread streams leave sockets open (socket exhaustion):

```js
const { Body } = await client.send(new GetObjectCommand({ Bucket: "b", Key: "k" }));
const str = await Body.transformToString();       // read as string
const bytes = await Body.transformToByteArray();  // read as Uint8Array
// or discard:
await (Body.destroy?.() ?? Body.cancel?.());
```

Streams can only be read once.

## Paginators

Use `paginate*` functions instead of manual token handling:

```js
import { DynamoDBClient, paginateListTables } from "@aws-sdk/client-dynamodb";

const client = new DynamoDBClient({});

const tableNames = [];
for await (const page of paginateListTables({ client }, {})) {
  // page contains a single paginated output.
  tableNames.push(...page.TableNames);
}
```

## DynamoDB DocumentClient

Use `@aws-sdk/lib-dynamodb` to work with native JS types instead of AttributeValues:

```js
import { DynamoDBClient } from "@aws-sdk/client-dynamodb";
import { DynamoDBDocumentClient, GetCommand, PutCommand } from "@aws-sdk/lib-dynamodb";

const client = DynamoDBDocumentClient.from(new DynamoDBClient({}));
await client.send(new PutCommand({ TableName: "T", Item: { id: "1", name: "Alice" } }));
const { Item } = await client.send(new GetCommand({ TableName: "T", Key: { id: "1" } }));
```

For marshall options, large numbers (NumberValue), pagination, and aggregated client → see `references/dynamodb.md`.

## S3: Presigned URLs, Multipart Upload, Waiters

```js
// Presigned GET URL
import { getSignedUrl } from "@aws-sdk/s3-request-presigner";
const url = await getSignedUrl(client, new GetObjectCommand({ Bucket: "b", Key: "k" }), { expiresIn: 3600 });

// Multipart upload (large files / streams)
import { Upload } from "@aws-sdk/lib-storage";
const upload = new Upload({ client, params: { Bucket: "b", Key: "k", Body: stream } });
await upload.done();

// Waiters
import { waitUntilObjectExists } from "@aws-sdk/client-s3";
await waitUntilObjectExists({ client, maxWaitTime: 120 }, { Bucket: "b", Key: "k" });
```

For presigned POST, signed headers, waiter options → see `references/s3.md`.

## Error Handling

```js
import { S3ServiceException } from "@aws-sdk/client-s3";

try {
  await client.send(new GetObjectCommand({ Bucket: "b", Key: "k" }));
} catch (e) {
  if (e?.$metadata) {
    // SDK service error — has $metadata.httpStatusCode, e.name, e.$response
    console.error(e.name, e.$metadata.httpStatusCode);
  }
}
```

Check `e.name` or `instanceof` for specific error types. See `references/error-handling.md` for full patterns.

For **runtime validation, serialization to non-default formats, or questions about what schemas are** in jsv3 → see `references/schemas.md`.

## Performance: Parallel Workloads

```js
// Configure maxSockets to match your parallel batch size
const client = new S3Client({
  requestHandler: { httpsAgent: { maxSockets: 50 } },
  cacheMiddleware: true, // skip if using custom middleware
});
```

**Streaming deadlock warning**: with limited sockets, don't `await` the request and stream body separately — chain them. See `references/performance.md`.

## Middleware

Add custom logic to all commands on a client:

```js
client.middlewareStack.add(
  (next, context) => async (args) => {
    console.log(context.commandName, args.input);
    const result = await next(args);
    return result;
  },
  { name: "MyMiddleware", step: "build", override: true }
);
```

Steps (in order): `initialize` → `serialize` → `build` → `finalizeRequest` → `deserialize`

## Abort Controller

```js
const { AbortController } = require("@aws-sdk/abort-controller");
const { S3Client, CreateBucketCommand } = require("@aws-sdk/client-s3");

const abortController = new AbortController();
const client = new S3Client(clientParams);

const requestPromise = client.send(new CreateBucketCommand(commandParams), {
  abortSignal: abortController.signal,
});

// The request will not be created if abortSignal is already aborted.
// The request will be destroyed if abortSignal is aborted before response is returned.
abortController.abort();

// This will fail with "AbortError" as abortSignal is aborted.
await requestPromise;
```

## Lambda Best Practices

Initialize clients **outside** the handler (container reuse), make API calls **inside**. For one-time async setup, use a lazy init flag inside the handler:

```js
import { S3Client } from "@aws-sdk/client-s3";

const client = new S3Client({}); // outside — reused across invocations

let ready = false;
export const handler = async (event) => {
  if (!ready) { await prepare(); ready = true; } // lazy one-time setup inside handler
  // ... API calls here
};
```

See `references/lambda.md` for Lambda layers and versioning.

## Node.js Version Requirements

- v3.968.0+ requires Node.js >= 20
- v3.723.0+ requires Node.js >= 18

## TypeScript

Response fields are typed as `T | undefined` by default. Use `AssertiveClient` from `@smithy/types` to remove `| undefined`, or `NodeJsClient` / `BrowserClient` to narrow streaming blob types. See `references/typescript.md`.

## SigV4a (S3 Multi-Region Access Points)

S3 MRAP and certain other features require SigV4a. You must install and side-effect-import exactly one of:

- `@aws-sdk/signature-v4-crt` — Node.js only, better performance
- `@aws-sdk/signature-v4a` — Node.js + browsers, pure JS

```js
import "@aws-sdk/signature-v4a"; // side-effect only — no exported values needed
```

See `references/sigv4a.md` for full details and MRAP ARN format.
