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I had some issues getting the v3 AWS SDK for JavaScript to communicate with localstack S3, but I found a solution! With the V2 JS SDK, the configuration object for the S3 client looks like:

{
  "region": "eu-west-1",
  "endpoint": "http://localhost:4566",
  "s3ForcePathStyle": true
}

The last field tells the sdk not to use <bucket>.hostname style connections, and instead puts the bucket in the path. For local dev this is important because otherwise the SDK tries to make connections to <bucket>.localhost. This'll work when managing buckets, but not when dealing with their contents (unless you add an entry to your /etc/hosts file, which often isn't practical).

Unfortunately this field doesn't exist for v3, and after searching I didn't find all that much except for a couple of mentions of the v3 JS SDK no longer supporting it.

Fortunately it's not actually the case. It was just renamed to forcePathStyle!

const { S3Client } = require('@aws-sdk/client-s3');

const s3 = new S3Client({
  region: 'eu-west-1', // The value here doesn't matter.
  endpoint: 'http://localhost:4566', // This is the localstack EDGE_PORT
  forcePathStyle: true
});

For reference, I boot localstack with this docker-compose configuration:

version: "3.9"
services:
  mock-s3:
    image: localstack/localstack:latest
    ports:
      - "4566-4583:4566-4583"
    environment:
      - AWS_DEFAULT_REGION=eu-west-1
      - EDGE_PORT=4566
      - SERVICES=s3
      - DEBUG=1
      - DATA_DIR=/tmp/localstack/data
    volumes:
      - "./.localstack:/tmp/localstack"
      - "/var/run/docker.sock:/var/run/docker.sock"