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Backblaze B2 uses the S3 API with AWS Signature V4 against https://s3.<region>.backblazeb2.com. MIRAGE derives this endpoint from your region automatically. Credentials (B2 application key, the keyID is the access key and the applicationKey is the secret) are created the same way in both runtimes, see Backblaze B2 Credentials.

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Node (server-side)

pnpm add @struktoai/mirage-node

import { BackblazeResource, MountMode, Workspace } from '@struktoai/mirage-node'

const b2 = new BackblazeResource({
  bucket: process.env.B2_BUCKET!,
  region: process.env.B2_REGION!,
  accessKeyId: process.env.B2_ACCESS_KEY_ID!,
  secretAccessKey: process.env.B2_SECRET_ACCESS_KEY!,
})

const ws = new Workspace({ '/bucket/': b2 }, { mode: MountMode.READ })
const res = await ws.execute('ls /bucket/')
console.log(res.stdoutText)

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Browser (presigned URLs)

pnpm add @struktoai/mirage-browser
The browser BackblazeResource is secret-free, your backend signs each operation using your B2 application key and returns a URL. B2 accepts AWS Signature V4, so @aws-sdk/s3-request-presigner works, pointed at the B2 endpoint.

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1. Server: sign URLs with the B2 endpoint

import {
  CopyObjectCommand,
  DeleteObjectCommand,
  GetObjectCommand,
  HeadObjectCommand,
  ListObjectsV2Command,
  PutObjectCommand,
  S3Client,
} from '@aws-sdk/client-s3'
import { getSignedUrl } from '@aws-sdk/s3-request-presigner'

const REGION = process.env.B2_REGION!

const client = new S3Client({
  region: REGION,
  endpoint: `https://s3.${REGION}.backblazeb2.com`,
  credentials: {
    accessKeyId: process.env.B2_ACCESS_KEY_ID!,
    secretAccessKey: process.env.B2_SECRET_ACCESS_KEY!,
  },
})
const BUCKET = process.env.B2_BUCKET!

app.post('/presign/b2', async (req, res) => {
  const { path, op, opts } = req.body
  const key = path.replace(/^\/+/, '')
  const ttl = typeof opts?.ttlSec === 'number' ? opts.ttlSec : 300
  let cmd
  switch (op) {
    case 'GET':    cmd = new GetObjectCommand({ Bucket: BUCKET, Key: key }); break
    case 'PUT':    cmd = new PutObjectCommand({ Bucket: BUCKET, Key: key, ContentType: opts?.contentType }); break
    case 'HEAD':   cmd = new HeadObjectCommand({ Bucket: BUCKET, Key: key }); break
    case 'DELETE': cmd = new DeleteObjectCommand({ Bucket: BUCKET, Key: key }); break
    case 'LIST':   cmd = new ListObjectsV2Command({
      Bucket: BUCKET,
      Prefix: opts?.listPrefix,
      Delimiter: opts?.listDelimiter,
      ContinuationToken: opts?.listContinuationToken,
    }); break
    case 'COPY':   cmd = new CopyObjectCommand({
      Bucket: BUCKET,
      Key: key,
      CopySource: `${BUCKET}/${opts?.copySource}`,
    }); break
  }
  res.json({ url: await getSignedUrl(client, cmd, { expiresIn: ttl }) })
})

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2. Browser: wire it up

import { BackblazeResource, MountMode, Workspace } from '@struktoai/mirage-browser'

const b2 = new BackblazeResource({
  bucket: 'my-bucket',
  region: 'us-west-004',
  presignedUrlProvider: async (path, op, opts) => {
    const r = await fetch('/presign/b2', {
      method: 'POST',
      headers: { 'Content-Type': 'application/json' },
      body: JSON.stringify({ path, op, opts }),
    })
    const { url } = await r.json()
    return url
  },
})

const ws = new Workspace({ '/bucket/': b2 }, { mode: MountMode.READ })
region on the browser config is only used for display/logging; the actual endpoint is baked into the presigned URLs your backend returns.

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3. CORS

B2 manages CORS through its native API, not the S3 PutBucketCors call. Set CORS rules in the Backblaze web console (Bucket Settings -> CORS Rules) or with the b2 CLI: 
b2 bucket update my-bucket \
  --cors-rules '[{"corsRuleName":"mirage","allowedOrigins":["http://localhost:5173","https://app.example.com"],"allowedOperations":["s3_get","s3_put","s3_head","s3_delete","s3_post"],"allowedHeaders":["*"],"exposeHeaders":["etag","content-length","content-type","last-modified"],"maxAgeSeconds":3000}]'
See the Backblaze resource docs for the equivalent Python wiring.