Download a blob with JavaScript or TypeScript

Article
11/25/2024

This article shows how to download a blob using the Azure Storage client library for JavaScript. You can download blob data to various destinations, including a local file path, stream, or text string.

Prerequisites

The examples in this article assume you already have a project set up to work with the Azure Blob Storage client library for JavaScript. To learn about setting up your project, including package installation, importing modules, and creating an authorized client object to work with data resources, see Get started with Azure Blob Storage and JavaScript.
The authorization mechanism must have permissions to perform a download operation. To learn more, see the authorization guidance for the following REST API operation:
- Get Blob

Download a blob

You can use any of the following methods to download a blob:

BlobClient.download
BlobClient.downloadToBuffer (only available in Node.js runtime)
BlobClient.downloadToFile (only available in Node.js runtime)

Download to a file path

The following example downloads a blob by using a file path with the BlobClient.downloadToFile method. This method is only available in the Node.js runtime:

JavaScript
TypeScript

async function downloadBlobToFile(containerClient, blobName, localFilePath) {

    const blobClient = containerClient.getBlobClient(blobName);
    
    await blobClient.downloadToFile(localFilePath);
}

async function downloadBlobToFile(
  containerClient: ContainerClient,
  blobName: string,
  filePath: string
): Promise<void> {

  const blobClient = containerClient.getBlobClient(blobName);

  await blobClient.downloadToFile(filePath);
}

Download as a stream

The following example downloads a blob by creating a Node.js writable stream object and then piping to that stream with the BlobClient.download method.

JavaScript
TypeScript

async function downloadBlobAsStream(containerClient, blobName, writableStream) {

    const blobClient = containerClient.getBlobClient(blobName);

    const downloadResponse = await blobClient.download();

    downloadResponse.readableStreamBody.pipe(writableStream);
}

async function downloadBlobAsStream(
  containerClient: ContainerClient,
  blobName: string,
  writableStream: fs.WriteStream
) {
  
  const blobClient: BlobClient = containerClient.getBlobClient(blobName);

  const downloadResponse = await blobClient.download();

  if (!downloadResponse.errorCode && downloadResponse?.readableStreamBody) {
    downloadResponse.readableStreamBody.pipe(writableStream);
  }
}

Download to a string

The following Node.js example downloads a blob to a string with BlobClient.download method. In Node.js, blob data returns in a readableStreamBody.

JavaScript
TypeScript

async function downloadBlobToString(containerClient, blobName) {

    const blobClient = containerClient.getBlobClient(blobName);

    const downloadResponse = await blobClient.download();

    const downloaded = await streamToBuffer(downloadResponse.readableStreamBody);
    console.log('Downloaded blob content:', downloaded.toString());
}

function streamToBuffer(readableStream) {
    return new Promise((resolve, reject) => {
        const chunks = [];
        readableStream.on('data', (data) => {
            chunks.push(data instanceof Buffer ? data : Buffer.from(data));
        });
        readableStream.on('end', () => {
            resolve(Buffer.concat(chunks));
        });
        readableStream.on('error', reject);
    });
}

async function downloadBlobToString(
  containerClient: ContainerClient,
  blobName: string
): Promise<void> {

  const blobClient: BlobClient = containerClient.getBlobClient(blobName);

  const downloadResponse: BlobDownloadResponseParsed =
    await blobClient.download();

  if (!downloadResponse.errorCode && downloadResponse.readableStreamBody) {
    const downloaded = await streamToBuffer(
      downloadResponse.readableStreamBody
    );
    if (downloaded) {
      console.log('Downloaded blob content:', downloaded.toString());
    }
  }
}

function streamToBuffer(readableStream) {
  return new Promise((resolve, reject) => {
    const chunks: Buffer[] = [];

    readableStream.on('data', (data) => {
      const content: Buffer = data instanceof Buffer ? data : Buffer.from(data);
      chunks.push(content);
    });
    readableStream.on('end', () => {
      resolve(Buffer.concat(chunks));
    });
    readableStream.on('error', reject);
  });
}

If you're working with JavaScript in the browser, blob data returns in a promise blobBody. To learn more, see the example usage for browsers at BlobClient.download.

Resources

To learn more about how to download blobs using the Azure Blob Storage client library for JavaScript, see the following resources.

Code samples

View code samples from this article (GitHub):

Download to file for JavaScript or TypeScript
Download to stream for JavaScript or TypeScript
Download to string for JavaScript or TypeScript

REST API operations

The Azure SDK for JavaScript contains libraries that build on top of the Azure REST API, allowing you to interact with REST API operations through familiar JavaScript paradigms. The client library methods for downloading blobs use the following REST API operation:

Get Blob (REST API)

Client library resources

This article is part of the Blob Storage developer guide for JavaScript/TypeScript. To learn more, see the full list of developer guide articles at Build your JavaScript/TypeScript app.