Retrieve File

This script retrieves the contents of a file stored on the Hedera network using a file ID, saves it locally, and logs the size of the retrieved data to the console. It demonstrates how to use the FileContentsQuery to fetch file contents and perform file operations with Hedera.

Step 1: Imports

• The script imports necessary modules from the Hedera JavaScript SDK (@hashgraph/sdk): FileContentsQuery, LocalProvider, and Wallet.

• It also imports the fs module from Node.js for file system operations, and the exit method from the process module.

const {
    FileContentsQuery,
    LocalProvider,
    Wallet
} = require("@hashgraph/sdk");
const fs = require('fs');
const { exit } = require("process");

Step 2: Environment Variables Retrieval and Validation

• The script loads environment variables from a .env file located in the File_Service directory using dotenv.

• It checks whether the required environment variables (MY_ACCOUNT_ID and MY_PRIVATE_KEY) are present. If not, it throws an error.

• It retrieves the file ID (FILE_ID) from the environment variables.

require('dotenv').config({ path: 'File_Service/.env' });

if (process.env.MY_ACCOUNT_ID == null || process.env.MY_PRIVATE_KEY == null) {
    throw new Error(
        "Environment variables OPERATOR_ID, and OPERATOR_KEY are required."
    );
}

const fileId = process.env.FILE_ID;

Step 3: Wallet Initialization

• It creates a new Wallet instance using the account ID and private key retrieved from the environment variables.

• The wallet is initialized with a LocalProvider, indicating that it will be used for local signing of transactions.

const wallet = new Wallet(
    process.env.MY_ACCOUNT_ID,
    process.env.MY_PRIVATE_KEY,
    new LocalProvider()
);

Step 4 : Main Function

• It creates a new FileContentsQuery object to retrieve the contents of the file specified by the file ID.

• The file ID is set using the value retrieved from the environment variables.

• The query is executed with the signer (wallet) to retrieve the contents of the file.

• The size of the retrieved data is logged to the console.

• The retrieved contents are then written to a local file named hedera-retrieved.pdf in the File_Service directory using fs.writeFileSync().

• Finally, the script exits the process.

async function main() {
    const query = new FileContentsQuery()
        .setFileId(fileId); // Set the file ID to retrieve its contents

    const contents = await query.executeWithSigner(wallet); // Execute the query with signer

    console.log(`The size of the data is ${contents.length}`);

    fs.writeFileSync('File_Service/hedera-retrieved.pdf', contents); // Write the retrieved contents to a local file

    process.exit(); // Exit the process
}

Step 5: Execution

This line calls the main() function to start the execution of the script.

main();