> For the complete documentation index, see [llms.txt](https://abc.bharatblockchain.io/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://abc.bharatblockchain.io/hedera/tutorials/consensus-service/subscribe-to-a-topic.md).

# Subscribe to a topic

After you create the topic, you will want to subscribe to the topic via a Hedera mirror node. Subscribing to a topic via a Hedera mirror node allows you to receive the stream of messages that are being submitted to it.

## Step 1 : Imports

* The script imports necessary modules from the Hedera JavaScript SDK: `TopicMessageQuery`, `Client`, and `PrivateKey`.
* It also imports the `dotenv` module to load environment variables from a `.env` file located in the `Consensus_Service` directory.

```javascript
const {
    TopicMessageQuery,
    Client,
    PrivateKey
} = require("@hashgraph/sdk");
require('dotenv').config({ path: 'Consensus_Service/.env' });
```

## Step 2 : **Environment Variables Retrieval**

These lines retrieve environment variables such as `MY_ACCOUNT_ID`, `MY_PRIVATE_KEY`, and `TOPIC_ID` from the `.env` file using `process.env`.

```javascript
const myAccountId = process.env.MY_ACCOUNT_ID;
const myPrivateKey = PrivateKey.fromString(process.env.MY_PRIVATE_KEY);
const topicId = process.env.TOPIC_ID;
```

## Step 3: **Validation**

This block of code ensures that the required environment variables (`MY_ACCOUNT_ID` and `MY_PRIVATE_KEY`) are present. If not, it throws an error.

```javascript
if (myAccountId == null ||
    myPrivateKey == null ) {
    throw new Error("Environment variables myAccountId and myPrivateKey must be present");
}
```

## Step 3 : Main Function

* This is the main function named `main()`.
* It creates a client instance for the Hedera testnet and sets the operator account using the user's account ID and private key.
* It creates a new `TopicMessageQuery` object to subscribe to a topic.
* The topic ID and start time are set using the retrieved environment variables.
* The script subscribes to the topic using the `subscribe()` method. Incoming messages are logged to the console.

```javascript
async function main() {
    const client = Client.forTestnet(); // Create a client for the Hedera testnet
    client.setOperator(myAccountId, myPrivateKey); // Set the operator account for the client

    // Create the query to subscribe to a topic
    new TopicMessageQuery()
        .setTopicId(topicId) // Set the topic ID to subscribe to
        .setStartTime(0) // Set the start time of the subscription
        .subscribe( // Subscribe to the topic
            client,
            (message) => console.log(Buffer.from(message.contents, "utf8").toString()) // Log incoming messages to the console
        );
}

main();
```

This script subscribes to a consensus topic on the Hedera network and listens for incoming messages. When a message is received, it is logged to the console.