> ## Documentation Index
> Fetch the complete documentation index at: https://docs.chainstack.com/llms.txt
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# subscribe ("newBlockHeaders") | Polygon

> Reference docs for the subscribe ("newBlockHeaders") JSON-RPC method on the Polygon blockchain, available via Chainstack JSON-RPC nodes.

The ethers.js subscription equivalent to [eth\_newBlockFilter](/reference/polygon-newblockfilter). `subscribe("newBlockHeaders")` allows developers to subscribe to real-time updates about new block headers on the Polygon blockchain; the application will receive notifications whenever a new block is added to the blockchain. The notification will include information about the new block, such as its block number, hash, and timestamp.

## Parameters

* `string` — a keyword identifying the type of event to subscribe to, `newBlockHeaders` in this case.
* `function` — (optional) a callback function that will be called every time a new event of the specified type is received. This function takes two parameters: `error` and `result`. The error parameter contains any error that occurred while subscribing to the event, and the result parameter contains the data for the event that was received.

## Response

* `object`— a block object with the following fields:

  * `number` — the block number of the requested block, encoded as hexadecimal. `null` if the block is pending.
  * `hash` — the block hash of the requested block. `null` if the block pending.
  * `parenthash` — hash of the previous block used to generate the current block. Also known as the 'parent block'.
  * `nonce` — the hash used to demonstrate proof-of-work. `null` if the block pending. It returns `0x0000000000000000` when the consensus is proof-of-stake.
  * `sha3uncles` — the hash of the list of uncles included in the block. It is used to identify the block uniquely and to verify the integrity of the block's data.
  * `logsbloom` — the bloom filter for the logs of the block, a data structure that allows for efficient membership testing of elements in a set, in this case, the logs included in the block. `null` if pending.
  * `transactionsroot` — the root of the transaction trie of the block. The `transactionsRoot` field allows Polygon nodes to verify the integrity of the transactions in a block.
  * `stateroot` — the root of the final state trie of the block. The `stateroot` field is included in the block header and is used to verify the integrity of the state at the time the block was processed
  * `receiptsroot` — the root of the receipts trie of the block. A 32-byte hash of the root node of the receipts trie of all transactions in the block. It is used to verify the integrity of the receipts data for all transactions in the block.
  * `miner` — the address of the miner receiving the reward.
  * `difficulty` — a measure of how hard it is to find a valid block for the Polygon blockchain. It is a number that increases as more miners join the network and more blocks are added to the chain, encoded as hexadecimal.
  * `totaldifficulty` — the cumulative sum of the difficulty of all blocks that have been mined in the Polygon network since the inception of the network. It measures the overall security and integrity of the Polygon network.
  * `extradata` — extra data included in a block by the miner who mined it. It often includes messages or other information related to the block.
  * `size` — the size of this block in bytes as an integer value, encoded as hexadecimal.
  * `gaslimit` — the maximum gas allowed in this block, encoded as hexadecimal.
  * `gasused` — the total used gas by all transactions in this block, encoded as hexadecimal.
  * `timestamp` — the Unix timestamp for when the block was collated.
  * `transactions` — an array of transaction objects. See [eth\_getTransactionByHash](/reference/ethereum-gettransactionbyhash) for the exact shape.
  * `uncles` — an array of uncle hashes.

## `subscribe("newBlockHeaders")` code example

<Info>
  Note that subscriptions require a WebSocket connection. Use the `ethers.WebSocketProvider` class to set one up.
</Info>

Use the provider event listeners to react to new block events:

* `provider.on("block", listener)` — registers a listener that activates for each new block; the listener receives the new block number.
* `provider.off("block", listener)` — removes the listener and stops the subscription.

<CodeGroup>
  ```javascript index.js theme={"system"}
  const { ethers } = require("ethers");
  const NODE_URL = "CHAINSTACK_WSS_URL";
  const provider = new ethers.WebSocketProvider(NODE_URL);

  async function subscribeToNewBlocks() {
      try {
          // Register a listener for the 'block' event
          provider.on('block', handleNewBlock);
          console.log("Subscribed to new blocks");
      } catch (error) {
          console.error(`Error subscribing to new blocks: ${error}`);
      }
  }

  /* Listener to react to new block events */

  // Event listener that fetches and logs the received block data
  async function handleNewBlock(blockNumber) {
      const block = await provider.getBlock(blockNumber);
      console.log(block);
  }

  subscribeToNewBlocks();
  ```
</CodeGroup>

## Use case

A practical use case for `subscribe("newBlockHeaders")` is a DApp that continuously listens for new block headers, then isolates the block number and `baseFeePerGas` for analytics purposes.

The following is an implementation of this concept using ethers.js subscriptions:

<CodeGroup>
  ```javascript index.js theme={"system"}
  const { ethers } = require("ethers");
  const NODE_URL = "CHAINSTACK_WSS_URL";
  const provider = new ethers.WebSocketProvider(NODE_URL);

  let blockCount = 0;

  async function subscribeToNewBlocks() {
      try {
          // Register a listener for the 'block' event
          provider.on('block', handleNewBlock);
          console.log("Subscribed to new blocks");
      } catch (error) {
          console.error(`Error subscribing to new blocks: ${error}`);
      }
  }

  function unsubscribeFromNewBlocks() {
      provider.off('block', handleNewBlock);
      console.log("Successfully unsubscribed!");
      return process.exit(1);
  }

  /* Fallback functions to react to the different events */

  // Event listener that extracts block number and base fee per gas from the received block header data
  async function handleNewBlock(blockNumber) {

      const block = await provider.getBlock(blockNumber);
      const baseFeeGwei = ethers.formatUnits(block.baseFeePerGas, 'gwei');

      blockCount++;
      console.log(`Block number: ${blockNumber} \n Base Fee per Gas: ${baseFeeGwei} Gwei \n`);
      if (blockCount === 100) {
          unsubscribeFromNewBlocks();
      }
  }

  subscribeToNewBlocks();
  ```
</CodeGroup>

This code creates a new subscription to new block events using the `provider.on("block", listener)` method. This method registers a listener that activates for each new block and receives the new block number.

The code defines the `handleNewBlock` listener function that is registered for the `block` event. The `handleNewBlock` function is called when a new block is received; it fetches the full block with `provider.getBlock`, then extracts the block number and base fee per gas from the block data using the `ethers.formatUnits` method to convert `baseFeePerGas` to gwei.

This code includes an `unsubscribeFromNewBlocks` function which removes the listener with `provider.off("block", listener)` and quits the program using `return process.exit(1)` after it received 100 new blocks. This is to demonstrate its use.

Finally, the code calls the `subscribeToNewBlocks` function, which creates the subscription and attaches the event listeners. When a new block header is received, the `handleNewBlock` function is called to extract the block data and log it to the console.

## Try the `subscribe("newBlockHeaders")` RPC method yourself