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8.1.0 • Public • Published


This module walks the Ethereum blockchain, keeping track of the latest block. It uses a web3 provider as a data source and will continuously poll for the next block.


yarn add eth-block-tracker


npm install eth-block-tracker


const createInfuraProvider = require('eth-json-rpc-infura');
const { PollingBlockTracker } = require('eth-block-tracker');

const provider = createInfuraProvider({
  network: 'mainnet',
  projectId: process.env.INFURA_PROJECT_ID,
const blockTracker = new PollingBlockTracker({ provider });

blockTracker.on('sync', ({ newBlock, oldBlock }) => {
  if (oldBlock) {
    console.log(`sync #${Number(oldBlock)} -> #${Number(newBlock)}`);
  } else {
    console.log(`first sync #${Number(newBlock)}`);



new PollingBlockTracker({ provider, pollingInterval, retryTimeout, keepEventLoopActive, usePastBlocks })

  • Creates a new block tracker with provider as a data source and pollingInterval (ms) timeout between polling for the latest block.
  • If an error is encountered when fetching blocks, it will wait retryTimeout (ms) before attempting again.
  • If keepEventLoopActive is false, in Node.js it will unref the polling timeout, allowing the process to exit during the polling interval. Defaults to true, meaning the process will be kept alive.
  • If usePastBlocks is true, block numbers less than the current block number can used and emitted. Defaults to false, meaning that only block numbers greater than the current block number will be used and emitted.


Synchronously returns the current block. May be null.


async getLatestBlock()

Asynchronously returns the latest block. if not immediately available, it will fetch one.

async checkForLatestBlock()

Tells the block tracker to ask for a new block immediately, in addition to its normal polling interval. Useful if you received a hint of a new block (e.g. via tx.blockNumber from getTransactionByHash). Will resolve to the new latest block when done polling.



The latest event is emitted for whenever a new latest block is detected. This may mean skipping blocks if there were two created since the last polling period.

blockTracker.on('latest', (newBlock) => console.log(newBlock));


The sync event is emitted the same as "latest" but includes the previous block.

blockTracker.on('sync', ({ newBlock, oldBlock }) =>
  console.log(newBlock, oldBlock),


The error event means an error occurred while polling for the latest block.

blockTracker.on('error', (err) => console.error(err));



  • Install Node.js version 16 or greater
    • If you are using nvm (recommended) running nvm use will automatically choose the right node version for you.
  • Install Yarn v1
  • Run yarn setup to install dependencies and run any requried post-install scripts
    • Warning: Do not use the yarn / yarn install command directly. Use yarn setup instead. The normal install command will skip required post-install scripts, leaving your development environment in an invalid state.

Testing and Linting

Run yarn test to run the tests once. To run tests on file changes, run yarn test:watch.

Run yarn lint to run the linter, or run yarn lint:fix to run the linter and fix any automatically fixable issues.

Release & Publishing

The project follows the same release process as the other libraries in the MetaMask organization. The GitHub Actions action-create-release-pr and action-publish-release are used to automate the release process; see those repositories for more information about how they work.

  1. Choose a release version.

    • The release version should be chosen according to SemVer. Analyze the changes to see whether they include any breaking changes, new features, or deprecations, then choose the appropriate SemVer version. See the SemVer specification for more information.
  2. If this release is backporting changes onto a previous release, then ensure there is a major version branch for that version (e.g. 1.x for a v1 backport release).

    • The major version branch should be set to the most recent release with that major version. For example, when backporting a v1.0.2 release, you'd want to ensure there was a 1.x branch that was set to the v1.0.1 tag.
  3. Trigger the workflow_dispatch event manually for the Create Release Pull Request action to create the release PR.

    • For a backport release, the base branch should be the major version branch that you ensured existed in step 2. For a normal release, the base branch should be the main branch for that repository (which should be the default value).
    • This should trigger the action-create-release-pr workflow to create the release PR.
  4. Update the changelog to move each change entry into the appropriate change category (See here for the full list of change categories, and the correct ordering), and edit them to be more easily understood by users of the package.

    • Generally any changes that don't affect consumers of the package (e.g. lockfile changes or development environment changes) are omitted. Exceptions may be made for changes that might be of interest despite not having an effect upon the published package (e.g. major test improvements, security improvements, improved documentation, etc.).
    • Try to explain each change in terms that users of the package would understand (e.g. avoid referencing internal variables/concepts).
    • Consolidate related changes into one change entry if it makes it easier to explain.
    • Run yarn auto-changelog validate --rc to check that the changelog is correctly formatted.
  5. Review and QA the release.

    • If changes are made to the base branch, the release branch will need to be updated with these changes and review/QA will need to restart again. As such, it's probably best to avoid merging other PRs into the base branch while review is underway.
  6. Squash & Merge the release.

    • This should trigger the action-publish-release workflow to tag the final release commit and publish the release on GitHub.
  7. Publish the release on npm.

    • Be very careful to use a clean local environment to publish the release, and follow exactly the same steps used during CI.
    • Use npm publish --dry-run to examine the release contents to ensure the correct files are included. Compare to previous releases if necessary (e.g. using[package name]@[package version]/).
    • Once you are confident the release contents are correct, publish the release using npm publish.




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