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Resources common to all EthereumJS implementations.

Note: this README reflects the state of the library from v2.0.0 onwards. See README from the standalone repository for an introduction on the last preceding release.


To obtain the latest version, simply require the project using npm:

npm install @ethereumjs/common


import / require

import (ESM, TypeScript):

import { Chain, Common, Hardfork } from '@ethereumjs/common'

require (CommonJS, Node.js):

const { Common, Chain, Hardfork } = require('@ethereumjs/common')


All parameters can be accessed through the Common class, instantiated with an object containing either the chain (e.g. 'Chain.Mainnet') or the chain together with a specific hardfork provided:

// ./examples/common.ts#L1-L7

import { Chain, Common, Hardfork } from '@ethereumjs/common'

// With enums:
const commonWithEnums = new Common({ chain: Chain.Mainnet, hardfork: Hardfork.London })

// (also possible with directly passing in strings:)
const commonWithStrings = new Common({ chain: 'mainnet', hardfork: 'london' })

If no hardfork is provided, the common is initialized with the default hardfork.

Current DEFAULT_HARDFORK: Hardfork.Shanghai

Here are some simple usage examples:

// ./examples/common.ts#L9-L23

// Instantiate with the chain (and the default hardfork)
let c = new Common({ chain: Chain.Mainnet })
console.log(`The gas price for ecAdd is ${c.param('gasPrices', 'ecAddGas')}`) // 500

// Chain and hardfork provided
c = new Common({ chain: Chain.Mainnet, hardfork: Hardfork.Byzantium })
console.log(`The miner reward under PoW on Byzantium us ${c.param('pow', 'minerReward')}`) // 3000000000000000000

// Get bootstrap nodes for chain/network
console.log('Below are the known bootstrap nodes')
console.log(c.bootstrapNodes()) // Array with current nodes

// Instantiate with an EIP activated
c = new Common({ chain: Chain.Mainnet, eips: [4844] })
console.log(`EIP 4844 is active -- ${c.isActivatedEIP(4844)}`)


With the breaking release round in Summer 2023 we have added hybrid ESM/CJS builds for all our libraries (see section below) and have eliminated many of the caveats which had previously prevented a frictionless browser usage.

It is now easily possible to run a browser build of one of the EthereumJS libraries within a modern browser using the provided ESM build. For a setup example see ./examples/browser.html.



See the API documentation for a full list of functions for accessing specific chain and depending hardfork parameters. There are also additional helper functions like paramByBlock (topic, name, blockNumber) or hardforkIsActiveOnBlock (hardfork, blockNumber) to ease blockNumber based access to parameters.

Generated TypeDoc API Documentation

Hybrid CJS/ESM Builds

With the breaking releases from Summer 2023 we have started to ship our libraries with both CommonJS (cjs folder) and ESM builds (esm folder), see package.json for the detailed setup.

If you use an ES6-style import in your code files from the ESM build will be used:

import { EthereumJSClass } from '@ethereumjs/[PACKAGE_NAME]'

If you use Node.js specific require, the CJS build will be used:

const { EthereumJSClass } = require('@ethereumjs/[PACKAGE_NAME]')

Using ESM will give you additional advantages over CJS beyond browser usage like static code analysis / Tree Shaking which CJS can not provide.

Buffer -> Uint8Array

With the breaking releases from Summer 2023 we have removed all Node.js specific Buffer usages from our libraries and replace these with Uint8Array representations, which are available both in Node.js and the browser (Buffer is a subclass of Uint8Array).

We have converted existing Buffer conversion methods to Uint8Array conversion methods in the @ethereumjs/util bytes module, see the respective README section for guidance.

BigInt Support

Starting with v4 the usage of BN.js for big numbers has been removed from the library and replaced with the usage of the native JS BigInt data type (introduced in ES2020).

Please note that number-related API signatures have changed along with this version update and the minimal build target has been updated to ES2020.


The Common class has a public property events which contains an EventEmitter. Following events are emitted on which you can react within your code:

Event Description
hardforkChanged Emitted when a hardfork change occurs in the Common object



The chain can be set in the constructor like this:

const c = new Common({ chain: Chain.Mainnet })

Supported chains:

  • mainnet (Chain.Mainnet)
  • goerli (Chain.Goerli)
  • sepolia (Chain.Sepolia) (v2.6.1+)
  • holesky (Chain.Holesky) (v4.1.0+)
  • Private/custom chain parameters

The following chain-specific parameters are provided:

  • name
  • chainId
  • networkId
  • consensusType (e.g. pow or poa)
  • consensusAlgorithm (e.g. ethash or clique)
  • consensusConfig (depends on consensusAlgorithm, e.g. period and epoch for clique)
  • genesis block header values
  • hardforks block numbers
  • bootstrapNodes list
  • dnsNetworks list (EIP-1459-compliant list of DNS networks for peer discovery)

To get an overview of the different parameters have a look at one of the chain configurations in the chains.ts configuration file, or to the Chain type in ./src/types.ts.

Working with private/custom chains

There are two distinct APIs available for setting up custom(ized) chains.

Basic Chain Customization / Predefined Custom Chains

There is a dedicated Common.custom() static constructor which allows for an easy instantiation of a Common instance with somewhat adopted chain parameters, with the main use case to adopt on instantiating with a deviating chain ID (you can use this to adopt other chain parameters as well though). Instantiating a custom common instance with its own chain ID and inheriting all other parameters from mainnet can now be as easily done as:

// ./examples/common.ts#L25-L27

// Instantiate common with custom chainID
const commonWithCustomChainId = Common.custom({ chainId: 1234 })
console.log(`The current chain ID is ${commonWithCustomChainId.chainId}`)

The custom() method also takes a string as a first input (instead of a dictionary). This can be used in combination with the CustomChain enum dict which allows for the selection of predefined supported custom chains for an easier Common setup of these supported chains:

const common = Common.custom(CustomChain.ArbitrumRinkebyTestnet)

The following custom chains are currently supported:

  • PolygonMainnet
  • PolygonMumbai
  • ArbitrumRinkebyTestnet
  • xDaiChain
  • OptimisticKovan
  • OptimisticEthereum

Common instances created with this simplified custom() constructor can't be used in all usage contexts (the HF configuration is very likely not matching the actual chain) but can be useful for specific use cases, e.g. for sending a tx with @ethereumjs/tx to an L2 network (see the Tx library README for a complete usage example).

Activate with a single custom Chain setup

If you want to initialize a Common instance with a single custom chain which is then directly activated you can pass a dictionary - conforming to the parameter format described above - with your custom chain values to the constructor using the chain parameter or the setChain() method, here is some example:

// ./examples/customChain.ts

import { Common } from '@ethereumjs/common'
import myCustomChain1 from './genesisData/testnet.json'

// Add custom chain config
const common1 = new Common({ chain: myCustomChain1 })
console.log(`Common is instantiated with custom chain parameters - ${common1.chainName()}`)

Initialize using customChains Array

A second way for custom chain initialization is to use the customChains constructor option. This option comes with more flexibility and allows for an arbitrary number of custom chains to be initialized on a common instance in addition to the already supported ones. It also allows for an activation-independent initialization, so you can add your chains by adding to the customChains array and either directly use the chain option to activate one of the custom chains passed or activate a build in chain (e.g. mainnet) and switch to other chains - including the custom ones - by using Common.setChain().

// ./examples/customChains.ts

import { Common } from '@ethereumjs/common'
import myCustomChain1 from './genesisData/testnet.json'
import myCustomChain2 from './genesisData/testnet2.json'
// Add two custom chains, initial mainnet activation
const common1 = new Common({ chain: 'mainnet', customChains: [myCustomChain1, myCustomChain2] })
console.log(`Common is instantiated with mainnet parameters - ${common1.chainName()}`)
console.log(`Common is set to use testnet parameters - ${common1.chainName()}`)
// Add two custom chains, activate customChain1
const common2 = new Common({
  chain: 'testnet2',
  customChains: [myCustomChain1, myCustomChain2],

console.log(`Common is instantiated with testnet2 parameters - ${common1.chainName()}`)

Starting with v3 custom genesis states should be passed to the Blockchain library directly.

Initialize using Geth's genesis json

For lots of custom chains (for e.g. devnets and testnets), you might come across a genesis json config which has both config specification for the chain as well as the genesis state specification. You can derive the common from such configuration in the following manner:

// ./examples/fromGeth.ts

import { Common } from '@ethereumjs/common'
import { hexToBytes } from '@ethereumjs/util'

import genesisJson from './genesisData/post-merge.json'

const genesisHash = hexToBytes('0x3b8fb240d288781d4aac94d3fd16809ee413bc99294a085798a589dae51ddd4a')
// Load geth genesis json file into lets say `genesisJson` and optional `chain` and `genesisHash`
const common = Common.fromGethGenesis(genesisJson, { chain: 'customChain', genesisHash })
// If you don't have `genesisHash` while initiating common, you can later configure common (for e.g.
// after calculating it via `blockchain`)

console.log(`The London forkhash for this custom chain is ${common.forkHash('london')}`)


The hardfork can be set in constructor like this:

// ./examples/common.ts#L1-L4

import { Chain, Common, Hardfork } from '@ethereumjs/common'

// With enums:
const commonWithEnums = new Common({ chain: Chain.Mainnet, hardfork: Hardfork.London })

Active Hardforks

There are currently parameter changes by the following past and future hardfork by the library supported:

  • chainstart (Hardfork.Chainstart)
  • homestead (Hardfork.Homestead)
  • dao (Hardfork.Dao)
  • tangerineWhistle (Hardfork.TangerineWhistle)
  • spuriousDragon (Hardfork.SpuriousDragon)
  • byzantium (Hardfork.Byzantium)
  • constantinople (Hardfork.Constantinople)
  • petersburg (Hardfork.Petersburg) (aka constantinopleFix, apply together with constantinople)
  • istanbul (Hardfork.Instanbul)
  • muirGlacier (Hardfork.MuirGlacier)
  • berlin (Hardfork.Berlin) (since v2.2.0)
  • london (Hardfork.London) (since v2.4.0)
  • merge (Hardfork.Merge) (DEFAULT_HARDFORK) (since v2.5.0)
  • shanghai (Hardfork.Shanghai) (since v3.1.0)
  • cancun (Hardfork.Cancun) (since v4.0.0)

Future Hardforks

The next upcoming HF Hardfork.Prague is currently not yet supported by this library.

Parameter Access

For hardfork-specific parameter access with the param() and paramByBlock() functions you can use the following topics:

  • gasConfig
  • gasPrices
  • vm
  • pow
  • sharding

See one of the hardfork configurations in the hardforks.ts file for an overview. For consistency, the chain start (chainstart) is considered an own hardfork.

The hardfork configurations above chainstart only contain the deltas from chainstart and shouldn't be accessed directly until you have a specific reason for it.


Starting with the v2.0.0 release of the library, EIPs are now native citizens within the library and can be activated like this:

const c = new Common({ chain: Chain.Mainnet, eips: [4844] })

The following EIPs are currently supported:

  • EIP-1153 - Transient storage opcodes (Cancun)
  • EIP-1559 - Fee market change for ETH 1.0 chain
  • EIP-2315 - Simple subroutines for the EVM (outdated)
  • EIP-2537 - BLS precompiles (removed in v4.0.0, see latest v3 release)
  • EIP-2565 - ModExp gas cost
  • EIP-2718 - Transaction Types
  • EIP-2929 - gas cost increases for state access opcodes
  • EIP-2930 - Optional access list tx type
  • EIP-3074 - AUTH and AUTHCALL opcodes
  • EIP-3198 - Base fee Opcode
  • EIP-3529 - Reduction in refunds
  • EIP-3540 - EVM Object Format (EOF) v1 (outdated)
  • EIP-3541 - Reject new contracts starting with the 0xEF byte
  • EIP-3554 - Difficulty Bomb Delay to December 2021 (only PoW networks)
  • EIP-3607 - Reject transactions from senders with deployed code
  • EIP-3651 - Warm COINBASE (Shanghai)
  • EIP-3670 - EOF - Code Validation (outdated)
  • EIP-3675 - Upgrade consensus to Proof-of-Stake
  • EIP-3855 - Push0 opcode (Shanghai)
  • EIP-3860 - Limit and meter initcode (Shanghai)
  • EIP-4345 - Difficulty Bomb Delay to June 2022
  • EIP-4399 - Supplant DIFFICULTY opcode with PREVRANDAO (Merge)
  • EIP-4788 - Beacon block root in the EVM (Cancun)
  • EIP-4844 - Shard Blob Transactions (Cancun) (experimental)
  • EIP-4895 - Beacon chain push withdrawals as operations (Shanghai)
  • EIP-5133 - Delaying Difficulty Bomb to mid-September 2022 (Gray Glacier)
  • EIP-5656 - MCOPY - Memory copying instruction (Cancun)
  • EIP-6780 - SELFDESTRUCT only in same transaction (Cancun)
  • EIP-7516 - BLOBBASEFEE opcode (Cancun)

Bootstrap Nodes

You can use common.bootstrapNodes() function to get nodes for a specific chain/network.


See our organizational documentation for an introduction to EthereumJS as well as information on current standards and best practices. If you want to join for work or carry out improvements on the libraries, please review our contribution guidelines first.



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