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caver-js (1.5.0 or later)

caver-js is a JavaScript API library that allows developers to interact with a Klaytn node using a HTTP or Websocket connection. It is available on npm.


  • Complete implementation of Klaytn’s JSON-RPC client API over HTTP and Websocket
  • Support of Klaytn transaction, account, and account key types
  • JavaScript smart contract package to deploy and execute a smart contract on the Klaytn network
  • In-memory wallet for managing Klaytn accounts
  • Support of fee-delegation
  • Support of the Klaytn wallet key format
  • Encoding/decoding of a transaction object in RLP
  • Signing of a transaction object
  • Easy to port web3-js application to caver-js

Packages in caver-js

Below are packages provided in caver-js.

Error Code Improvement

The error messages from Ethereum via web3.js are hardly figuring out where the error occurs. caver-js improves the interface to catch error messages from Klaytn.

More details can be found in the value of txError of the transaction receipt like the below:

Error: runtime error occurred in interpreter
"blockHash": "0xe7ec35c9fff1178d52cee1d46d40627d19f828c4b06ad1a5c3807698b99acb20",
"blockNumber": 7811,
"contractAddress": null,
"from": "0xa8a2d37727197cc0eb827f8c5a3a3aceb26cf59e",
"gasUsed": 9900000000,
"logsBloom": "0x00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000",
"status": false,
"to": "0xf8425b0f65147969621f9390ca06139c7b439497",
"transactionHash": "0x85ce2b307899c90144442d9b3236827ac57375c522be2435093aebfd920b8c58",
"transactionIndex": 0,
"txError": "0x2",
"events": {}

Caution when Sending a Transaction to Klaytn

Klaytn has a new gas price policy since the Magma hard fork which enabled the KIP-71.

Therefore, you need to set the gasPrice logic differently when sending a transaction, depending on whether the hard fork is applicable or not.

Until the Magma hard fork, transactions on Klaytn have been subject to a "fixed gas price". Therefore, transactions with any other price submitted to the network are rejected. If gasPrice is not defined when you sign or submit a transaction, caver-js uses caver.rpc.klay.getGasPrice RPC call to set the gas price.

After the Magma hard fork, Klaytn uses a "dynamic gas fee pricing mechanism". The gas price of the transaction should be higher than the base fee of the Klaytn network. If gasPrice is not defined when you sign or submit a transaction, caver-js sets the gasPrice field of the transaction using caver.rpc.klay.getGasPrice.

How to set gasPrice field

caver-js provides various ways to set the gasPrice. Ways to set the gasPrice field when using caver-js are suggested below. The methods described here can be used regardless of the hard fork.

Do not define gasPrice field

If you create an instance without defining the gasPrice field, the gasPrice field is automatically set when you call tx.sign or tx.signAsFeePayer to sign a transaction.

const tx = caver.transaction.valueTransfer.create({ from, to, value, gas })
await tx.sign(from, tx) // Before signing, gasPrice is set inside `tx.sign`.

Use tx.fillTransaction method

You can use tx.fillTransaction, a function that fills the optional fields of a transaction with appropriate values when they are omitted.

const tx = caver.transaction.valueTransfer.create({ from, to, value, gas })
await tx.fillTransaction() // Fill the optional tx fields.

Use tx.suggestGasPrice method

You can set the gasPrice with the result of tx.suggestGasPrice which returns the recommended gas price.

const tx = caver.transaction.valueTransfer.create({ from, to, value, gas })
tx.gasPrice = await tx.suggestGasPrice()

For more information about the gas price, see GasPrice Overview The price of gas used in the network can be obtained by using caver.rpc.klay.getGasPrice.

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