# caver-js
`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](https://www.npmjs.com/package/caver-js).
## Features
* 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`.
* [caver.account](/content/dapp/sdk/caver-js/api-references/caver.account.md)
* [caver.wallet.keyring](/content/dapp/sdk/caver-js/api-references/caver.wallet/keyring.md)
* [caver.wallet](https://github.com/klaytn/klaytn-docs/blob/main/docs/dapp/sdk/caver-js/api-references/caver.wallet)
* [caver.transaction](https://github.com/klaytn/klaytn-docs/blob/main/docs/dapp/sdk/caver-js/api-references/caver.transaction)
* [caver.rpc](https://github.com/klaytn/klaytn-docs/blob/main/docs/dapp/sdk/caver-js/api-references/caver.rpc)
* [caver.contract](/content/dapp/sdk/caver-js/api-references/caver.contract.md)
* [caver.abi](/content/dapp/sdk/caver-js/api-references/caver.abi.md)
* [caver.kct](https://github.com/klaytn/klaytn-docs/blob/main/docs/dapp/sdk/caver-js/api-references/caver.kct)
* [caver.validator](/content/dapp/sdk/caver-js/api-references/caver.validator.md)
* [caver.utils](/content/dapp/sdk/caver-js/api-references/caver.utils.md)
* [caver.ipfs](/content/dapp/sdk/caver-js/api-references/caver.ipfs.md)
## 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](https://kips.klaytn.foundation/KIPs/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](/content/dapp/sdk/caver-js/api-references/caver.rpc/klay.md#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 [Gas and Unit Price Overview](/content/klaytn/design/transaction-fees.md#gas-and-unit-price-overview) The price of gas used in the network can be obtained by using [caver.rpc.klay.getGasPrice](/content/dapp/sdk/caver-js/api-references/caver.rpc/klay.md#caver-rpc-klay-getgasprice).
## Links
* caver-js [GitHub repository](https://github.com/klaytn/caver-js)
* caver-js on [npm](https://www.npmjs.com/package/caver-js)
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