SDK
Writing a blockchain application without an SDK is … painful. It is doable, but only few people like it. To help developers build blockchain-based applications, Ethereum community provides a suite of SDKs including web3.js.
Klaytn has its own SDK suite: caver-js and caver-java. Since Klaytn transactions are not 100% compatible with those of Ethereum, it is recommended to use Klaytn SDKs when you use Klaytn.