Ethers.js / Web3.js
The standard JavaScript libraries for Ethereum interaction are fully compatible with the Avalanche C-Chain's EVM API, making them common choices for frontend and backend development.
Links
Docs: https://docs.ethers.org/
GitHub: https://github.com/ethers-io/ethers.js
Docs: https://web3js.readthedocs.io/
GitHub: https://github.com/web3/web3.js