What Is Web3.js?
Also known as: web3.js, Web3 JavaScript Library
In short
Web3.js is an open-source library that lets applications written in JavaScript communicate with nodes on Ethereum and EVM-compatible networks over JSON-RPC. It is used for tasks such as querying balances, sending transactions and calling smart contracts.
Web3.js is one of the first JavaScript libraries of the Ethereum ecosystem, and for many years it was used as the standard tool in the front ends of decentralized applications (DApps). Using a contract's ABI definition, the library makes it easy to call its functions like JavaScript objects, listen for events, perform address and unit conversions, and connect to browser wallets such as MetaMask.
For example, after connecting a user's wallet, a developer can use Web3.js to read the balance of an ERC-20 token or send a transfer transaction with the user's approval. Version 4.x of the library brought TypeScript support and a modular structure.
ChainSafe, which maintained the library, announced in 2025 that it was ending active development of Web3.js. For this reason, maintained alternatives such as Ethers.js or viem are generally preferred for new projects. Web3.js is still frequently encountered in existing projects; however, it should be kept in mind that a dependency that no longer receives updates carries the risk of security vulnerabilities and incompatibility with network developments.
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