Solidity Events

Definition

Mechanism for contracts to broadcast logs, cheaper than storing state.

core usage

Frontend / off-chain apps listen to events and react.

indexed keyword: up to 3 parameters can be indexed, useful for filtering & searching

contract Auction {
   event Bid(address indexed bidder, uint amount);
   function placeBid() public payable {
       // ... logic ...
       emit Bid(msg.sender, msg.value);
   }
}

// ethers.js
import { ethers } from "ethers";

const contract = new ethers.Contract(contractAddress, abi, provider);

contract.on("Bid", (bidder, amount, event) => {
    console.log("📢 New Bid:", bidder, amount.toString());
    console.log("Full event object:", event);
});

// web3.js
contract.events.Bid()
  .on("data", (event) => {
      console.log("📢 New Bid:", event.returnValues);
  })
  .on("error", console.error);

practical effect

The frontend can show newest bid price, don't need to call contract function period.

Event data can see in the Etherscan