# Callbacks Promises Events

**Published by:** [web3.js中文学习文档](https://paragraph.com/@web3-js/)
**Published on:** 2023-06-13
**URL:** https://paragraph.com/@web3-js/callbacks-promises-events

## Content

To help web3 integrate into all kind of projects with different standards we provide multiple ways to act on asynchronous functions. Most web3.js objects allow a callback as the last parameter, as well as returning promises to chain functions. Ethereum as a blockchain has different levels of finality and therefore needs to return multiple “stages” of an action. To cope with requirement we return a “promiEvent” for functions like web3.eth.sendTransaction or contract methods. This “promiEvent” is a promise combined with an event emitter to allow acting on different stages of action on the blockchain, like a transaction. PromiEvents work like a normal promises with added on, once and off functions. This way developers can watch for additional events like on “receipt” or “transactionHash”.web3.eth.sendTransaction({from: '0x123...', data: '0x432...'}) .once('transactionHash', function(hash){ ... }) .once('receipt', function(receipt){ ... }) .on('confirmation', function(confNumber, receipt){ ... }) .on('error', function(error){ ... }) .then(function(receipt){ // will be fired once the receipt is mined });

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