Note

This documentation is work in progress and web3.js 1.0 is not yet released! You can find the current documentation for web3 0.x.x at github.com/ethereum/wiki/wiki/JavaScript-API.

Callbacks Promises Events

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 its mined
});