Note

This documentation is under construction and documents the 2.x alpha versions of web3.js. The current stable version of web3.js is 1.0 and should get preferred for production use cases.

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. These stages are encapsulated into a “PromiEvent”, which combines a promise with an event emitter. The event emitter fires an event for each of the finality stages.

An example of a function that benefits from a PromiEvent is the web3.eth.sendTransaction method.

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
});