.. _promiEvent:

.. include:: include_announcement.rst

=========================
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 :ref:`web3.eth.sendTransaction <eth-sendtransaction-return>` 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 :ref:`web3.eth.sendTransaction <eth-sendtransaction-return>` method.

.. code-block:: javascript

    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
    });