[Ethereum] How to parse and deal with BigNumber (BN) args in Solidity events

Topics are indexed parameters to an event.

topic[0] always refers to the hash of the hash of the event itself, and can have up to 3 indexed arguments, which will each be reflected in the topics.

EVM uses low-level primitives called logs to map them to high-level Solidity construct called Event. Logs may contain different topics that are indexed arguments.

Consider Event:

  event PersonCreated(uint indexed age, uint height);

And you fire it the foobar function of MyContract:

  function foobar() {
        emit PersonCreated(26, 176);
  }

This will create a low-level EVM log entry with topics

• 0x6be15e8568869b1e100750dd5079151b32637268ec08d199b318b793181b8a7d (Keccak-256 hash of PersonCreated(uint256,uint256))

• 0x36383cc9cfbf1dc87c78c2529ae2fcd4e3fc4e575e154b357ae3a8b2739113cf (Keccak-256 hash of age), value 26

You'll notice that height will not be a topic, but it will be included in the data section of the event.

Internally, your Ethereum node (Geth / Parity) will index arguments to build on indexable search indexes, so that you can easily do look ups by value later. Because creating indexes takes additional disk space, indexed parameters in events have additional gas cost. However, indexed are required to any meaningful look up in scale of events by value later.

Now in the web3 client you want to watch for creation events of all persons that are age of 26, you can simply do:

var createdEvent = myContract.PersonCreated({age: 26});
createdEvent.watch(function(err, result) {
  if (err) {
    console.log(err)
    return;
  }
  console.log("Found ", result);
})

Or you could filter all past events in similar fashion.

More information here

• http://solidity.readthedocs.io/en/develop/miscellaneous.html?highlight=topic#modifiers

• http://solidity.readthedocs.io/en/develop/contracts.html?highlight=topic#events

• https://media.consensys.net/technical-introduction-to-events-and-logs-in-ethereum-a074d65dd61e#.7w96id6rs

• https://emn178.github.io/online-tools/keccak_256.html

In Javascript, the maximum safe integer is 2^53-1.

So you should generally avoid using numbers like 100000000000000000000.

For example, in your code, you should change this:

myToken.transfer(acc[3], 100000000000000000000, {from:acc[0]})

To this:

myToken.transfer(acc[3], "100000000000000000000", {from:acc[0]})

In addition to that, the usage of toNumber() is also unsafe, as you have noticed yourself.

When you call from the off-chain an on-chain function which returns uint256, you get a BigNumber object. If this object represents a number smaller than Number.MAX_SAFE_INTEGER, then it is safe to use toNumber() on it. But since you do not know that for sure, you should avoid using toNumber().

Also, please note that it is better to use toFixed() than toString(), because the latter may return the scientific notation of the number (e.g. 1+e18), which may give you unexpected problems when you later use it in other contexts.