Web3.js – Parsing Transaction Receipt Logs

eventsjavascriptlogsreceiptsweb3js

The event parser in web3 provides a nice parsing functionality for events, and I use it for logging all events to a file, but it's very difficult to use for looking at individual events for a particular transaction for automated testing because it induces an unneeded and difficult to manage concurrency aspect when there doesn't need to be one.

When I get a transaction result with web3, I have the transaction receipt in hand and that's the perfect time to synchronously examine the result and apply pass/fail test criteria.

I would like to parse the log section of the receipt but can't find a function in web3 to do so. Does it exist?

Best Answer

Do this: You'll need to pull code from web3, and it works best if your frontend is bundled using something like webpack or browserify:

var SolidityCoder = require("web3/lib/solidity/coder.js");
var log = receipt.logs[0];
var data = SolidityCoder.decodeParams(["string", "uint"], log.data.replace("0x", ""));

In this case, we're decoding log data that contains two variables, one a string type and one a uint.

EDIT:

If you have the ABI available, you can detect which event it is related to that ABI:

var SolidityCoder = require("web3/lib/solidity/coder.js");

// You might want to put the following in a loop to handle all logs in this receipt.
var log = receipt.logs[0];
var event = null;

for (var i = 0; i < abi.length; i++) {
  var item = abi[i];
  if (item.type != "event") continue;
  var signature = item.name + "(" + item.inputs.map(function(input) {return input.type;}).join(",") + ")";
  var hash = web3.sha3(signature);
  if (hash == log.topics[0]) {
    event = item;
    break;
  }
}

if (event != null) {
  var inputs = event.inputs.map(function(input) {return input.type;});
  var data = SolidityCoder.decodeParams(inputs, log.data.replace("0x", ""));
  // Do something with the data. Depends on the log and what you're using the data for.
}

Related Solutions

events – How to Access the Event Log by Knowing the Contract Address

Take a look at web3.eth.filter and watch.

Something like this:

const filter = web3.eth.filter({
  fromBlock: 0,
  toBlock: 'latest',
  address: contractAddress,
  topics: [web3.sha3('newtest(string,uint256,string,string,uint256)')]
})

filter.watch((error, result) => {
   //
})

Note the part that "In Solidity: The first topic is the hash of the signature of the event." Canonical types, such as uint256 have to be used in signatures.

EDIT: Per @plingamp's comment web3.sha3 now includes the '0x'.

[Ethereum] Transaction receipt contains all log entries, but only the last two are decoded by web3

OK, I understand the reason:

All the events generated during the execution of the function called by web3 are included in the receipt logs, but only the events generated in the scope of the contract in which the function is implemented are included in the decoded logs.

For example, given the following on-chain code:

File IContract1.sol:

pragma solidity ^0.4.18;

interface IContract1 {
    function func(uint256 x) external returns (uint256);
}

File Contract1.sol:

pragma solidity ^0.4.18;
import "./IContract1.sol";

contract Contract1 is IContract1{
    event Event1(uint256 x);
    function func(uint256 x) external returns (uint256) {
        Event1(x);
        return x;
    }
}

File Contract2.sol:

pragma solidity ^0.4.18;
import "./IContract1.sol";

contract Contract2 {
    IContract1 private contract1;
    function Contract2(IContract1 _contract1) public {
        contract1 = _contract1;
    }
    event Event2(uint256 x);
    function func(uint256 x) external returns (uint256) {
        Event2(x);
        return contract1.func(x);
    }
}

If I execute the following off-chain code:

let contract1 = await artifacts.require("Contract1.sol").new();
let contract2 = await artifacts.require("Contract2.sol").new(contract1.address);

let hash = await contract2.func(42);

Then hash.receipt.logs will include both Event1 and Event2.

But hash.logs will include only Event2.

Best Answer

Related Solutions

events – How to Access the Event Log by Knowing the Contract Address

[Ethereum] Transaction receipt contains all log entries, but only the last two are decoded by web3

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