How do I parse the transaction receipt log with web3.js?

Question

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?

Answer 1

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.
}

Answer 2

You can now use the web3.eth.abi.decodeLog function (web3 1.0).

Example from the documentation:

web3.eth.abi.decodeLog([{
    type: 'string',
    name: 'myString'
},{
    type: 'uint256',
    name: 'myNumber',
    indexed: true
},{
    type: 'uint8',
    name: 'mySmallNumber',
    indexed: true
}],
'0x0000000000000000000000000000000000000000000000000000000000000020000000000000000000000000000000000000000000000000000000000000000748656c6c6f252100000000000000000000000000000000000000000000000000',
['0x000000000000000000000000000000000000000000000000000000000000f310', '0x0000000000000000000000000000000000000000000000000000000000000010']);
> Result {
    '0': 'Hello%!',
    '1': '62224',
    '2': '16',
    myString: 'Hello%!',
    myNumber: '62224',
    mySmallNumber: '16'
}

Answer 3

Tim:

Thanks so much for the pointer. You forced me to finally understand some of the internals of web3.js.

I found a cleaner way to do this that covers all the corner cases of the actually fairly complicated log message format (e.g. indexing). I just used SolidityEvent from web3 to do the already-tested work for me.

Below is the code. I have this code on github, including test code.

// XXX move this to a hook function
var SolidityEvent = require("web3/lib/web3/event.js");
Pudding.logParser = function (logs, abi) {

    // pattern similar to lib/web3/contract.js:  addEventsToContract()
    var decoders = abi.filter(function (json) {
        return json.type === 'event';
    }).map(function(json) {
        // note first and third params required only by enocde and execute;
        // so don't call those!
        return new SolidityEvent(null, json, null);
    });

    return logs.map(function (log) {
        return decoders.find(function(decoder) {
            return (decoder.signature() == log.topics[0].replace("0x",""));
        }).decode(log);
    })
}

Answer 4

For web3.js 1.0 use the following:

contractInstance.inputs = [{"indexed": false, "name": "_id", "type": "uint256"}]; //event abi
contractInstance._decodeEventABI({data: '0x0'}); //event raw data

output

{
  returnValues: 
   Result {
     '0': '1',
     _id: '1',
  },
  raw: {
    data: '0x0'
  }
}

Answer 5

Once you have transaction receipt (tr), you know block number of the transaction (tr.blockNumber). So, you may do the following:

myContract.MyEvent (
  {},
  {fromBlock: tr.blockNumber, toBlock: tr.blockNumber}).
    get ().
      filter (function (e) {
        return e.transactionHash == tr.transactionHash
      });

This will return an array of all events of type MyEvent generated by contract myContract in transaction referred by transaction receipt tr.

Answer 6

With web3.js 0.20.6.

$ node
> var AllEvents = require('web3/lib/web3/allevents')
undefined
> var decodeEventsForContract = (C, tr) => {
    const ae = new AllEvents(C._web3, C.abi, C.address);

    // ae.decode mutates the args, so we deep copy
    return JSON.parse(JSON.stringify(tr)) 
      .logs
      .filter(l => l.address === C.address)
      .map(l => ae.decode(l));
  }
undefined
> decodeEventsForContract(MyTokenContract, txreceipt);
[ { logIndex: 0,
    transactionIndex: 0,
    transactionHash: '0xbc68d5ddc391fab84cd633a77dbc815cbc42546a13de9d123f7a5b820faa3cb4',
    blockHash: '0xb604998aa0b6bece492530c9cbab494349a31b18a34067f225e1b59613952051',
    blockNumber: 21,
    address: '0x345ca3e014aaf5dca488057592ee47305d9b3e10',
    type: 'mined',
    event: 'Transfer',
    args: 
     { from: '0x0000000000000000000000000000000000000000',
       to: '0x627306090abab3a6e1400e9345bc60c78a8bef57',
       value: [BigNumber] } },
  { logIndex: 1,
    transactionIndex: 0,
    transactionHash: '0xbc68d5ddc391fab84cd633a77dbc815cbc42546a13de9d123f7a5b820faa3cb4',
    blockHash: '0xb604998aa0b6bece492530c9cbab494349a31b18a34067f225e1b59613952051',
    blockNumber: 21,
    address: '0x345ca3e014aaf5dca488057592ee47305d9b3e10',
    type: 'mined',
    event: 'Mint',
    args: 
     { to: '0x627306090abab3a6e1400e9345bc60c78a8bef57',
       value: [BigNumber],
       minter: '0x627306090abab3a6e1400e9345bc60c78a8bef57' } } ]