Adding new array entries to object only if value doesnt exist

Question

I have written the following code to store a data retrieve values and store data into an object:

var contract = new web3.eth.Contract(contractAbi, contractAddress);
var accounts = {};
var balance = {};


// Crawl the Chain from the Contract Deployment Date till the latest block
contract.getPastEvents(
    'Transfer',
    {
      fromBlock: 8437000,
      toBlock: 'latest'
    },(error, events) => { 

    if (!error){
      var obj=JSON.parse(JSON.stringify(events));

      for (const [key, val] of Object.entries(obj)) {
        accounts[val.blockNumber] = {
            _from: val.returnValues._from,
            _to: val.returnValues._to,
            _value: parseInt(val.returnValues._value, 10)
            // _value: val.returnValues._value
        };
    }

    }
    else {
      console.log(error)
}

Object.entries(accounts).forEach(([key, val]) => console.log(key, val)); 

})

This works fine and the output of this is:

8441134 { _from: '0x34872874b65E12408eC0265E9cf0a35FA6c8D13E',
  _to: '0x39BC998bD7DC71885661B1062CC2baa9fbe94F45',
  _value: 76470000000000000000 }
8441135 { _from: '0x85C5c26DC2aF5546341Fc1988B9d178148b4838B',
  _to: '0x2E642b8D59B45a1D8c5aEf716A84FF44ea665914',
  _value: 947018889517274100000 }
8441142 { _from: '0xda7C4aAb9bb74710498485ed01CB4D521f7b4456',
  _to: '0x24fa98CA3aB52D8CF562B641AbC4b12861e991Cf',
  _value: 321279880100000000000 }
8441157 { _from: '0x46340b20830761efd32832A74d7169B29FEB9758',
  _to: '0xeE7D76D5B00A10B49e37a9e26a7678be1EE607F4',
  _value: 29500000000000000000 }
8441167 { _from: '0x2baa435Ca4B9dE4a135cAE11f4c04C4a1d334089',
  _to: '0x9fa7f05C9AaE89752fa9273CFAcADF602657599d',
  _value: 1.5043600286e+21 }

I would like to transform this into a format , where each of the accounts (_to and _from) and adding as account to the indexes over the blocks i.e.

// The _to and _from fields become account entries
8441142 { Accounts: '0xda7C4aAb9bb74710498485ed01CB4D521f7b4456',
                    '0x24fa98CA3aB52D8CF562B641AbC4b12861e991Cf'}
// Add accounts that did not previously exist, ignore ones already in here
8441157 { Accounts: 0xda7C4aAb9bb74710498485ed01CB4D521f7b4456',
                  '0x24fa98CA3aB52D8CF562B641AbC4b12861e991Cf', '0x46340b20830761efd32832A74d7169B29FEB9758','0xeE7D76D5B00A10B49e37a9e26a7678be1EE607F4'}
// Add accounts that did not previously exist, ignore ones already in here
8441167 { Accounts: '0x2baa435Ca4B9dE4a135cAE11f4c04C4a1d334089','0x9fa7f05C9AaE89752fa9273CFAcADF602657599d'}

I would appreciate pointers on how I can do this efficiently.

Answer 1

Use a variable outside the loop to hold the account list from the previous block. Make a copy of it and push the new accounts onto it.

  let last_accounts = [];
  for (const [key, {blockNumber, returnValues: {_from, _to}}] of Object.entries(obj)) {
    let these_accounts = last_accounts.slice(); // copy accounts from previous block
    if (!these_accounts.includes(_from)) {
        these_accounts.push(_from);
    }
    if (!these_accounts.includes(_to)) {
        these_accounts.push(_to);
    }
    accounts[blockNumber] = {Accounts: these_accounts};
    last_accounts = these_accounts;
  }