Web3.js - How to sign an approve transaction for a swap?

Question

I want to programmatically swap two tokens. I have to approve the amount first. How do I approve using web3.js ?

Here's what I've come up with so far but I get the error Error: Returned error: nonce too low

const myAddress = "my-wallet-address"
const privateKey = Buffer.from('my-private-key', 'hex');

const test = async () => {
    const allowed = await tokenContract.methods.allowance(myAddress, 'UniswapV2Router02ContractAddress').call()

    const encodedABI = tokenContract.methods.approve('UniswapV2Router02ContractAddress', amountIn).encodeABI();
    const tx = {        
        from: myAddress,
        to: 'UniswapV2Router02ContractAddress',
        gas: 2000000,
        data: encodedABI
    };
    
    const customCommon = Common.default.forCustomChain(
        'mainnet',
        {
          name: 'SAMPLE testnet',
          networkId: custom-testnet-id,
          chainId: custom-testnet-id,
        },
        'petersburg',
      )

    const txTx = new Tx(tx, {common: customCommon});
    txTx.sign(privateKey);

    // below gives true, true
    console.log(txTx.validate(), ethUtil.bufferToHex(txTx.getSenderAddress()) === ethUtil.bufferToHex(ethUtil.privateToAddress(privateKey)))
    
    const serializedTx = txTx.serialize();

    // below line results in error
    await web3.eth.sendSignedTransaction('0x' + serializedTx.toString('hex'))

    // await uniswapV2Router02Contract.methods.swapExactTokensForTokensSupportingFeeOnTransferTokens(amountIn, amountOutMin, path, myAddress, deadline).send({from: myAddress})
}

test()

The Error: Returned error: nonce too low error was discussed in the last answer in this post: web3.eth.sendSignedTransaction() always return "Returned error: nonce too low" however I've verified that my private key belongs to the sender (me) of the transaction so I'm clueless

Answer 1

Here's the code:

var Web3 = require('web3');
var web3 = new Web3("rpc-node-link");
var Tx = require('ethereumjs-tx').Transaction;
var Common = require('ethereumjs-common');
var ethUtil = require('ethereumjs-util')

web3.eth.getTransactionCount(myAddress, async (err, txCount) => {
        const encodedABI = contractWhichisApproving.methods.approve(contractThatWantsToSpendContractWhichisApprovingsToken, amountIn).encodeABI();
        const tx = {
            nonce: web3.utils.toHex(txCount),
            from: myAddress,
            to: contractWhichisApprovingAddress,
            gas: 2000000,
            data: encodedABI
        };

        const customCommon = Common.default.forCustomChain(
            'mainnet',
            {
            name: 'Example testnet',
            networkId: custom-testnet-id,
            chainId: custom-testnet-id,
            },
            'petersburg',
        )

        const txTx = new Tx(tx, {common: customCommon});
        txTx.sign(privateKey);

        // following line logs true, true
        console.log(txTx.validate(), ethUtil.bufferToHex(txTx.getSenderAddress()) === ethUtil.bufferToHex(ethUtil.privateToAddress(privateKey)))

        const serializedTx = txTx.serialize();
        console.log('sending')
        await web3.eth.sendSignedTransaction('0x' + serializedTx.toString('hex')).on('receipt', console.log);
        console.log('done')
    });

Reference using Custom Chain: https://github.com/ethereumjs/ethereumjs-tx/blob/master/examples/custom-chain-tx.ts

Answer 2

Let's first understand what's Nonce in Ethereum.

Based on the Ethereum paper: Nonce is the scalar value equal to number of transactions sent from the address (wallet)...

The transaction including: transfer, create contract...

The Nonce too low error can be raise when you use your account in different places. Especially when you use the private key to sign the transaction.

Example:

• An address have 5 transactions, sent from MM (PC1). Nonce is 5.
• You perform a transaction at another place, using private key (PC2). Nonce is 6. But, the MM doesn't have knowledge about this.
• Back to PC1, perform another transaction on MM, the Nonce should be 7, but MM say it 6. This lead to the Nonce too low issue.

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For a fix, we can simply reset MM wallet:

From MM menu choose Setting => Advance Setting => Reset.

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Why reset MM can solve the nonce problems?

• By reset, the MM can sync the number of transactions of address. So the nonce is increased nicely.