How to approve and swap?

[unsphere]

Hey,

I am struggling with swapping tokens back to WBNB. Approval is working fine, but I get
Fail with error 'TransferHelper: TRANSFER_FROM_FAILED' for swapExactTokensForTokens.

When I check PancakeSwap after approval it still shows me "Approve" for Safemoon or any other token that I tested.

Swapping WBNB to any token is working fine.

Can someone point me in the right direction?

const ethers = require('ethers');

const tokenAddress = '0x8076c74c5e3f5852037f31ff0093eeb8c8add8d3'; //e.g. Safemoon
const amountToSwap = 1000;

const walletAddress = 'WALLET_ADDRESS';
const mnemonic = 'SEED';

const wbnbAddress = '0xbb4CdB9CBd36B01bD1cBaEBF2De08d9173bc095c'; //WBNB
const routerAddress = '0x10ED43C718714eb63d5aA57B78B54704E256024E'; //PancakeSwap V2 Router

const slipPage = 0;
const gasLimit = 345684;
const gasPrice = 10;

const provider = new ethers.providers.JsonRpcProvider(
  'https://bsc-dataseed1.binance.org:443'
);
provider.removeAllListeners();

const wallet = ethers.Wallet.fromMnemonic(mnemonic);
const account = wallet.connect(provider);

const router = new ethers.Contract(
  routerAddress,
  [
    'function swapExactTokensForTokens(uint amountIn, uint amountOutMin, address[] calldata path, address to, uint deadline) external returns (uint[] memory amounts)',
  ],
  account
);

const token = new ethers.Contract(
  tokenAddress,
  ['function approve(address spender, uint256 amount) external returns (bool)'],
  account
);

const main = async () => {
  console.log('Start approving...');

  const approve = await token.approve(
    routerAddress,
    ethers.constants.MaxUint256,
    {
      gasLimit,
      gasPrice: ethers.utils.parseUnits(gasPrice.toString(), 'gwei'),
    }
  );

  console.log(approve);

  console.log('Waiting for confirmation...');

  const receiptApprove = await approve.wait();

  console.log(receiptApprove);

  console.log('Start swapping...');

  if (receiptApprove) {
    const swap = await router.swapExactTokensForTokens(
      ethers.utils.parseUnits(amountToSwap.toString(), 'ether'),
      ethers.utils.parseUnits(slipPage.toString(), 'ether'),
      [tokenAddress, wbnbAddress],
      walletAddress,
      Date.now() + 1000 * 60 * 10,
      {
        gasLimit,
        gasPrice: ethers.utils.parseUnits(gasPrice.toString(), 'gwei'),
      }
    );

    console.log(swap);

    console.log('Waiting for confirmation...');

    const receiptSwap = await swap.wait();
    console.log(receiptSwap);

    console.log('Swap finished!');
    process.exit();
  }
};

main();

Link to example transaction: https://bscscan.com/tx/0x89e7049152c96dd2c9400eb43829ac11cff7c6b55c76ac1308749bb38e4c7d81

[zemse]

I just checked your transactions' execution trace, it fails when it is trying to subtract balance. Can you once check if you have enough balance of the asset you are swapping?

[zemse]

I'm seeing your transaction in tenderly and it shows that it went out of gas during a message call / internal tx.

Can you try giving in a bit more gas gasLimit + 30000

[unsphere]

Never knew about tenderly. Thanks for the tip! But unfortunately, after increasing gasLimit, it is still not working. Now I get a "Pancake: K" error: see here.

[nader8]

is this the same code you used to swap WBNB to any token?

[zemse]

The k error comes the constant product formula in uniswap doesn't hold. This can happen for some ERC20 implementations that actually transfer less tokens to receipent than sent from sender. Example USDT has a code to charge fees for ERC20 transfer (however they have set it to 0 for now).

Take a case when transfer charges 1% fee. So if you transfer 100 then receiver gets 99. So you have to calculate the amount to send such that receiver gets 100, i.e. (100 / 0.99) tokens. So in your case you have to find amount of fees charged by the token, and send more tokens to accommodate for that.

[vlbhtcno1]

@zemse Sorry, I have a question. What's the difference between JsonRpcProvider and WebSocketProvider?
For the performance ( Example: Listen to event from Contract) Which one is better performance?