How to approve swapExactTokensForETH from UniSwapRouter V2, before invoking it?

Question

I am trying to create a simple DAI/ETH swapper which lets me swap some DAI for ETH using Uniswap Router V2. I am creating it using React and uniswapSDK. So far, I have managed to connect to my Metamask wallet, get the ABI for the Uniswap contract so I can invoke the method.

However, I am finding it difficult to understand how do I approve this method before calling it? I understand that sending a transaction over the chain needs to first signed, then be called. I am not sure 'how' to approve it? This is my code till now for swapping

export const makeSwap = async (
  contract: any,
  account: any,
  library: any,
  amount: any
) => {
  const DAI: Token = await Fetcher.fetchTokenData(chainId, DAItokenAddress);
  const pair: Pair = await Fetcher.fetchPairData(DAI, WETH[chainId]);
  const route: Route = new Route([pair], DAI);
  const trade = new Trade(
    route,
    new TokenAmount(DAI, library.utils.toWei(amount)),
    TradeType.EXACT_INPUT
  );

  const slippageTolerance = new Percent("50", "10000"); //0.5%
  const amountOutMin = ethers.BigNumber.from(
    trade.minimumAmountOut(slippageTolerance).raw.toString()
  ).toHexString();
  const path = [DAI.address, WETH[chainId].address];
  const to = account;
  const deadline = Math.floor(Date.now() / 1000) + 60 * 1;
  const value = ethers.BigNumber.from(
    trade.inputAmount.raw.toString()
  ).toHexString();
  await contract.methods.approve(value, account);
  const tx = await contract.methods
    .swapExactTokensForETH(value, amountOutMin, path, to, deadline)
    .call();


   //Fails above this line


  console.log(`Transaction hash: ${tx.hash}`);
  const receipt = await tx.wait();
  console.log(`Transaction was mined in block ${receipt.blockNumber}`);
};

Thank you for your help!

Answer 1

First of all .call() is used when you need to read data from the contract. As you need to send a transaction so use .send() instead for .swapExactTokensForETH.

For this to work, you need to approve the Uniswap Router to send DAI from our account.

await daiContract.methods.approve(UniswapRouterAddress, tokenAmount).send();

Answer 2

You need to call the DAI contract with approve(UniRouterAddress, amount) not sure about the syntax since im not familiar at all with web3.js but i think you can figure it out ( if this helps understanding better in solidity that would be IERC20(DAIContractAddress).approve(UniRouterAddress, amount) )