Uniswap V2 removeLiquidity

Question

I am able to add liquidity and remove liquidity on uniswap, but I am having trouble calculating the min values for the token pairs.

For removing liquidity I am trying to get the amount of each token by doing the following:

const pathA = [liquidityToken.address, tokenA.address];
  const amountsA = await uniswap.getAmountsOut(liquidityAmount, pathA);
  const desiredAmtA = amountsA[1];

  const pathB = [liquidityToken.address, tokenB.address];
  const amountsB = await uniswap.getAmountsOut(liquidityAmount, pathA);
  const desiredAmtB = amountsB[1];

This is throwing an error, but I'm not sure how to get the amount of tokena/b that my liquidity token represents. And consequently, I"m trying to calculate the min from it like this. I don't understand how these slippage tolerances get calculated.

const minAmtA = desiredAmtA.mul(BigNumber.from(9)).div(BigNumber.from(10));
const minAmtB = desiredAmtB.mul(BigNumber.from(9)).div(BigNumber.from(10));

It works when I put 0, but obviously I don't want 100% slippage

    tokenA.address,
    tokenB.address,
    liquidityAmount,
    BigNumber.from(0), // don't know how to calculate this
    BigNumber.from(0), // don't know how to calculate this
    wallet.address,
    deadline,
    {
      gasLimit: process.env.GAS_LIMIT,
      gasPrice: 20e9,
    }
  );

Answer 1

This is wrong for two reasons:

1. You are not trading liquidity tokens for pool tokens, you're removing liquidity which is different. amountADesired & amountBDesired are not maintained as pairs with liquidity pool tokens.
2. getAmountsOut and getAmountsIn are used for calculating trade prices, which is not the case in adding/removing liquidity.

Your liquidity tokens represent your share in the pool. So the formula for calculating amount of pooled tokens for certain liquidity would be :

amountADesired = (liquidity* reserveA)/totalSupply

This is what happens behind the scene in burn function of Pool Smart contract. Uniswap Pair burn method

Here's a code sample for removing liquidity, Rest of it is easy if you've come this far. Comment down if you need further help

    const toBN = web3.utils.toBN;
    const owner = '<Your address>'
    const pairContract = new web3.eth.Contract(getPairABI(), pairAddress);
    
    const reserves = await getReserves(tokenAName, tokenBName);// calling pairContract.getResrves()
    const totalSupply = await pairContract.methods.totalSupply().call();
    const ownerBalance = await pairContract.methods.balanceOf(owner).call();

    console.log('LIQUIDITY SHARE', web3.utils.toBN(ownerBalance).div(toBN(totalSupply)).mul(toBN(100)).toNumber());
    const liquidityPerc = 50;
    const liquidity = toBN(ownerBalance).mul(toBN(liquidityPerc)).div(toBN(100));

    const amountA = liquidity.mul(toBN(reserves[0])).div(toBN(totalSupply));
    const amountB = liquidity.mul(toBN(reserves[1])).div(toBN(totalSupply));
    console.log('HOlDING AMOUNTS', amountA.toString(), amountB.toString());
    //5% slippage
    var amountAMin = toBN(amountA).div(toBN(100)).mul(toBN(95));
    var amountBMin = toBN(amountB).div(toBN(100)).mul(toBN(95));
    console.log('MIN AMOUNTS', amountAMin.toString(), amountBMin.toString());
    
    var routerAbi = getRouterABI();
    var contract = new web3.eth.Contract(routerAbi, getRouterAddress(dex));

    var data = contract.methods.removeLiquidity(
        getTokenAddress(tokenAName),
        getTokenAddress(tokenBName),
        web3.utils.toHex(liquidity),
        web3.utils.toHex(amountAMin),
        web3.utils.toHex(amountBMin),
        targetAccount.address,
        web3.utils.toHex(Math.round(Date.now()/1000)+60*200), // in seconds
    );