Uniswap – Understanding Fees on Transfer Tokens and Uniswap V2

uniswap

I am implementing a uniswap v2 front end interface for a UniswapV2Router contract. I was wondering how does uniswap identifies when swapExactETHSupportingFeeOnTransferTokens needs to be used instead of swapExactETHForTokens. In other words, how does the uniswap interface identifies fee on transfer tokens? Also for both these functions does the user only need approve the router ?

Best Answer

For your two questions:

How does uniswap identifies when swapExactETHSupportingFeeOnTransferTokens needs to be used instead of swapExactETHForTokens.

As their name, if the swapped token have the function of, for example, take 10% fee when transfering token. Thus, you have to use swapExactETHSupportingFeeOnTransferTokens. For the Frontend code of Uniswap you can refer to their sdk at https://github.com/Uniswap/v2-sdk. In their code, they also define logic of using these two functions as (https://github.com/Uniswap/v2-sdk/blob/81948482505b09da3282c7bcd93945eef1992f71/src/router.ts#L114)

const useFeeOnTransfer = Boolean(options.feeOnTransfer)

const methodName = useFeeOnTransfer ? 'swapExactETHForTokensSupportingFeeOnTransferTokens' : 'swapExactETHForTokens'

Both these functions does the user only need approve the router ?

The answer is No if you swap from native token to standard token, eg. swapExactETH... However, if you swap from standard token to native token (i.e., WETH to LDO), you have to approve the router to use the token.

Hope it helps you.

Related Solutions

[Ethereum] Uniswap on Ganache

have you considered using Hardhat to fork mainnet so you can test your contracts against a more real-world version of uniswap?

Take a look at this. https://hardhat.org/guides/mainnet-forking.html

I use this approach when testing contracts that involve creating pairs, adding/removing liquidity, arbitrage etc.

Uniswap – How Does Uniswap Interact with Token Contract When Users Swap?

When you swap an ERC20 token for ETH on Uniswap, the token's transfer function is called - see IERC20.transfer. IIRC when you "buy" your wallet is the recipient and the Token-WETH pair contract (created by the Uniswap Factory) is the sender. On the other hand, when you "sell" your wallet is the sender and the pair is the recipient.

If you'd like to get the address of the pair (your token-WETH) you can use the factory to get it:

IUniswapV2Router router = IUniswapV2Router(uniswapV2RouterAddress);
IUniswapV2Pair pair = IUniswapV2Factory(router.factory())
    .getPair(address(this), router.WETH());

See Uniswap interface code and this answer (for example) to find the Uniswap router address (or google it)

Best Answer

Related Solutions

[Ethereum] Uniswap on Ganache

Uniswap – How Does Uniswap Interact with Token Contract When Users Swap?

Related Topic