How to get uniswap quote using a fractional input amount

Question

i have a question about interacting with the Uniswap quoter contract.

I am able to get a quote using quoteExactInputSingle. The method has 1 parameter for the input amount, which works fine when it is an integer like 100. The contract parameter is a uint (therefore we must use something equivalent in javascript that can be converted using jsbn/jsbi).

Does anyone know how to get quotes using fractional input amounts instead? E.g. wanting to swap 0.00014 UNI in javascript:

const amountIn = 0.00014;
await quoterContract.callStatic.quoteExactInputSingle(
  token0Address,
  token1Address,
  poolFee,
  amountIn,  // The parameter for input amount
  0
);

produces:

Error: invalid BigNumber string (argument="value", value="0.00014", code=INVALID_ARGUMENT, version=bignumber/5.7.0)
  reason: 'invalid BigNumber string',
  code: 'INVALID_ARGUMENT',
  argument: 'value',
  value: '0.00014'
}

---

I have tried finding the answer in the uniswap source code:

• front-end, inc. the widgets repo
• AWS router
• their SDK docs
• their guide on making trades

I did notice that the uniswap product multiplies decimal amounts by 20 decimal places (before sending the amount in a http request to their API), which is then converted "back" to a decimal in a smart contract (after they identify the number of decimal places for the input token). This is obviously because the EVM doesn't support fractions, however i can't find an explanation for why the multiplication is done to exactly 20 decimal places.

I do also know about the Token sdk class, e.g.:

const token = new Token(5, token0Address, 18, 'UNI', 'Uniswap');

However i haven't found a way to use the Token instance with the quoter contract.

Answer 1

i just needed to convert the token balance to token amount in the contract call, silly me :). Theres no such thing as fractional tokens.

const amountIn = 0.00014;  // before
const amountIn = new Decimal('0.00014e18').toString();  // after. This specific token in this example has 18 decimal places

await quoterContract.callStatic.quoteExactInputSingle(
  token0Address,
  token1Address,
  poolFee,
  amountIn,
  0
);

I also figured out that the uniswap front-end caches quotes to an extent, hence i was getting thrown off with seeing token amount being passed in as 0.00014e20 instead of 0.00014e18 in the http call history.