Uniswap V2, getAmountsOut and toWei/fromWei

Question

I have a simple code to get prices from Uniswap V2

const getTokenExchangeRate = async (tokenA, tokenB) => {
  return (
    await dexRouter.methods.getAmountsOut(toWei("1"), [tokenA, tokenB]).call()
  )[1];
};

// usage

const price = await getTokenExchangeRate(LINK, DAI)
priceNormal = web3.utils.fromWei(price)

console.log(priceNormal)

If both coins have equal decimals it works normal, but if there is a difference in decimals I get a huge number. For example

const price = await getTokenExchangeRate(LINK, USDC)
priceNormal = web3.utils.fromWei(price)

console.log(priceNormal)
PS C:\Users\Viktor\Desktop\Uniswap3JS> node test.js
0.000000000009015961

What should I add to get human readable numbers for all possible pairs? Thanks.

Answer 1

1. Create an instance for ERC20 contract.
2. Create instance for the UniswapPair contract
3. Call decimals() of that tokens.
4. Create a logic like:

if(tokenDecimals < 18){
 const amountOut = await pair.methods.getReserves().call()
 const parsedAmount = Number(amountOut.tokenB._hex).toFixed(18)
 await dexRouter.methods.getAmountsOut(parsedAmount, [tokenA,tokenB]).call()
}

Can't test if this code will compile but here's is the north.

Answer 2

Well, I found an answer, though I don't like it. And now I don't understand why getAmountsOut (WETH, USDC) gives a real price and getAmountsOut (USDC, WETH) gives some weird numbers. But that's another story.

const getTokenExchangeRate = async (tokenA, tokenB) => {
  return (
    await dexRouter.methods.getAmountsOut(toWei("1"), [tokenA, tokenB]).call()
  )[1];
};
const price = await getTokenExchangeRate(USDC, WETH9)

// DECIMALS
const abi2 = [
  {
    "constant": true,
    "inputs": [],
    "name": "decimals",
    "outputs": [
      {
        "name": "",
        "type": "uint8"
      }
    ],
    "payable": false,
    "type": "function"
  },
];


const check = new web3.eth.Contract(
  abi2,
  WETH9 //TokenB
);

const result = await check.methods.decimals().call()
let humanPrice = price/(10**result)

Answer 3

I would recommend to convert all numbers to a common denominator (i.e. 18 decimal places precision) and perform the analysis with these decimal places.

The Aave V2 repo has some nice utility functions for this purpose.