Possible unintended behavior when converting FixedNumber to BigNumber

[danielblnc]

Describe the bug
Using BigNumber.from() on a FixedNumber ignores the comma, treats the whole number and decimals as an int. This can cause issues, for example passing a FixedNumber in a contract call will internally convert it to BigNumber, resulting in the contract being called with a much larger value than intended.

Reproduction steps

const ETH_AMOUNT = '1'
var SWAP_AMOUNT_IN_WEI = ethers.utils.parseUnits(ETH_AMOUNT, 'ether')

const inputAmountFixedNumber = ethers.FixedNumber.from(SWAP_AMOUNT_IN_WEI)
const inputAmountBigNumber = ethers.BigNumber.from(inputAmountFixedNumber)

console.log(inputAmountFixedNumber.toString(), inputAmountBigNumber.toString()) // The second amount is the first*10^18

Environment:
Latest Node.js Docker image

Search Terms
BigNumber, FixedNumber, decimal, conversion

[danielblnc]

Is this intended behavior? If it is, what would be the correct way to convert a FixedNumber to a BigNumber?

[ricmoo]

The parseEthers method converts a value from ether to Wei, so 1 becomes 1e18. It doesn’t matter what you do with it after that, it is already 1e18.

By default the decimals component is 0, which means the value passed in is the whole component.

If you are trying to reposition the decimal point with respect to the value, you need to specify your own decimals offset. See here: https://github.com/ethers-io/ethers.js/blob/master/packages/bignumber/src.ts/fixednumber.ts#L328

[danielblnc]

I realize I chose a bad example.

The behavior happens with any value stored in a FixedNumber and converted to a BigNumber. A FixedNumber contains 18 decimals by default, so a 1 will be converted into a 1e18, and a 1e18 (such as in my example) to a 1e36.

This line here is the culprit:
https://github.com/ethers-io/ethers.js/blob/master/packages/bignumber/src.ts/bignumber.ts#L268

It takes only the Hex value from the FixedNumber to create the BigNumber, and the point position is ignored.

Bug:
console.log(ethers.BigNumber.from(ethers.FixedNumber.from(1)).toString())

Workaround by having a FixedNumber with no decimals:
console.log(ethers.BigNumber.from(ethers.FixedNumber.from(1).round().toFormat("fixed128x0")).toString())

[danielblnc]

Perhaps I was unclear:

SWAP_AMOUNT_IN_WEI is a 1 followed by 18 zeros, as you mentioned.
inputAmountFixedNumber = ethers.FixedNumber.from(SWAP_AMOUNT_IN_WEI) // Again a 1 followed by 18 zeros, which is correct
inputAmountBigNumber = ethers.BigNumber.from(inputAmountFixedNumber) // Creates a 1 followed by 36 zeros, I think this is not intended

Easier to see here:

  const oneFixedNumber = ethers.FixedNumber.from('1')
  const oneBigNumber = ethers.BigNumber.from(oneFixedNumber)

  console.log(oneFixedNumber.toString()) // Prints 1.0
  console.log(oneBigNumber.toString()) // Prints 1000000000000000000

BigNumber.from(aFixedNumber) always adds 18 zeros to whatever the value of aFixedNumber was

[danielblnc]

I did some digging, when calling BigNumber.from() , the function goes to if (anyValue) and then into if (anyValue.toHexString). There it returns a new BigNumber taking the hex value from the FixedNumber, ignoring the format (which contains 18 decimals by default), hence the result being the original value times 10^18.

This is the line:
https://github.com/ethers-io/ethers.js/blob/master/packages/bignumber/src.ts/bignumber.ts#L268