Potential bug when using "FixedNumber.fromValue" with large number of decimals

While working with FixedNumber, I got this error after setting the number of decimals to 77:

fractional component exceeds decimals (fault=“underflow”, operation=“parseFixed”, code=NUMERIC_FAULT, version=bignumber/5.4.2)

Here’s my code:

const foo = "115792089237316195423570985008687907853269984665640564039457584007913129639935";
const decimals = 77;
const result = FixedNumber.fromValue(foo, decimals);
console.log(result.toString());

I then peaked into the guts of the FixedNumber implementation, and noticed that the number of decimals gets overridden to 18 in this function:

https://github.com/ethers-io/ethers.js/blob/4166b2753d0d6b230a3a22b5a2b6c4e1906a7af7/packages/bignumber/src.ts/fixednumber.ts#L342-L364

That is, even if it’s 77 when it enters the fromValue function, it gets set to 18 by the time it is used in fromString.

My request is to document the maximum number of decimals that can be set when working with FixedNumber, plus any other constraints that there might be.

[ricmoo]

I think there is some confusion as to what the decimals and format mean.

The decimals indicates the number of decimals to assume the input value is operating under. For example, "10" with 0 decimals is 10, while "10" with 2 decimals is 0.1. It only impacts the interpretation of the value.

The format parameter indicates the internal representation used during fixed-point mathematical operations.

So, the default decimals of 0 and the default format of fixed128x18 indicates that whatever value is passed in should be taken literally and that 18 decimal places are used during maths.

So, the value "123" is represented internally as 123.000_000_000_000_000_000 (underscores added for readability).

Using a value "123" with a decimals of 2 and the default format is represented internally as 1.230_000_000_000_000_000.

Does that make sense?

[ricmoo]

Just discussing this with my partner, and this example came up, which might help explain where the logic and names stem from.

const balance = await erc20Contract.getBalance();
const decimals = await erc20Contract.decimals();

const value = FixedNumber.fromValue(balance, decimals);

Hopefully that helps future peeps who happen across this issue too. 😃