formatUnits fraction length

[babyface001]

Describe the Feature

current implementation is full length, but for most token 18 decimals is too long.

Code Example

formatUnits(amount, 18, 3)

// only get 3 fraction.

[ricmoo]

I don't think I would make this change. The complexities around what is intended in rounding would mean the behaviour is not what many would expect, depending on the rounding strategy they expected. Also, formatting of significant digits is very case dependant.

For example, some would want 3.4566 to be rounded to "3.456" and others "3.457". And in the case of 3.0004, some would want "3.000", others, "3.0" and others still "3". Basically, the needs of string formatting a decimal value are vast.

Which is why the output format is intended to be very predictable (i.e. always exactly one decimal point and at least one of each whole and fractional components), which make it easy to to reprocess as a string if desired. The string can always be split using "." and get exactly two components, which can then be refined as desired. The exception to this is if decimals is 0, in which case only a whole component is presented.

Otherwise, the FixedNumber class enables simplifying converting a value by using any rounding strategy first to the desired resolution and then formatting to the target depth.

Does that make sense?

[babyface001]

it's an optional parameter.

and in ethereum world, cast is best practice.

[lomocc]

I use this code:

import { BigNumberish, ethers } from 'ethers';

export default function formatUnits(
  value: BigNumberish,
  decimals: string | BigNumberish = 0,
  maxDecimalDigits?: number
) {
  return ethers.FixedNumber.from(ethers.utils.formatUnits(value, decimals))
    .round(maxDecimalDigits ?? ethers.BigNumber.from(decimals).toNumber())
    .toString();
}

formatUnits(ethers.BigNumber.from('3224686211026'), 8)
// '32246.86211026'
formatUnits(ethers.BigNumber.from('3224686211026'), 8, 2)
// '32246.86'
formatUnits(ethers.BigNumber.from('3224680211026'), 8, 2)
// '32246.8'

[ricmoo]

I likely won't make any formatting API changes, especially to v5, but the solution from @lomocc looks good.

Moving this to Q&A Discussions, so it's easier for people to find in the future. ;)