Ethers.js: BigNumber error on small value

[abrikos]

ethers.utils.parseEther('0.0007000000000000001')
throws Error: underflow occurred (operation="division", fault="underflow", version=4.0.39)
How can I fix it?

[ricmoo]

That is arithmetic underflow though, you can’t have 0.1 wei. :)

[ricmoo]

I'm going to close this now, as this is expected behaviour, but please feel free to re-open or continue discussion here.

Thanks! :)

[paulrberg]

I think it's a rather common use case to convert a string value from an input html tag to a parsed big number, so it would be nice to have a regular expression as part of the constants object exported by ethers. Here's what I use in my app:

^[0-9]+.?[0-9]{0,18}$

Which matches:

• 5
• 5.12
• 5.123456789101112131

But doesn't match:

• Any kind of characters other than Arabic numerals (from 0 to 9)
• Any number with more than 18 decimals
• The empty string (""), so you may want to pick a different initialisation value

[ricmoo]

It also doesn't match .4, which should be valid and doesn't match negative numbers like -1, which should sometimes be valid.

The regular expression that matches a fully qualified (but still flexible) number is a bit more complicated, but it may make sense to include.

In v5, it would be more useful to probably use something like:

function valid(value) {
    try {
        ethers.FixedNumber.fromString(value, "fixed128x18");
        return true;
    } catch (e) { }
    return false; 
}

as this allows checking for total bit width as well. Other common things may be "ufixed128x18" for positive-only, or for satoshis, "ufixed128x8" could be used. In general, this function would be called text.oninput and used to update any "submit" button, or highlight the invalid field.

[jewpaltz]

For the simpleminded folk amongst us.
The issue is that ether (and most ERC20 tokens) only support 18 decimals.
So what I use is a simple function to round the string down before I pass it to ethers.utils.parseEther

function roundCryptoValueString(str, decimalPlaces=18){
    const arr = str.split(".");
    const fraction = arr[1] .substr(0, decimalPlaces);
    return arr[0] + "." + fraction;
}

[paulrberg]

For what it's worth, I ended up building my own solution for this problem.

It's a tiny npm package called evm-fp, which works like this;

import { BigNumber } from "@ethersproject/bignumber";
import fp from "evm-fp";

const foo: BigNumber = fp("3.1415");
const bar: BigNumber = fp("115792089237316195423570985008687907853269984665640564039457.584007913129639935");
const baz: BigNumber = fp("100e6", 6);