BigNumber.toString and BigNumber._hex.toString are not the same

[emclab]

There is a BigNumber 10000 returned in the app and needs to be converted to number. Here are 2 ways to convert:

a = parseInt(a._hex.toString(), 16)/10**18; //<<==10000. Correct

and 2nd:

a = parseInt(a.toString(), 16)/10**18; //<<==309485009.8213451. NOT correct

The 2nd way of converting is in Ethers document. Why it didn't yield correct number?

[emclab]

But parseInt(a.toString())/10**18 works correct.

[ricmoo]

This is still not recommended; checkout my other answer. :)

[ricmoo]

Do not use numbers! This will almost guarantee you get rounding errors. See: https://docs.ethers.io/v5/api/utils/bignumber/#BigNumber--notes

Also, you should not access _hex. That is internal. The .toString() returns base-10, so the parseInt(base10, 16) will not do what you want. The .toHexString() returns a hex string. Make sure you check out the docs on the BigNumber.

If you need a normal JavaScript IEEE 754 number, you can use BigNumber.toNumber() which will throw if rounding will occur. But you generally do not need normal JavaScript numbers. What are you trying to do?

[emclab]

The purpose is to process the BigNumber returned from a = await contractFYX.balanceOf(acctAddress);. Here contractFYX is an ERC20 contract. a is the BigNumber returned from the balanceOf in wei. The app needs to do a/10**18 to return balance in Ether.

[ricmoo]

Then you can use ethers.utils.formatUnits(a) which will give you a string representation of the value. Or if you need to do math, BigNumber supports all the primitive math operations.

Make sure you read the above notes, as the numbers in Ethereum are generally well beyond what JavaScript can handle safely. But there are a plethora of operations in ethers to facilitate anything you should need to do with those numbers. :)

[emclab]

Or if you need to do math, BigNumber supports all the primitive math operations, do you mean BigNumber can be used in same way as a number? ex, do a/10**18 directly and convert a to ether? I was not very clear about this statement in the doc.

[ricmoo]

For example, if you have gasPrice and gasLimit you can compute maxFee = gasPrice.mul(gasLimit) keeping everything as a BigNumber. The parseUnits will allow you to create a string with a decimal point.