Error in converting ERC20 tokens with decimals to full coins without rounding errors?

Question

How to convert token to full token without having any rounding errors.

     web3.utils.toBN(6221221111111110001).div(10**18).toFixed()

TypeError: num.isZero is not a function at BN.divmod (node_modules/number-to-bn/node_modules/bn.js/lib/bn.js:2329:17) at BN.div (node_modules/number-to-bn/node_modules/bn.js/lib/bn.js:2425:17) at Promise.all.then () at at process._tickCallback (internal/process/next_tick.js:182:7)

        bal=6221221111111110001/ (10**(18))

It also has some precision errors.

Any help is appreciated.

Answer 1

First of all, regarding the specific error in your question, change this:

div(10**18)

To this:

divn(10**18)

Or to this:

div(web3.utils.toBN(10).pow(web3.utils.toBN(18)))

---

Second, since class BN supports only integer-arithmetic, the result of your computation is 6.

in order to perform non-integer computation, you will need to use class Decimal instead:

const Decimal = require('decimal.js');
const str = Decimal(6221221111111110001).div(Decimal(10).pow(18)).toFixed();

Or simply:

const str = Decimal('6221221111111110001e-18').toFixed();

Note that you will not be able to send this result to a contract function.