how to convert "0.5" number knowing decimals to a valid bignumber

Question

I am trying to create a transaction where the users sends "0.5" USDT token to a smart contract. My issue is that i can't convert "0.5" to a BigNumber in order to pass it in the transaction using web3 contract methods as BigNumber.js doesn't accept decimals. for example this method will throw an error saying that bigNumber doesn't support decimals.

Contract.method(web3.utils.toBN("0.5")).send()

I can use the toWei method of web3 but it doesn't work if the decimals are not equal to 18. How can i do this ? is there a workaround ?

Answer 1

That is usually how I calculate half of a token. but I don't guarantee that it works correctly :) so please check the code and make sure it works and is correct and use it only on your own responsibility. Web3js (I'm using "web3": "^1.6.0") use this https://github.com/indutny/bn.js/ for BigNumbers

function getHalfCoin() {
    const decimal = 6; // you can get it from the contract;
    var BN = web3.utils.BN;
    const decimalAsBigNumber = new BN(decimal);
    console.log(decimalAsBigNumber); // BN { negative: 0, words: [ 6 ], length: 1, red: null }

    const numberOneAsBigNumber = new BN(1);
    const numberTowAsBigNumber = new BN(2);
    const numberTenAsBigNumber = new BN(10);

    const oneCoin = numberOneAsBigNumber.mul(numberTenAsBigNumber.pow(decimalAsBigNumber));
    console.log(oneCoin); // BN { negative: 0, words: [ 1000000, <1 empty item> ], length: 1, red: null}
    console.log(oneCoin.toString()); // 1000000

    const halfCoin = oneCoin.div(numberTowAsBigNumber);
    console.log(halfCoin); // BN { negative: 0, words: [ 500000 ], length: 1, red: null }
    console.log(halfCoin.toString()); // 500000
}

https://bybit-exchange.github.io/erc20-faucet/

here mint 1 token on rinkeby

and here transfer 0.5 on rinkeby