How would I safely calcuate a percentage of a BigNumber?

Question

I need to multiple and divide a big number in a safe way - without overlow.

It won't have to result in an accurate amount to suit me; namely, if it gets round by some function to remain integer rather than more precise float or decimal, it'd still do for me.

Currently this code will result in overflow:

  let amountToSendPercent = provider.getBalance(wallet.address);
  //let amountToSendPercent = contract.balanceOf(wallet.address);

  //minus gas.... - doesn't matter here....


  let amountToSendPercent = 45;

  // !!!
  //sometimes - overflow
  // !!
  if (amountToSendPercent < 100) {
     amountToSend = Math.round(amountToSend / 100 * amountToSendPercent);
  }

I'm using ethers. I'd prefer to refrain from using any additional library, if possible.

---

I need this for both ERC20 tokens and ETH.

Answer 1

ethers.js is good here, I have modified your code slightly to show you how you could do it.

const ethers = require("ethers");

// ... (Set up your provider, wallet, and contract as needed)

let balance = await provider.getBalance(wallet.address); // Replace with contract.balanceOf for ERC20 tokens
let amountToSendPercent = 45;

if (amountToSendPercent < 100) {
  let amountToSend = balance.mul(ethers.BigNumber.from(amountToSendPercent)).div(100);
}