Ethers JS BigNumber Errors

Question

I'm trying to pass the following number to the approve() function of a standard ERC20 Token contract:

2^256-1

I read that this should be done using BigNumber, using the following code:

const maxInt = new BN("2").pow(new BN("256").sub(new BN("1"))); 
// Will pass into solidity as uint 2**256 - 1

Well I've gotten ALL SORTS of errors using that code - for starters it looks like the syntax and commands for BigNumber are quite different depending on the version of ethers you happen to be using - and I happen to be on V. 4.0 - and would very much prefer to stay on it right now to avoid other potential headaches.

So I've tried all sorts of variations on the above code - here's one where I broke down each component of that formula into it's own individual piece - just to see what's up:

// ==>Using 'bigNumberify'

let bigNum2 = ethers.utils.bigNumberify("2");
console.log("'bigNum2'  -> '2' = ", bigNum2);

let bigNum256 = ethers.utils.bigNumberify("256");
console.log("'bigNum256'  -> '256' = ", bigNum256);

let bigNum2power256 = ethers.utils.bigNumberify(bigNum2.pow(bigNum256));
console.log("'bigNum2power256'  -> 2^256= ", bigNum2power256);

let bigNum1 = ethers.utils.bigNumberify("1");
console.log("'bigNum1'  -> '1' = ", bigNum1);

let finalNumber = ethers.utils.bigNumberify(bigNum2power256 - bigNum1);
console.log("'finalNumber' = ", finalNumber);

Well when I pass this finalNumber into approve() I get this error:

Uncaught (in promise) Error: underflow (operation="setValue", fault="underflow", value=1.157920892373162e+77, outputValue=1, version=4.0.45)

I've tried all sorts of variations on this code - like replacing this line:

let bigNum2power256 = ethers.utils.bigNumberify(bigNum2.pow(bigNum256));

with:

let bigNum2power256 = bigNum1.pow(bigNum2);

but that doesn't work either.

Every variation I tried - and there've been lots - seems to give me yet another sort of error - why is this is difficult? What's the right way to do this?

=========================

UPDATE:

To be clear, the value of 2^256-1 - which I'm storing in my bigNum2power256 variable, ultimately needs to be passed-in as the 2nd argument of the following contract function call:

tokenContractInstance.approve(contractAddress, bigNum2power256, { gasLimit: 500_000 }).then((txResult) => {
    console.log("Back from 'tokenContractInstance' 'approve'");
    console.log(">Here's the 'approve' txResult = \n", txResult);
     .....
}

Answer 1

If you are using ethers.js, there is a constants module you can use directly: here

const maxNumber = ethers.constants.MaxUint256