How to avoid rounding number incorrectly when using Number or BigNumber library from JavaScript for web3.js?

Question

I have the following function to get the token balance from a web3 contract with web3.js The contract has 18 decimal point and interacting via Metamask:

 async function getTokensBalance() {
  const w3 = await new Web3(window.ethereum);
  window.contract = await new w3.eth.Contract(myContr.abi, myContr.addr);
  const balanceOf = await window.contract.methods.balanceOf(ethereum.selectedAddress).call();
  console.log(balanceOf);
 }

This returns the following string:

1399999999999999989100000000000006785

I want to now convert those to tokens number, but the rounding up creates an issue because it gives more token that it actually has.

My expected results are either this:

1399999999999999989.100000000000006785

or this worst case scenario this:

1399999999999999989

But instead I get this when trying to convert with either Number() or w3.utils.fromWei() this:

1400000000000000000 

or

1.4e+36

Which obviously throws an error since the user does not have that much tokens.

I also tried to pass directly the string form of balanceOf but get the error:

Uncaught (in promise) Error: invalid BigNumber string How do I solve this?