JavaScript is natively poor at handling big numbers correctly. Therefore, applications that require you to deal with big numbers and need perfect calculations use the BigNumber.js library to work with big numbers.

web3.js also depends on BigNumber.js. It adds it automatically. web3.js always returns the BigNumber object for number values. It can take JavaScript numbers, number strings, and BigNumber instances as input.

Here is an example to demonstrate this:

web3.eth.getBalance("0x27E829fB34d14f3384646F938165dfcD30cFfB7c").toString(); 

Here, we use the web3.eth.getBalance() method to get the balance of an address. This method returns a BigNumber object. We need to call toString() on a BigNumber object to convert it into a number string.

BigNumber.js fails to correctly handle numbers with more than 20 floating point digits; therefore, it is recommended that you store the balance in a wei unit and while displaying, convert it to other units. web3.js itself always returns and takes the balance in wei. For example, the getBalance() method returns the balance of the address in the wei unit.