What is the input parameter required for a Web3 callback function?

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The input parameter required for a Web3 callback function is usually an error object and a result object. The error object will contain any error message or code if the request fails, while the result object will contain the result of the successful request.

For example, in the following code snippet, the callback function takes two parameters, "err" and "result":

web3.eth.getBalance(address, function(err, result) {
  if (err) {
    console.log(err);
  } else {
    console.log(result);
  }
});

If the getBalance request fails, the callback function will log the error message. If the request succeeds, the callback function will log the balance of the specified address.