What happens when we use "callback" in a function ? and when do we need to use it?

Question

What happens when we use "callback" as a keyword in a function ? And when do we need to use it ? and what happens if we do not use it ? For example in the following JavaScript code:

function signPayment(recipient, amount, nonce, contractAddress, callback) {
  var hash = "0x" + ethereumjs.ABI.soliditySHA3(
    ["address", "uint256", "uint256", "address"],
    [recipient, amount, nonce, contractAddress]
  ).toString("hex");

  web3.personal.sign(hash, web3.eth.defaultAccount, callback);
}

Answer 1

Callback basically is a function which is passed to other function and executed there when you call this "callback". Very simple example can be found on mozilla docs.
If you don't pass any callback, depending on function code it may or may not affect your program, since callback will be null.

Answer 2

Some web3 methods are asynchronous and require a callback.

The callback function is executed once the original function is done and takes a transaction hash or similar as an argument.

You can avoid callbacks entirely by using an async/await pattern.

For instance:

async function signPayment(recipient, amount, nonce, contractAddress) {
  var hash = "0x" + ethereumjs.ABI.soliditySHA3(
    ["address", "uint256", "uint256", "address"],
    [recipient, amount, nonce, contractAddress]
  ).toString("hex");

  var signed = await web3.personal.sign(hash, web3.eth.defaultAccount);
  console.log(signed);
}