🌟 assert

Purpose:

• Used to check invariants — conditions that should never ever be false under any circumstances.
• If assert fails → there is a bug in your code.
• Consumes all remaining gas and reverts the transaction.
• Should only be used for critical internal checks.

Example:

uint public balance;

function deposit(uint _amount) public {
    balance += _amount;

    // critical invariant check
    assert(balance >= _amount); // balance cannot possibly go below amount after adding
}

👉 If assert fails, something is horribly wrong — logic bug, overflow (pre-Solidity 0.8), etc.

Use assert sparingly, for impossible-to-fail conditions.

🌟 require

Purpose:

• Used to validate:
  • function inputs
  • conditions
  • permissions
• If require fails → transaction is reverted
• Consumes only the remaining gas stipend (gas is refunded for unused)
• Optionally shows a custom error message

Example:

function deposit(uint _amount) public {
    require(_amount > 0, "Amount must be positive");
    require(balance + _amount >= balance, "Overflow"); // precondition check

    balance += _amount;
}