Why testing prevents costly bugs in Blockchain / Solidity - Challenge Your Understanding

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a smart contract function without testing

Consider this simple smart contract function that transfers tokens. What will be the output when calling transfer(50) if the sender has only 30 tokens?

Blockchain / Solidity

contract Token {
    mapping(address => uint) balances;

    function transfer(uint amount) public returns (string memory) {
        if (balances[msg.sender] >= amount) {
            balances[msg.sender] -= amount;
            return "Transfer successful";
        } else {
            return "Insufficient balance";
        }
    }
}

A "Insufficient balance"

B "Transfer failed due to gas limit"

C Transaction reverts with error

D "Transfer successful"

💡 Hint

Think about what happens if the sender does not have enough tokens.

The function checks if the sender's balance is enough. Since the sender has only 30 tokens but tries to transfer 50, the condition fails and the function returns "Insufficient balance".

🧠 Conceptual

intermediate

Why is testing important in blockchain development?

Which of the following best explains why testing smart contracts before deployment is crucial?

A Because bugs in smart contracts can cause irreversible financial loss

B Because testing increases the speed of blockchain transactions

C Because smart contracts can be easily updated after deployment

D Because testing reduces the size of the blockchain

💡 Hint

Think about the nature of blockchain transactions and immutability.

Smart contracts are immutable once deployed, so bugs cannot be fixed easily. Bugs can cause permanent loss of funds or unintended behavior, making testing essential.

🔧 Debug

advanced

Identify the bug causing incorrect token balance update

This smart contract function is supposed to transfer tokens from sender to receiver. What is the bug causing incorrect balances after transfer?

Blockchain / Solidity

function transfer(address to, uint amount) public {
    require(balances[msg.sender] >= amount, "Not enough tokens");
    balances[to] += amount;
    balances[msg.sender] -= amount;
}

A The order of updating balances is wrong; sender balance should be reduced before receiver is increased

B The function lacks a return statement indicating success or failure

C The function does not check for overflow when increasing receiver balance

D The require statement should check if receiver has enough tokens

💡 Hint

Consider what happens if the receiver's balance is very large before adding amount.

Without checking for overflow, adding amount to receiver's balance can wrap around causing incorrect balances. This is a common bug in smart contracts.

📝 Syntax

advanced

Syntax error in smart contract function

Which option contains the correct syntax for a Solidity function that returns the balance of an address?

A function getBalance(address user) public returns (uint) { return balances[user]; }

B function getBalance(address user) public view returns (uint) { return balances[user]; }

C function getBalance(address user) public pure returns (uint) { return balances[user]; }

D function getBalance(address user) returns uint { return balances[user]; }

💡 Hint

Think about the function type that reads state without modifying it.

The function reads the contract state without modifying it, so it must be marked as 'view'. Option B is correct syntax. Option B misses 'view', C uses 'pure' which disallows state access, D misses visibility and parentheses.

🚀 Application

expert

Effect of missing tests on deployed smart contract

A developer deployed a smart contract without thorough testing. After deployment, a bug caused incorrect token transfers. What is the most likely consequence?

A The developer can easily patch the contract and fix the bug immediately

B The bug will only affect test networks and not the main blockchain

C The blockchain network will automatically detect and correct the bug

D Users may lose tokens permanently and the bug cannot be fixed without redeploying

💡 Hint

Remember that smart contracts are immutable once deployed on mainnet.

Smart contracts on blockchain are immutable. Bugs cannot be fixed by patching the contract. Users may lose tokens permanently, and fixing requires redeploying a new contract and migrating users.