Reading contract state in Blockchain / Solidity - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to read the contract's stored value.

Blockchain / Solidity

const value = await contract.[1]();

Drag options to blanks, or click blank then click option'

AstoreValue

BreadValue

CfetchValue

DgetValue

Attempts:

3 left

💡 Hint

Common Mistakes

Using a function that writes data instead of reading.

Calling a function that does not exist in the contract.

The function getValue is the correct contract method to read the stored value.

2fill in blank

medium

Complete the code to connect to the contract using ethers.js.

Blockchain / Solidity

const contract = new ethers.Contract(contractAddress, abi, [1]);

Drag options to blanks, or click blank then click option'

Aprovider

Bsigner

Cwallet

Daccount

Attempts:

3 left

💡 Hint

Common Mistakes

Using a signer when only reading data.

Passing an undefined variable.

To read contract state, a provider is used to connect without signing transactions.

3fill in blank

hard

Fix the error in the code to properly await the contract call.

Blockchain / Solidity

const balance = await contract.[1]();

Drag options to blanks, or click blank then click option'

AgetBalance

Bawait getBalance

Cawait balance

Dbalance

Attempts:

3 left

💡 Hint

Common Mistakes

Including 'await' inside the method name.

Not using 'await' at all causing a Promise instead of value.

The await keyword should be used outside the function call, not inside the method name.

4fill in blank

hard

Fill both blanks to create a dictionary of token balances greater than zero.

Blockchain / Solidity

const balances = { [1]: [2] for (const token of tokens) if (await contract.balanceOf(token) > 0) };

Drag options to blanks, or click blank then click option'

Atoken

Bawait contract.balanceOf(token)

Ctoken.balance

Dbalance

Attempts:

3 left

💡 Hint

Common Mistakes

Using a property that does not exist on token.

Not awaiting the balanceOf call.

Use token as the key and the awaited balance as the value in the dictionary comprehension.

5fill in blank

hard

Fill all three blanks to filter and map contract events for transfers above 100 tokens.

Blockchain / Solidity

const largeTransfers = events.filter(e => e.args.[1] > [2]).map(e => e.args.[3]);

Drag options to blanks, or click blank then click option'

Avalue

B100

Cto

Dfrom

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'from' instead of 'to' in the map.

Comparing with a string instead of a number.

Filter events where value is greater than 100, then map to the to address.

Practice

1. What is the main purpose of using .call() when interacting with a blockchain smart contract?

easy

A. To send tokens to another address

B. To read data from the contract without changing its state

C. To deploy a new smart contract

D. To mine a new block on the blockchain

Solution

1. Step 1: Understand what .call() does

   .call() is used to read data from a smart contract without creating a transaction or changing the blockchain state.

2. Step 2: Compare with other blockchain actions

   Sending tokens or deploying contracts changes state and requires transactions, unlike .call().

3. Final Answer:

   To read data from the contract without changing its state -> Option B

4. Quick Check:

   .call() reads state without transactions [OK]

Hint: Use .call() only to read data, not to write [OK]

Common Mistakes:

• Thinking .call() sends transactions
• Confusing .call() with contract deployment
• Assuming .call() changes contract state

2. Which of the following is the correct syntax to read a contract's public variable balance using .call() in JavaScript?

easy

A. const bal = contract.methods.balance().call();

B. const bal = contract.call.methods.balance();

C. const bal = contract.methods.balance();

D. const bal = contract.balance.call();

Solution

1. Step 1: Recall the correct method call pattern

   To read a contract variable, use contract.methods.variableName().call() in JavaScript.

2. Step 2: Check each option's syntax

   const bal = contract.methods.balance().call(); matches the correct pattern. Options B, C, and D have incorrect method chaining or missing .call().

3. Final Answer:

   const bal = contract.methods.balance().call(); -> Option A

4. Quick Check:

   Correct syntax = const bal = contract.methods.balance().call(); [OK]

Hint: Remember: contract.methods.<name>().call() reads state [OK]

Common Mistakes:

• Placing .call() before .methods
• Omitting .call() when reading
• Using contract.balance.call() directly

3. Given the following Solidity contract snippet:

contract Wallet {
    uint public balance = 100;
    function getBalance() public view returns (uint) {
        return balance;
    }
}

What will be the output of this JavaScript code?

const bal = await contract.methods.getBalance().call();
console.log(bal);

medium

A. 100

B. undefined

C. Error: getBalance is not a function

D. 0

Solution

1. Step 1: Understand the Solidity function

   The getBalance() function returns the current balance value, which is 100.

2. Step 2: Analyze the JavaScript call

   The JavaScript code calls getBalance() using .call(), which reads the value without changing state, returning 100.

3. Final Answer:

   100 -> Option A

4. Quick Check:

   Calling view function returns stored value [OK]

Hint: View functions return stored values via .call() [OK]

Common Mistakes:

• Expecting a transaction receipt instead of value
• Confusing .call() with sending a transaction
• Assuming default value is zero

4. You try to read a contract's state variable using contract.methods.value.call (without parentheses). What error will you most likely encounter?

medium

A. TypeError: contract.methods.value.call is not a function, missing parentheses

B. SyntaxError: Unexpected token

C. No error, returns the value directly

D. TypeError: contract.methods.value.call is not a function

Solution

1. Step 1: Identify the missing parentheses issue

   .call is a function and must be invoked with parentheses ().

2. Step 2: Understand the error message

   Without parentheses, JavaScript treats .call as a property, causing a TypeError indicating it's not a function call.

3. Final Answer:

   TypeError: contract.methods.value.call is not a function -> Option D

4. Quick Check:

   Missing () on .call() causes TypeError [OK]

Hint: Always add () after .call to execute it [OK]

Common Mistakes:

• Forgetting parentheses on .call()
• Assuming .call is a property, not a function
• Ignoring JavaScript function call syntax

5. You want to read multiple state variables owner (address) and totalSupply (uint) from a deployed contract efficiently. Which approach is best?

hard

A. Call contract.methods.owner().call() and contract.methods.totalSupply().call() separately

B. Use contract.methods.owner.call and contract.methods.totalSupply.call without parentheses

C. Create a new contract function that returns both variables in a tuple and call it once

D. Read owner with .call() and read totalSupply from the blockchain directly

Solution

1. Step 1: Understand multiple calls cost

   Calling each variable separately sends multiple requests, which is less efficient.

2. Step 2: Use a combined function

   Creating a contract function that returns both variables together reduces calls and improves efficiency.

3. Final Answer:

   Create a new contract function that returns both variables in a tuple and call it once -> Option C

4. Quick Check:

   Batch reading state reduces calls and improves performance [OK]

Hint: Batch reads in one call for efficiency [OK]

Common Mistakes:

• Making multiple separate calls unnecessarily
• Trying to read blockchain data outside contract calls
• Forgetting parentheses on .call()