Reading contract state in Blockchain / Solidity - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does 'reading contract state' mean in blockchain?

It means checking the current data stored inside a smart contract without changing it. It's like looking at a saved note without erasing or writing anything new.

beginner

Which method is commonly used to read state from a smart contract?

A 'call' method is used to read state. It asks the contract for information without making a transaction or spending gas.

intermediate

Why is reading contract state usually free on blockchains like Ethereum?

Because reading does not change anything on the blockchain, it doesn't require miners to update the chain, so no gas fees are needed.

intermediate

What is the difference between a 'call' and a 'transaction' when interacting with a contract?

'Call' reads data without changing state and costs no gas. 'Transaction' changes state and requires gas fees.

beginner

How can you read a public variable from a smart contract using web3.js?

You use the contract's method for that variable with .call(), like: contract.methods.variableName().call()

What does reading contract state NOT do?

A Retrieve current data from the contract

B Change the data stored in the contract

C Use a call method

D Cost no gas

Reading contract state only retrieves data; it does not change anything.

Which of these is used to read data from a smart contract?

A deploy

B sendTransaction

C mine

D call

The 'call' method reads data without changing the blockchain.

Why does reading contract state usually cost no gas?

A Because it does not change blockchain state

B Because it uses a special token

C Because it is done by miners

D Because it requires a transaction

No gas is needed since reading does not update the blockchain.

In web3.js, how do you read a public variable named 'balance'?

A contract.methods.balance().call()

B contract.methods.balance().send()

C contract.balance()

D contract.call.balance()

The correct syntax uses methods.balance() with .call() to read.

Which statement is true about 'call' and 'transaction'?

A Neither 'call' nor 'transaction' change data

B 'call' changes data; 'transaction' reads data

C 'call' reads data; 'transaction' changes data

D Both 'call' and 'transaction' change data

'call' is for reading; 'transaction' is for writing.

Explain in simple terms how you would read the state of a smart contract without paying gas.

Think about asking a question without changing the answer.

Describe the difference between reading contract state and sending a transaction to a contract.

One is like looking, the other is like writing.

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()