Reading contract state in Blockchain / Solidity - Step-by-Step Execution

Concept Flow - Reading contract state

Start

↓

Connect to blockchain node

↓

Identify contract address

↓

Call contract's read function

↓

Receive state data

↓

Use or display data

↓

End

This flow shows how a program connects to a blockchain, calls a contract's read-only function, and gets the current state data.

Execution Sample

Blockchain / Solidity

const contract = new ethers.Contract(address, abi, provider);
const value = await contract.getValue();
console.log(value);

This code connects to a contract and reads a stored value without changing the blockchain.

Execution Table

Step	Action	Evaluation	Result
1	Create contract object with address, ABI, and provider	contract = new ethers.Contract(...)	Contract instance ready
2	Call read function getValue()	await contract.getValue()	Promise pending
3	Blockchain node processes call	Read-only call executed	Returns stored value
4	Receive value from contract	value resolved	value = 42
5	Print value to console	console.log(value)	Output: 42

💡 Read function completes and returns current contract state value

Variable Tracker

Variable	Start	After Step 2	After Step 4	Final
contract	undefined	Contract instance	Contract instance	Contract instance
value	undefined	Promise pending	42	42

Key Moments - 3 Insights

Why does calling getValue() not change the blockchain?

Because getValue() is a read-only function (view or pure), it only reads state without sending a transaction, as shown in execution_table step 3.

What is the role of the provider in creating the contract object?

The provider connects to the blockchain node to read data; without it, the contract call cannot fetch state (see execution_table step 1).

Why is the value initially a Promise before resolving?

Because contract calls are asynchronous, the call returns a Promise that resolves when the blockchain responds (execution_table step 2 and 4).

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'value' after step 2?

A Promise pending

B 42

C undefined

D Contract instance

💡 Hint

Check the 'value' variable in variable_tracker after step 2

At which step does the contract call return the actual stored value?

A Step 1

B Step 4

C Step 3

D Step 5

💡 Hint

Look at execution_table rows where 'value resolved' happens

If the provider was missing when creating the contract, what would happen?

A The contract call would still return the value

B The value would be undefined after step 4

C The contract instance would not connect to blockchain

D The console.log would print 'undefined'

💡 Hint

Refer to key_moments about the provider's role and execution_table step 1

Concept Snapshot

Reading contract state:
- Connect to blockchain with provider
- Create contract object with address and ABI
- Call read-only function (view/pure)
- Await result (async call)
- Use returned data without changing blockchain

Full Transcript

Reading contract state means connecting to a blockchain node and calling a contract's read-only function to get current data. The program creates a contract object using the contract's address, ABI, and a provider that connects to the blockchain. When calling the read function, it returns a Promise because the call is asynchronous. The blockchain node processes the call and returns the stored value without changing the blockchain. Finally, the program receives the value and can display or use it. This process does not send a transaction or cost gas because it only reads data. The provider is essential to connect and fetch data from the blockchain.

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()