Flash loans in Blockchain / Solidity - Mini Project: Build & Apply

Flash Loans Basics

📖 Scenario: You are learning how flash loans work in blockchain smart contracts. Flash loans let you borrow tokens without collateral, but you must pay back the loan in the same transaction.We will create a simple smart contract that requests a flash loan, uses it, and repays it immediately.

🎯 Goal: Build a basic smart contract that requests a flash loan from a lending pool, performs a simple action, and repays the loan in the same transaction.

📋 What You'll Learn

Create a smart contract with a function to request a flash loan

Set up a variable to store the flash loan amount

Implement the callback function that executes after receiving the loan

Print/log the success message after repaying the loan

💡 Why This Matters

🌍 Real World

Flash loans are used in decentralized finance (DeFi) to perform arbitrage, refinancing, or collateral swaps without upfront capital.

💼 Career

Understanding flash loans is important for blockchain developers working on DeFi protocols, smart contract security, and financial applications.

1

Create the flash loan contract and amount variable

Create a smart contract named FlashLoanExample and declare a public variable loanAmount of type uint256 set to 1000 ether.

Blockchain / Solidity

contract FlashLoanExample {
    // Your code here
}

Hint

Use contract FlashLoanExample {} to create the contract and declare uint256 public loanAmount = 1000 ether; inside it.

2

Add the flash loan request function

Inside the FlashLoanExample contract, add a public function requestFlashLoan() that will be used to start the flash loan process. For now, just add the function signature and an empty body.

Blockchain / Solidity

pragma solidity ^0.8.0;

contract FlashLoanExample {
    uint256 public loanAmount = 1000 ether;

    // Add the requestFlashLoan function below
    function requestFlashLoan() public {
        // Your code here
    }
}

Hint

Define function requestFlashLoan() public {} inside the contract.

3

Implement the flash loan callback function

Add a function executeOperation() inside FlashLoanExample that takes parameters uint256 amount, uint256 fee, and returns bool. Inside it, write code to simulate using the loan and then returning true.

Blockchain / Solidity

pragma solidity ^0.8.0;

contract FlashLoanExample {
    uint256 public loanAmount = 1000 ether;

    function requestFlashLoan() public {
        // Flash loan request logic will go here
    }

    // Add executeOperation function below
    function executeOperation(uint256 amount, uint256 fee) public returns (bool) {
        // Your code here
    }
}

Hint

Define executeOperation with the correct parameters and return true inside.

4

Print success message after loan repayment

Add a public function printSuccess() that prints the message "Flash loan repaid successfully!" using emit and an event named Success. Declare the event Success(string message) at the top of the contract. Then call printSuccess() at the end of executeOperation().

Blockchain / Solidity

pragma solidity ^0.8.0;

contract FlashLoanExample {
    uint256 public loanAmount = 1000 ether;

    // Declare Success event here

    function requestFlashLoan() public {
        // Flash loan request logic will go here
    }

    function executeOperation(uint256 amount, uint256 fee) public returns (bool) {
        // Simulate using the loan

        // Call printSuccess here

        // Return true to indicate success
        return true;
    }

    // Add printSuccess function here
}

Hint

Declare event Success(string message); at the top. Use emit Success("Flash loan repaid successfully!"); inside printSuccess(). Call printSuccess() inside executeOperation().

Practice

1. What is the main feature of a flash loan in blockchain?

easy

A. You can borrow funds without collateral but must repay within the same transaction

B. You borrow funds with collateral and repay anytime

C. You borrow funds and repay after 30 days

D. You borrow funds only for staking purposes

Solution

1. Step 1: Understand flash loan basics

   Flash loans allow borrowing without collateral but require repayment in the same transaction.

2. Step 2: Compare options

   Only You can borrow funds without collateral but must repay within the same transaction correctly states no collateral and instant repayment.

3. Final Answer:

   You can borrow funds without collateral but must repay within the same transaction -> Option A

4. Quick Check:

   Flash loan = no collateral + instant repayment [OK]

Hint: Flash loans = borrow now, repay instantly [OK]

Common Mistakes:

• Thinking collateral is required
• Assuming repayment can be delayed
• Confusing flash loans with regular loans

2. Which of the following is the correct Solidity function signature to implement a flash loan callback?

easy

A. function repayLoan(uint256 amount) external

B. function flashLoan(address borrower, uint256 amount) public

C. function startLoan(address asset, uint256 amount) external returns (bool)

D. function executeOperation(address[] calldata assets, uint256[] calldata amounts, uint256[] calldata premiums, address initiator, bytes calldata params) external returns (bool)

Solution

1. Step 1: Identify the standard flash loan callback

   The Aave protocol requires implementing executeOperation with specific parameters.

2. Step 2: Match function signature

   function executeOperation(address[] calldata assets, uint256[] calldata amounts, uint256[] calldata premiums, address initiator, bytes calldata params) external returns (bool) matches the exact signature needed for flash loan execution and repayment.

3. Final Answer:

   function executeOperation(address[] calldata assets, uint256[] calldata amounts, uint256[] calldata premiums, address initiator, bytes calldata params) external returns (bool) -> Option D

4. Quick Check:

   executeOperation signature = function executeOperation(address[] calldata assets, uint256[] calldata amounts, uint256[] calldata premiums, address initiator, bytes calldata params) external returns (bool) [OK]

Hint: Flash loan callback is always executeOperation with specific params [OK]

Common Mistakes:

• Using incorrect function names
• Missing required parameters
• Wrong return type

3. Given this simplified Solidity snippet inside executeOperation:

uint256 amountOwing = amounts[0] + premiums[0];
IERC20(assets[0]).approve(address(LENDING_POOL), amountOwing);
return true;

What does this code do?

medium

A. Transfers the loan amount to the borrower

B. Approves the lending pool to withdraw the loan plus fee for repayment

C. Withdraws the loan amount from the lending pool

D. Rejects the flash loan request

Solution

1. Step 1: Understand the approval call

   The code approves the lending pool contract to spend the loan amount plus premium from this contract.

2. Step 2: Interpret the purpose

   This approval is necessary so the lending pool can pull repayment automatically after the operation.

3. Final Answer:

   Approves the lending pool to withdraw the loan plus fee for repayment -> Option B

4. Quick Check:

   approve() = allow repayment withdrawal [OK]

Hint: approve() lets lending pool pull repayment [OK]

Common Mistakes:

• Confusing approve with transfer
• Thinking it sends funds to borrower
• Missing the premium fee in amount

4. Identify the error in this simplified flash loan executeOperation snippet:

function executeOperation(address[] calldata assets, uint256[] calldata amounts, uint256[] calldata premiums, address initiator, bytes calldata params) external returns (bool) {
    uint256 amountOwing = amounts[0] + premiums[0];
    IERC20(assets[0]).transferFrom(msg.sender, address(this), amountOwing);
    return true;
}

medium

A. Incorrect function parameters

B. Missing return statement

C. Using transferFrom instead of approve for repayment

D. Not calling the lending pool to borrow funds

Solution

1. Step 1: Analyze repayment method

   The code tries to pull repayment using transferFrom from msg.sender, which is incorrect.

2. Step 2: Correct repayment approach

   Flash loans require approving the lending pool to pull funds, not transferring from msg.sender.

3. Final Answer:

   Using transferFrom instead of approve for repayment -> Option C

4. Quick Check:

   Repayment needs approve(), not transferFrom() [OK]

Hint: Repay by approve(), not transferFrom() [OK]

Common Mistakes:

• Confusing transferFrom with approve
• Forgetting to approve lending pool
• Misunderstanding msg.sender role

5. You want to use a flash loan to perform arbitrage between two decentralized exchanges (DEXs). Which sequence correctly describes the steps inside executeOperation to profit and repay the loan?

hard

A. Borrow funds -> Buy low on DEX1 -> Sell high on DEX2 -> Approve repayment -> Return true

B. Borrow funds -> Approve repayment -> Buy low on DEX1 -> Sell high on DEX2 -> Return true

C. Approve repayment -> Borrow funds -> Buy low on DEX1 -> Sell high on DEX2 -> Return true

D. Buy low on DEX1 -> Borrow funds -> Sell high on DEX2 -> Approve repayment -> Return true

Solution

1. Step 1: Understand flash loan flow

   You first borrow funds, then use them to buy low on one DEX and sell high on another to gain profit.

2. Step 2: Approve repayment and finish

   After trading, approve the lending pool to pull the loan plus fee, then return true to complete.

3. Final Answer:

   Borrow funds -> Buy low on DEX1 -> Sell high on DEX2 -> Approve repayment -> Return true -> Option A

4. Quick Check:

   Arbitrage flow = borrow -> trade -> approve -> return [OK]

Hint: Trade first, then approve repayment [OK]

Common Mistakes:

• Approving repayment before trading
• Trying to trade before borrowing
• Not approving repayment at all