Event testing in Blockchain / Solidity - Mini Project: Build & Apply

Event Testing in Blockchain Smart Contracts

📖 Scenario: You are building a simple blockchain smart contract that tracks product sales. Each time a product is sold, the contract emits an event with the product name and price. You want to test that this event is emitted correctly when a sale happens.

🎯 Goal: Create a smart contract that emits a ProductSold event when a product is sold. Then write a test script to check that the event is emitted with the correct data.

📋 What You'll Learn

Create a Solidity smart contract with a ProductSold event

Add a sellProduct function that emits the event with product name and price

Write a JavaScript test using Hardhat or similar framework to call sellProduct

Verify the ProductSold event is emitted with the expected product name and price

💡 Why This Matters

🌍 Real World

Events in blockchain smart contracts are like notifications that something important happened. They help users and other programs know when actions occur, such as a product sale or token transfer.

💼 Career

Blockchain developers must write and test events to ensure smart contracts communicate correctly with users and other systems. Event testing is a key skill for building reliable decentralized applications.

1

Create the Solidity contract with the ProductSold event

Create a Solidity contract called Store. Inside it, declare an event called ProductSold that takes a string productName and a uint price. Also add a public function called sellProduct that takes string memory productName and uint price as parameters but leave the function body empty for now.

Blockchain / Solidity

pragma solidity ^0.8.0;

contract Store {
    // Declare the ProductSold event here

    // Define the sellProduct function here
    
    // Your code here
}

Hint

Remember to use event keyword to declare events in Solidity.

2

Emit the ProductSold event inside sellProduct

Inside the sellProduct function, emit the ProductSold event with the productName and price parameters.

Blockchain / Solidity

pragma solidity ^0.8.0;

contract Store {
    event ProductSold(string productName, uint price);

    function sellProduct(string memory productName, uint price) public {
        // Emit the ProductSold event here
        // Your code here
    }
}

Hint

Use the emit keyword followed by the event name and parameters.

3

Write a JavaScript test to call sellProduct and check the event

Create a JavaScript test file. Import the contract factory and deploy the Store contract. Call the sellProduct function with "Laptop" and 1500. Use expect with to.emit to check that the ProductSold event is emitted with "Laptop" and 1500.

Blockchain / Solidity

// Import necessary libraries
const { expect } = require("chai");

// Your test code here

Hint

Use Hardhat's expect and to.emit syntax to test events.

4

Run the test and print success message

Add a console.log statement after the test call to print "Event test passed!" if the event is emitted correctly.

Blockchain / Solidity

// Import necessary libraries
const { expect } = require("chai");
const { ethers } = require("hardhat");

describe("Store contract", function () {
  it("should emit ProductSold event with correct values", async function () {
    const Store = await ethers.getContractFactory("Store");
    const store = await Store.deploy();
    await store.deployed();

    await expect(store.sellProduct("Laptop", 1500))
      .to.emit(store, "ProductSold")
      .withArgs("Laptop", 1500);

    // Print success message here
    // Your code here
  });
});

Hint

Use console.log("Event test passed!") to print the message.

Practice

1.

What is the main purpose of event testing in blockchain smart contracts?

easy

A. To verify that the contract emits the correct events during execution

B. To check the contract's gas usage

C. To test the contract's user interface

D. To optimize the contract's storage

Solution

1. Step 1: Understand event testing purpose

   Event testing focuses on confirming that smart contracts emit the right signals (events) when certain actions happen.

2. Step 2: Compare options with event testing goal

   Only To verify that the contract emits the correct events during execution matches this purpose, as it talks about verifying emitted events.

3. Final Answer:

   To verify that the contract emits the correct events during execution -> Option A

4. Quick Check:

   Event testing = Verify emitted events [OK]

Hint: Event testing checks emitted signals, not gas or UI [OK]

Common Mistakes:

• Confusing event testing with gas optimization
• Thinking event testing checks UI elements
• Assuming event testing measures storage use

2.

Which of the following is the correct syntax to listen for an event named Transfer in a Solidity test using JavaScript?

contractInstance.Transfer({fromBlock: 0, toBlock: 'latest'}).???

easy

A. on

B. watch

C. emit

D. listen

Solution

1. Step 1: Recall event listening syntax in Solidity tests

   In older Solidity test frameworks, watch is used to listen for events on contract instances.

2. Step 2: Match options with correct method

   Only watch fits the syntax shown; on, emit, and listen are incorrect here.

3. Final Answer:

   watch -> Option B

4. Quick Check:

   Event listening method = watch [OK]

Hint: Use 'watch' to listen for events in Solidity JS tests [OK]

Common Mistakes:

• Using 'emit' which triggers events, not listens
• Confusing 'on' with event listening in other frameworks
• Using 'listen' which is not a Solidity test method

3.

Given this Solidity event and test snippet, what will be the output of console.log(events.length) after calling transfer() once?

event Transfer(address indexed from, address indexed to, uint amount);

// In test:
const events = await contractInstance.getPastEvents('Transfer', {fromBlock: 0, toBlock: 'latest'});
await contractInstance.transfer(receiver, 100);
console.log(events.length);

medium

A. undefined

B. 1

C. 0

D. 100

Solution

1. Step 1: Understand event fetching timing

   The getPastEvents call happens before transfer(), so it fetches events before the transfer.

2. Step 2: Analyze event count after transfer

   Since events are fetched before the transfer, events.length will be 0 because no Transfer event happened yet.

3. Final Answer:

   0 -> Option C

4. Quick Check:

   Events fetched before transfer = 0 [OK]

Hint: Fetch events after action to see new events [OK]

Common Mistakes:

• Assuming events array updates automatically
• Ignoring async order of calls
• Expecting event count before event occurs

4.

Identify the error in this test code snippet that listens for an event Approval:

contractInstance.Approval().watch((error, event) => {
  if (!error) {
    console.log('Approval event:', event);
  }
});

medium

A. Using watch instead of on

B. Using Approval() instead of Approval without parentheses

C. Incorrect callback function signature

D. Missing event filter parameters in the call

Solution

1. Step 1: Check event listening syntax

   The event constructor requires a filter object parameter (e.g., {fromBlock: 0}) to specify the event scope.

2. Step 2: Identify correct usage

   The correct syntax is contractInstance.Approval({fromBlock: 0}).watch(...); the code uses empty Approval().

3. Final Answer:

   Missing event filter parameters in the call -> Option D

4. Quick Check:

   Event filter params required [OK]

Hint: Include event filter parameters like {fromBlock: 0} [OK]

Common Mistakes:

• Omitting filter parameters in event call
• Confusing callback parameters
• Mixing event listening methods

5.

You want to test that a Deposit event is emitted only when the deposit amount is greater than zero. Which test approach correctly verifies this?

1. Call deposit(0) and check no event emitted
2. Call deposit(100) and check event emitted with amount 100
3. Call deposit(-50) and check event emitted
4. Call deposit(100) and check event emitted with amount 0

hard

A. Only steps 1 and 2 are correct test cases

B. All steps are correct to test event emission

C. Only step 3 is needed to test event emission

D. Only step 4 is correct to test event emission

Solution

1. Step 1: Understand event emission condition

   The event should emit only if deposit amount > 0, so deposit(0) should emit no event, deposit(100) should emit event with amount 100.

2. Step 2: Evaluate test steps

   Step 3 calls deposit(-50), which is invalid (negative deposit), so event emission is not expected. Step 4 expects event with amount 0, which contradicts the condition.

3. Final Answer:

   Only steps 1 and 2 are correct test cases -> Option A

4. Quick Check:

   Test event only for positive deposits [OK]

Hint: Test zero and positive amounts only for event emission [OK]

Common Mistakes:

• Testing negative amounts which are invalid
• Expecting event with zero amount
• Assuming all deposits emit events