Mocking methods in an existing lua file during Busted tests

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07-10-2024

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Mocking Methods in Lua: Power Up Your Busted Tests

Testing your Lua code effectively often requires isolating and controlling dependencies. Mocking methods – replacing actual code with controlled behavior – becomes crucial for this. This article guides you through mocking methods in Lua using the popular Busted testing framework.

The Problem:

Let's say you have a function calculateDiscount that relies on a third-party library function getCurrencyRate. During testing, you want to control the returned currency rate without actually calling the external library. This is where mocking comes in.

Scenario:

Imagine a Lua file shopping_cart.lua with the following code:

local shopping_cart = {}

local external_lib = require("external_library")

function shopping_cart.calculateDiscount(price, percentage)
  local currency_rate = external_lib.getCurrencyRate()
  local discount = price * percentage * currency_rate / 100
  return discount
end

return shopping_cart

We want to test shopping_cart.calculateDiscount without depending on external_lib.getCurrencyRate. This is where Busted's mocking capabilities shine.

Mocking with Busted:

Busted provides a powerful mocking mechanism through the mock module. Here's how you would implement the test using mocking:

local busted = require('busted')
local shopping_cart = require('shopping_cart')

-- Define your mock function
local mock_currency_rate = 1.1

local function mockGetCurrencyRate()
  return mock_currency_rate
end

-- Set up your mock
local mock = require('busted.mock')
mock.setup(function()
  mock('external_library', 'getCurrencyRate', mockGetCurrencyRate)
end)

-- Your test
busted.describe("Shopping Cart", function()
  busted.it("should calculate discount correctly with mocked currency rate", function()
    local expected_discount = 11
    local actual_discount = shopping_cart.calculateDiscount(100, 10)
    assert.are.equal(actual_discount, expected_discount)
  end)
end)

-- Cleanup mock after test
mock.teardown()

Explanation:

1. Setup: We define a mockGetCurrencyRate function to provide a controlled value for the currency rate. Using mock.setup, we tell Busted to replace the original getCurrencyRate in the external_library module with our mock function.

2. Testing: In our test, we can now confidently call shopping_cart.calculateDiscount knowing that it will use the mock currency rate, allowing us to test specific scenarios.

3. Teardown: Using mock.teardown, we clean up the mocks, ensuring that the original functionality is restored after the test.

Key Benefits:

• Controlled Environment: Mocks isolate the function under test from external dependencies, leading to more reliable and predictable results.
• Simplified Testing: Mocking allows you to test various scenarios without relying on external systems or potentially complex dependencies.
• Testability: Mocking makes it easier to test functions that have dependencies on external libraries or services.

Important Considerations:

• Cleanup: Always ensure to properly teardown your mocks to avoid unexpected behavior in subsequent tests.
• Maintainability: Keep your mocks organized and well-documented to enhance maintainability.
• Stubbing vs. Mocking: Understand the difference between stubbing (providing a predefined value) and mocking (controlling the behavior of a method).

Conclusion:

Mastering mocking with Busted is a crucial skill for developing robust and maintainable Lua code. By isolating your dependencies, you can create comprehensive and focused tests, ensuring your application behaves as expected.