Unit testing logic in Svelte - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the testing function from Svelte Testing Library.

Svelte

import { [1] } from '@testing-library/svelte';

Drag options to blanks, or click blank then click option'

Acheck

Btest

Cmount

Drender

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'test' instead of 'render' to display the component.

Trying to use 'mount' which is not from Svelte Testing Library.

The render function is used to render Svelte components in tests.

2fill in blank

medium

Complete the code to check if the component shows the text 'Hello World'.

Svelte

expect(getByText([1])).toBeInTheDocument();

Drag options to blanks, or click blank then click option'

A'World'

B'Hello'

C'Hello World'

D'hello world'

Attempts:

3 left

💡 Hint

Common Mistakes

Using partial text like 'Hello' which may match multiple elements.

Using lowercase text which does not match the exact case.

The exact text 'Hello World' must be matched to find the element.

3fill in blank

hard

Fix the error in the test by completing the assertion to check the button is enabled.

Svelte

expect(button).[1].toBeDisabled();

Drag options to blanks, or click blank then click option'

AtoBe

Bnot

CtoHaveAttribute

DtoBeTruthy

Attempts:

3 left

💡 Hint

Common Mistakes

Using toBe which expects a value, not a matcher.

Using toHaveAttribute without specifying attribute.

The not modifier with toBeDisabled() checks that the button is enabled (not disabled).

4fill in blank

hard

Fill both blanks to create a test that checks if clicking the button calls the mock function.

Svelte

const mockFn = vi.fn();

const { getByRole } = render(Component, { props: { onClick: [1] } });

const button = getByRole('button');

button.[2]();

expect(mockFn).toHaveBeenCalled();

Drag options to blanks, or click blank then click option'

AmockFn

Bclick

CfireEvent

DhandleClick

Attempts:

3 left

💡 Hint

Common Mistakes

Passing a wrong function name as the click handler.

Using fireEvent directly on the button instead of click().

The mock function mockFn is passed as the click handler, and click() simulates the button click.

5fill in blank

hard

Fill all three blanks to write a test that renders a component, updates a prop, and checks the updated text.

Svelte

const { getByText, component } = render(Component, { props: { message: [1] } });

expect(getByText([2])).toBeInTheDocument();

component.$set({ message: [3] });

expect(getByText('Updated')).toBeInTheDocument();

Drag options to blanks, or click blank then click option'

A'Initial'

C'Updated'

D'Changed'

Attempts:

3 left

💡 Hint

Common Mistakes

Using mismatched initial prop and text.

Not updating the prop correctly with $set.

The component starts with message 'Initial', which is checked, then updated to 'Updated' and checked again.

Practice

1. What is the main purpose of unit testing in Svelte applications?

easy

A. To write HTML templates

B. To style components with CSS

C. To check small parts of code for errors early

D. To deploy the app to production

Solution

1. Step 1: Understand unit testing purpose

   Unit testing focuses on testing small, isolated parts of code to find bugs early.

2. Step 2: Compare options to unit testing goal

   Only To check small parts of code for errors early describes checking small code parts for errors early, which matches unit testing.

3. Final Answer:

   To check small parts of code for errors early -> Option C

4. Quick Check:

   Unit testing = check small code parts early [OK]

Hint: Unit tests check small code parts, not styling or deployment [OK]

Common Mistakes:

• Confusing unit testing with styling or deployment
• Thinking unit tests write HTML templates
• Assuming unit tests run only in production

2. Which of the following is the correct syntax to define a test case in Svelte using Vitest or Jest?

easy

A. test('test case') { /* test code */ }

B. describe 'test group' { it 'test case' { /* test code */ } }

C. it 'test case' => { /* test code */ }

D. describe('test group', () => { it('test case', () => { /* test code */ }) })

Solution

1. Step 1: Recall correct test syntax

   In Vitest/Jest, tests use describe() and it() functions with arrow functions.

2. Step 2: Check each option's syntax

   describe('test group', () => { it('test case', () => { /* test code */ }) }) uses correct parentheses and arrow functions. Others miss parentheses or use wrong syntax.

3. Final Answer:

   describe('test group', () => { it('test case', () => { /* test code */ }) }) -> Option D

4. Quick Check:

   describe and it with arrow functions = correct syntax [OK]

Hint: Use parentheses and arrow functions with describe and it [OK]

Common Mistakes:

• Omitting parentheses around function arguments
• Using wrong arrow function syntax
• Confusing test and it keywords

3. Given this test code snippet, what will be the test result?

describe('sum function', () => {
  it('adds two numbers', () => {
    const sum = (a, b) => a + b;
    expect(sum(2, 3)).toBe(5);
  });
});

medium

A. The test will pass because 2 + 3 equals 5

B. The test will fail because sum is not defined

C. The test will fail because toBe is not a function

D. The test will throw a syntax error

Solution

1. Step 1: Analyze the sum function and test

   The sum function adds two numbers correctly. The test checks if sum(2, 3) equals 5.

2. Step 2: Evaluate the expect statement

   expect(sum(2, 3)).toBe(5) compares 5 to 5, which is true, so the test passes.

3. Final Answer:

   The test will pass because 2 + 3 equals 5 -> Option A

4. Quick Check:

   sum(2,3) = 5, expect toBe(5) passes [OK]

Hint: Check function output matches expected value exactly [OK]

Common Mistakes:

• Assuming sum is undefined
• Confusing toBe with other matchers
• Thinking syntax error occurs here

4. Identify the error in this Svelte unit test code:

describe('multiply function', () => {
  it('multiplies two numbers', () => {
    const multiply = (a, b) => a * b
    expect(multiply(2, 3)).toBe(6)
  }
});

medium

A. Missing closing parenthesis for the it block

B. multiply function is not defined

C. toBe matcher is used incorrectly

D. describe block is missing a callback function

Solution

1. Step 1: Check syntax of it block

   The it block starts with a parenthesis but lacks a closing parenthesis before the closing brace.

2. Step 2: Verify other parts

   multiply is defined correctly, toBe is used properly, and describe has a callback function.

3. Final Answer:

   Missing closing parenthesis for the it block -> Option A

4. Quick Check:

   Syntax error due to missing parenthesis [OK]

Hint: Count opening and closing parentheses carefully [OK]

Common Mistakes:

• Ignoring missing parentheses errors
• Thinking multiply is undefined
• Assuming toBe usage is wrong

5. You want to write a unit test in Svelte that checks if a function returns the correct value only when the input is positive. Which test logic correctly implements this condition?

function checkPositive(n) {
  return n > 0 ? 'positive' : 'not positive';
}

hard

A. expect(checkPositive(0)).toBe('positive'); expect(checkPositive(5)).toBe('not positive');

B. expect(checkPositive(5)).toBe('positive'); expect(checkPositive(-1)).toBe('not positive');

C. expect(checkPositive(-5)).toBe('positive'); expect(checkPositive(5)).toBe('positive');

D. expect(checkPositive(5)).toBe('not positive'); expect(checkPositive(-1)).toBe('positive');

Solution

1. Step 1: Understand function behavior

   checkPositive returns 'positive' if input > 0, else 'not positive'.

2. Step 2: Match test expectations to function logic

   expect(checkPositive(5)).toBe('positive'); expect(checkPositive(-1)).toBe('not positive'); tests positive input 5 returns 'positive' and negative input -1 returns 'not positive', matching function logic.

3. Final Answer:

   expect(checkPositive(5)).toBe('positive'); expect(checkPositive(-1)).toBe('not positive'); -> Option B

4. Quick Check:

   Test matches function output for positive and negative inputs [OK]

Hint: Test both positive and negative inputs to cover conditions [OK]

Common Mistakes:

• Testing zero as positive incorrectly
• Swapping expected results for inputs
• Testing only one input case