Writing test cases in Node.js - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main purpose of writing test cases in Node.js?

Test cases help check if your code works correctly by running small checks automatically. They catch errors early and make sure changes don’t break existing features.

beginner

What does a typical test case include?

A test case usually has three parts: setup (prepare data or state), execution (run the code), and assertion (check if the result matches what you expect).

beginner

In Node.js testing, what is an assertion?

An assertion is a statement that checks if a value or result is what you expect. If it’s not, the test fails and tells you there’s a problem.

intermediate

Why use a testing framework like Mocha or Jest in Node.js?

Testing frameworks give you tools to write, organize, and run tests easily. They handle running tests, showing results clearly, and help write assertions.

intermediate

What is a 'mock' in testing?

A mock is a fake version of a function or module used in tests to simulate behavior without running real code. It helps test parts of code in isolation.

Which part of a test case checks if the output is correct?

A Assertion

B Setup

C Execution

D Cleanup

The assertion checks if the output matches the expected result.

What is the benefit of using a testing framework like Jest?

A It helps write and run tests easily

B It makes your code run faster

C It automatically fixes bugs

D It replaces the need for writing code

Testing frameworks provide tools to write, organize, and run tests efficiently.

What does a 'mock' do in a test?

A Generates random data

B Deletes test files automatically

C Runs the entire application

D Simulates a part of code to isolate testing

Mocks simulate parts of code so you can test components without running real dependencies.

Which of these is NOT a typical part of a test case?

A Setup

B Assertion

C Compilation

D Execution

Compilation is not part of writing test cases; it relates to building code.

Why write test cases before fixing bugs?

A To make the code slower

B To confirm the bug exists and prevent future errors

C To avoid writing code

D To delete old files

Writing tests first helps confirm bugs and ensures fixes don’t break other parts.

Explain the three main parts of a test case and why each is important.

Think about preparing data, running code, and checking results.

Describe how using mocks can help when writing test cases in Node.js.

Imagine replacing a real part with a fake one to test only what you want.

Practice

1. What is the main purpose of writing test cases in Node.js projects?

easy

A. To change the code style automatically

B. To make the code run faster

C. To add more comments to the code

D. To check if the code works as expected

Solution

1. Step 1: Understand the role of test cases

   Test cases are written to verify that the code behaves correctly for different inputs and scenarios.

2. Step 2: Identify the main goal

   The main goal is to check if the code works as expected, not to improve speed or style.

3. Final Answer:

   To check if the code works as expected -> Option D

4. Quick Check:

   Test cases = Verify code correctness [OK]

Hint: Test cases verify code correctness, not speed or style [OK]

Common Mistakes:

• Thinking test cases improve code speed
• Confusing test cases with code comments
• Believing test cases change code style

2. Which of the following is the correct syntax to define a test case using Mocha in Node.js?

easy

A. describe('add function', () => { it('adds numbers', () => { /* test code */ }) })

B. function testAdd() { /* test code */ }

C. test('should add numbers', () => { /* test code */ })

D. runTest('add', () => { /* test code */ })

Solution

1. Step 1: Recall Mocha test structure

   Mocha uses describe to group tests and it to define individual test cases.

2. Step 2: Match syntax with options

   describe('add function', () => { it('adds numbers', () => { /* test code */ }) }) correctly uses describe and it with arrow functions.

3. Final Answer:

   describe('add function', () => { it('adds numbers', () => { /* test code */ }) }) -> Option A

4. Quick Check:

   Mocha uses describe and it [OK]

Hint: Use describe and it blocks for Mocha tests [OK]

Common Mistakes:

• Using undefined functions like test() or runTest()
• Not grouping tests inside describe()
• Writing test code outside it() blocks

3. Given the following test case using Jest, what will be the output when running the test?

describe('sum function', () => {
  it('adds 2 and 3', () => {
    expect(2 + 3).toBe(5);
  });
  it('adds 2 and 2', () => {
    expect(2 + 2).toBe(5);
  });
});

medium

A. First test passes, second test fails

B. Both tests pass

C. Both tests fail

D. Syntax error, tests do not run

Solution

1. Step 1: Analyze each test expectation

   The first test expects 2 + 3 to be 5, which is true. The second test expects 2 + 2 to be 5, which is false.

2. Step 2: Determine test results

   Therefore, the first test passes, and the second test fails.

3. Final Answer:

   First test passes, second test fails -> Option A

4. Quick Check:

   Correct expectations = test pass/fail [OK]

Hint: Check each expect() value carefully for pass or fail [OK]

Common Mistakes:

• Assuming all tests pass without checking values
• Confusing toBe() with toEqual()
• Ignoring failing test results

4. Identify the error in this Mocha test code snippet:

describe('multiply function', () => {
  it('multiplies 2 and 3', () => {
    const result = 2 * 3
    expect(result).to.equal(6);
  }
});

medium

A. Incorrect assertion method name

B. Missing closing parenthesis for it() function

C. Using arrow function instead of regular function

D. No error, code is correct

Solution

1. Step 1: Check parentheses and braces

   The it function is missing a closing parenthesis after the arrow function block.

2. Step 2: Verify assertion method and function type

   The assertion to.equal is valid in Chai, and arrow functions are allowed in Mocha tests.

3. Final Answer:

   Missing closing parenthesis for it() function -> Option B

4. Quick Check:

   Check matching parentheses in test functions [OK]

Hint: Count parentheses carefully in test blocks [OK]

Common Mistakes:

• Forgetting to close it() parentheses
• Confusing assertion method names
• Thinking arrow functions are disallowed

5. You want to write a test case that checks if a function filterPositive correctly returns only positive numbers from an array. Which test case correctly applies this in Jest?

hard

A. it('filters positive numbers', () => { expect(filterPositive([-1,0,2,3])).toBeTruthy(); });

B. it('filters positive numbers', () => { expect(filterPositive([-1,0,2,3])).toBe([2,3]); });

C. it('filters positive numbers', () => { expect(filterPositive([-1,0,2,3])).toEqual([2,3]); });

D. it('filters positive numbers', () => { expect(filterPositive([-1,0,2,3])).toContain(2,3); });

Solution

1. Step 1: Understand Jest matchers for arrays

   toEqual compares array contents deeply, while toBe checks reference equality, which fails for arrays.

2. Step 2: Evaluate each option

   it('filters positive numbers', () => { expect(filterPositive([-1,0,2,3])).toEqual([2,3]); }); uses toEqual correctly to check array contents. it('filters positive numbers', () => { expect(filterPositive([-1,0,2,3])).toBe([2,3]); }); uses toBe which fails for arrays. it('filters positive numbers', () => { expect(filterPositive([-1,0,2,3])).toContain(2,3); }); uses toContain incorrectly with multiple values. it('filters positive numbers', () => { expect(filterPositive([-1,0,2,3])).toBeTruthy(); }); only checks truthiness, not exact output.

3. Final Answer:

   it('filters positive numbers', () => { expect(filterPositive([-1,0,2,3])).toEqual([2,3]); }); -> Option C

4. Quick Check:

   Use toEqual for array content comparison [OK]

Hint: Use toEqual, not toBe, for comparing arrays in Jest [OK]

Common Mistakes:

• Using toBe for arrays instead of toEqual
• Using toContain with multiple values incorrectly
• Checking only truthiness instead of exact output