Writing test cases in Node.js

Introduction

Writing test cases helps check if your code works correctly. It finds mistakes early so your program runs smoothly.

When you want to make sure a function returns the right result.

Before changing code to avoid breaking existing features.

To check if your app handles errors properly.

When you want to automate checking your code instead of testing manually.

To help others understand how your code should behave.

Syntax

Node.js

import assert from 'assert';
describe('Feature or function name', () => {
  it('should do something expected', () => {
    const result = functionToTest();
    assert.strictEqual(result, expectedValue);
  });
});

describe groups related tests.

it defines a single test case with a clear goal.

Examples

This test checks if adding 2 and 3 equals 5.

Node.js

import assert from 'assert';

describe('Addition function', () => {
  it('adds two numbers correctly', () => {
    const sum = 2 + 3;
    assert.strictEqual(sum, 5);
  });
});

This test checks if the greet function returns the right message.

Node.js

import assert from 'assert';

function greet(name) {
  return `Hello, ${name}!`;
}

describe('Greet function', () => {
  it('returns greeting with name', () => {
    assert.strictEqual(greet('Alice'), 'Hello, Alice!');
  });
});

Sample Program

This test suite checks the multiply function with different inputs: positive, zero, and negative numbers.

Node.js

import assert from 'assert';

function multiply(a, b) {
  return a * b;
}

describe('multiply function', () => {
  it('multiplies positive numbers', () => {
    assert.strictEqual(multiply(3, 4), 12);
  });

  it('multiplies by zero', () => {
    assert.strictEqual(multiply(5, 0), 0);
  });

  it('multiplies negative numbers', () => {
    assert.strictEqual(multiply(-2, 3), -6);
  });
});

Important Notes

Use clear test names to understand what each test checks.

Run tests often to catch errors early.

Keep tests small and focused on one thing.

Summary

Test cases check if code works as expected.

Use describe and it to organize tests.

Write simple tests for different input cases.

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