4. Writing Unit Tests

As discussed earlier, unit tests are important to help maintain code quality and reduce bugs. But how do we write them?

Getting Started

Install Jest in your node project with npm install --save-dev jest
Create your test file (either in your __tests__ folder or with file extension: .test.js or .spec.js)
Run your tests with npm jest

Example 1: Palindromess

Write a function what checks if the provided string is the same forwards as it is backwards

Ignore spacing and punctuation

Finished Function

export function isPalindrome(str) {
    const cleanedStr = str.toLowerCase().replace(/[\W_]/g, '');
    return cleanedStr === cleanedStr.split('').reverse().join('');
}

Finished Tests

const isPalindrome = require('./isPalindrome');

test('returns true for a simple palindrome', () => {
    expect(isPalindrome('racecar')).toBe(true);
});

test('returns false for a non-palindrome', () => {
    expect(isPalindrome('hello')).toBe(false);
});

test('ignores spaces and punctuation', () => {
    expect(isPalindrome('A man, a plan, a canal, Panama')).toBe(true);
});

Example 2: Factorials

Write a function that calculates the factorial of a number.

Ensure a factorial of 0 properly returns 1
Factorials of negative numbers should return `undefined`

Finished Function

export function factorial(n) {
    if (n < 0) {
        return undefined;
    }
    if (n === 0) {
        return 1;
    }
    return n * factorial(n - 1);
}

Finished Tests

const factorial = require('./factorial');

test('calculates factorial of a positive number', () => {
    expect(factorial(5)).toBe(120);
});

test('returns 1 for factorial of 0', () => {
    expect(factorial(0)).toBe(1);
});

test('returns -1 for negative numbers', () => {
    expect(factorial(-3)).toBe(-1);
});

Example 3: FizzBuzz

Write a function that to compute the proper FizzBuzz word.

If a number is divisible by 3, return "Fizz"
If a number is divisible by 5, return "Buzz"
If a number is divisible by 3 and 5, return "FizzBuzz"
Otherwise, return the provided number

Finished Function

export function fizzBuzz(number) {
    if (numnber % 3 == 0 && number % 5 == 0) {
        return 'FizzBuzz';
    }
    if (numnber % 3 == 0) {
        return 'Fizz';
    }
    if (numnber % 5 == 0) {
        return 'Buzz';
    }
    
    return number.toString();
}

Finished Tests

const fizzBuzz = require('./fizzbuzz');

test('returns Fizz for multiples of 3', () => {
    expect(fizzBuzz(9)).toBe('Fizz');
});

test('returns Buzz for multiples of 5', () => {
    expect(fizzBuzz(10)).toBe('Buzz');
});

test('returns FizzBuzz for multiples of 3 and 5', () => {
    expect(fizzBuzz(15)).toBe('FizzBuzz');
});

test('returns the number if not divisible by 3 or 5', () => {
    expect(fizzBuzz(7)).toBe('7');
});

Example 4: Greeting Generator

Write a function that gets a greeting for a user based on the time of day.

If it's currently before 12 PM, return "Good Morning"
If it's between 12 and 6PM return "Good Afternoon"
Otherwise, return "Good Evening"

Finished Function

function generateGreeting() {
    const currentHour = new Date().getHours();

    if (currentHour < 12) {
        return 'Good morning!';
    }
    else if (currentHour < 18) {
        return 'Good afternoon!';
    }
    else {
        return 'Good evening!';
    }
}

Testable Function

// Function to get the current hour
function getCurrentHour() {
    return new Date().getHours();
}

// Function to generate the greeting based on the hour
function getGreetingMessage(hour) {
    if (hour < 12) {
        return 'Good morning!';
    }
    else if (hour < 18) {
        return 'Good afternoon!';
    }
    else {
        return 'Good evening!';
    }
}

Finished Tests

const { getGreetingMessage } = require('./greeting');

// Test greeting message based on different times of day
test('returns "Good morning!" for hours less than 12', () => {
    expect(getGreetingMessage(9)).toBe('Good morning!');
});

test('returns "Good afternoon!" for hours between 12 and 17', () => {
    expect(getGreetingMessage(15)).toBe('Good afternoon!');
});

test('returns "Good evening!" for hours greater than or equal to 18', () => {
    expect(getGreetingMessage(20)).toBe('Good evening!');
});