🧠 Learning Jest 🧠

• This repo was created for me to learn testing JavaScript.
• I am using a FreeCodeCamp Tutorial: https://www.freecodecamp.org/news/how-to-test-javascript-code-with-jest/

🟥 Setting Up Project and Jest 🟥

• I download Git onto to my computer so I can access the terminal
• I open Git bash in the root of this project, an executed the following:

npm init -y

• This created a package.json file

{
  "name": "learning-jest",
  "version": "1.0.0",
  "description": "* This repo was created for me to learn testing JavaScript.\r * I am using a FreeCodeCamp Tutorial: [https://www.freecodecamp.org/news/how-to-test-javascript-code-with-jest/](/https://www.freecodecamp.org/news/how-to-test-javascript-code-with-jest/)",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "keywords": [],
  "author": "",
  "license": "ISC"
}

• I runn the following command to add the Jest dependency into this project:

npm install --save-dev jest

• This creates a node_modules folder to the root of this project
• I can see Jest has been added to package.json:

{
   "name": "learning-jest",
   "version": "1.0.0",
   "description": "* This repo was created for me to learn testing JavaScript.\r * I am using a FreeCodeCamp Tutorial: [https://www.freecodecamp.org/news/how-to-test-javascript-code-with-jest/](/https://www.freecodecamp.org/news/how-to-test-javascript-code-with-jest/)",
   "main": "index.js",
   "scripts": {
      "test": "echo \"Error: no test specified\" && exit 1"
   },
   "keywords": [],
   "author": "",
   "license": "ISC",
   "devDependencies": {
      "jest": "^29.7.0"
   }
}

• I change the test key of scripts to jest:

"scripts": {
   "jest": "echo \"Error: no test specified\" && exit 1"
}

🟥 Basic Concepts in Jest 🟥

• I start with creating a simple unit test, I create sum.js and sum.test.js files.
• The sum.js file is defined as:

function sum(a,b) {
   return a + b;
}
module.exports = sum;

• The last line is to allow other JS files to access the sum() function
• In the sum.test.js file, I import the sum module using require:

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

• I create my first test by calling the test() function:

test('adds 1 and 2 to equal 3', () => {
   expect(sum(1,2)).toBe(3);
})

• I now need to run the test, so I execute npm test but I get the following error:

npm test
npm error Missing script: "test"
npm error
npm error Did you mean this?
npm error   npm run jest # run the "jest" package script
npm error
npm error To see a list of scripts, run:
npm error   npm run
npm error A complete log of this run can be found in: C:\Users\shiv_\AppData\Local\npm-cache\_logs\2024-09-22T13_52_25_097Z-debug-0.log

• I fix this by updating the package.json to:

"scripts": {
   "test": "jest"
},

• Now when I execute the test:

npm test

> [email protected] test
> jest

PASS ./sum.test.js
  √ adds 1 and 2 to equal 3 (1 ms)

Test Suites: 1 passed, 1 total
Tests:       1 passed, 1 total
Snapshots:   0 total
Time:        0.356 s
Ran all test suites.

🟥 Matchers in Jest 🟥

• .toBe() is one example of a matcher, there are various other matchers!
• .toBe() is for JavaScript PRIMITIVES (booleans, strings, numbers), not for objects like arrays:

test('two plus two is four', ()=> {
   expect(2+2).toBe([1,2,3]); // FAIL :(
})

• The above fails with the following error:

expect(received).toBe(expected) // Object.is equality

Expected: [1, 2, 3]
Received: 4

• We have the toEqual() matcher for objects:

test('objects need to use toEqual()', ()=> {
   const object = {one:1, two:2};
   expect(object).toEqual({one:1, two: 2}); // PASS
})

• We can also evaluate if a value is falsy using the toBeFalsy() matcher:

test('null is falsy', ()=> {
   const nullVar = null;
   expect(nullVar).toBeFalsy(); // PASS
})

• If we tried to use isEqual(false) or toBe(false), the test will fail⚠️⚠️⚠️
• We also have toBeTruthy():

const one = 1;
expect(one).toBeTruthy(); // PASS

• We also have a toThrow() matcher:

const n = () => {
   throw new Error()
}
expect(n).toThrow(); // PASS