ALX Backend — JavaScript

Description

Backend JavaScript track: ES6 Promises, ES6 Classes, ES6 Data manipulation, Node.js basics, and Unit testing (Mocha/Chai/Sinon). This README explains what each folder and file does and how to run the exercises so you can follow or redo them yourself.

Structure

• 0x01. ES6 Promise
• 0x02. ES6 Classes
• 0x03. ES6 Data manipulation
• 0x05. Node.js basic
• 0x06. Unittests in JS

Folder	Topic	What you'll practice
0x01-ES6_promise	Promises & async	Promise, then/catch, async/await, try/catch
0x02-ES6_classes	ES6 classes	class, constructor, getters/setters, inheritance
0x03-ES6_data_manipulation	Arrays & structures	map, filter, reduce, Set, Map, TypedArrays
0x05-Node_JS_basic	Node.js	Console, stdin, readFile (sync/async), HTTP server, Express
0x06-unittests_in_js	Testing	Mocha, Chai, Sinon (spies, stubs), async tests, skip

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Prerequisites

• Node.js 12.11.x (or compatible LTS)
• npm for installing dependencies

Per-project setup (typical):

cd <project_folder>
npm install

Many tasks use Babel for ES6+ and Jest or Mocha for tests; see each folder’s package.json and README.

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0x01 — ES6 Promises

Problem: Async operations (APIs, I/O) need a consistent way to handle success and failure and to chain or combine results. Promises (and later async/await) solve that. This project practices creating and consuming Promises.

Files and what they solve

File	Problem solved	How to run
0-promise.js	Return a Promise from a function	npm run dev 0-main.js
1-promise.js	Resolve or reject based on boolean (getFullResponseFromAPI)	npm run dev 1-main.js
2-then.js	Append .then/.catch to a promise; log "Got a response from the API"	npm run dev 2-main.js
3-all.js	handleProfileSignup: Promise.all(uploadPhoto, createUser); log body, firstName, lastName	npm run dev 3-main.js
4-user-promise.js	signUpUser(firstName, lastName) → resolved promise with { firstName, lastName }	npm run dev 4-main.js
5-photo-reject.js	uploadPhoto(fileName) → reject with Error	npm run dev 5-main.js
6-final-user.js	handleProfileSignup: Promise.allSettled(signUpUser, uploadPhoto); return status/value array	npm run dev 6-main.js
7-load_balancer.js	Return the value of the promise that resolves first (chinaDownload, USDownload)	npm run dev 7-main.js
8-try.js	divideFunction(numerator, denominator); throw if denominator === 0	npm run dev 8-main.js
9-try.js	guardrail(mathFunction): run function, push result or error message to queue, push "Guardrail was processed"	npm run dev 9-main.js
utils.js	uploadPhoto / createUser (return resolved promises) — provided for tasks	Used by 3-all, etc.

Doing the exercises yourself

1. 0: return new Promise((resolve) => resolve(...)); or return Promise.resolve(...).
2. 1: Inside Promise executor: if success then resolve with { status: 200, body: 'Success' }, else reject with new Error('...').
3. 2–3: Use .then() and .catch(); for 3, use Promise.all([uploadPhoto(), createUser()]) and log the required fields.
4. 4–6: Return promises (resolve/reject); for 6 use Promise.allSettled and map to { status, value } or { status, reason }.
5. 7: Promise.race([chinaDownload, USDownload]).
6. 8–9: Synchronous throw and try/catch; 9 builds an array with result or error message and "Guardrail was processed".

Run from project dir: npm run dev <file>.js.

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0x02 — ES6 Classes

Problem: Model real-world entities (classrooms, courses, pricing, buildings, airports, cars) with classes, getters/setters, and inheritance. This project practices ES6 class syntax and OOP in JavaScript.

Files and what they solve

File	Problem solved	How to run
0-classroom.js	ClassRoom class with maxStudentsSize	Import and instantiate; use in 1-make_classrooms
1-make_classrooms.js	Create array of ClassRoom instances (sizes 19, 20, 34)	npm run dev 1-main.js or test
2-hbtn_course.js	HolbertonCourse (name, length, students) with getters/setters	Jest/test
3-currency.js	Currency class (code, name)	Used by 4-pricing
4-pricing.js	Pricing (amount, currency); displayFullPrice method	Jest/test
5-building.js	Building (sqft); EvacuationWarningMessage abstract	Used by 6
6-sky_high.js	SkyHighBuilding extends Building (floors); evacuation message	Jest/test
7-airport.js	Airport (name, code); default string description	Jest/test
8-hbtn_class.js	HolbertonClass (size, location)	Used by 9
9-hoisting.js	Hoisting demo (function vs class)	Educational
10-car.js	Car (brand, motor, color); cloneCar method	Jest/test
100-evcar.js	EVCar extends Car (range); override description	Jest/test

Doing the exercises yourself

1. Define classes with class Name { constructor(...) { ... } }.
2. Use getters/setters where required: get name() { return this._name; }.
3. Use extends and super() for inheritance (e.g. SkyHighBuilding, EVCar).
4. Run tests: npm test in the project folder (if Jest is configured).

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0x03 — ES6 Data manipulation

Problem: Process lists of students, grades, and other data using map, filter, reduce, and structures like Set and Map. This project practices functional array methods and typed/weak structures.

Files and what they solve

File	Problem solved	How to run
0-get_list_students.js	Return array of student objects (id, firstName, location)	Used by other tasks
1-get_list_student_ids.js	Map list of students to array of ids	npm run dev 1-main.js or test
2-get_students_by_loc.js	Filter students by location	npm run dev 2-main.js or test
3-get_ids_sum.js	Sum of student ids (reduce)	npm run dev 3-main.js or test
4-update_grade_by_city.js	Update grades by city (map + find)	npm run dev 4-main.js or test
5-typed_arrays.js	Create TypedArray from buffer (Int8, etc.)	npm run dev 5-main.js or test
6-set.js	setFromArray(array) → Set	npm run dev 6-main.js or test
7-has_array_values.js	hasValuesFromArray(set, array) → boolean	npm run dev 7-main.js or test
8-clean_set.js	Clean Set: return string of values starting with given string (joined by '-')	npm run dev 8-main.js or test
9-groceries_list.js	Map: grocery name → quantity (from array of arrays)	npm run dev 9-main.js or test
10-update_uniq_items.js	Update map: set value 100 for keys that appear exactly once	npm run dev 10-main.js or test
100-weak.js	WeakMap / WeakSet usage (query by endpoint)	npm run dev 100-main.js or test

Doing the exercises yourself

1. Use array.map, array.filter, array.reduce for list transformations.
2. Use new Set(array) and Set methods; build Map from entries.
3. For TypedArrays: use ArrayBuffer and view constructors (Int8Array, etc.).
4. Run: npm test or npm run dev <n>-main.js.

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0x05 — Node.js basic

Problem: Use Node.js for console output, reading input, reading files (sync and async), and building HTTP servers (plain and with Express). This project builds small Node scripts and servers.

Files and what they solve

File	Problem solved	How to run
0-console.js	displayMessage(string) → console.log(string)	node 0-main.js or require and call
1-stdin.js	Read from stdin; display welcome message with name	node 1-stdin.js (type name and press Enter)
2-read_file.js	Read file synchronously; count students by field; throw if missing	node 2-main_0.js database.csv
3-read_file_async.js	Same as 2 but async (Promise-based)	node 3-main_0.js database.csv
4-http.js	HTTP server: GET / → "Hello Holberton School!"	node 4-http.js then curl localhost:port
5-http.js	HTTP server: GET / → HTML page with "Hello Holberton School!"	node 5-http.js
6-http_express.js	Express app: GET / → "Hello Holberton School!"	node 6-http_express.js
7-http_express.js	Express app: GET / → same; read path and respond	node 7-http_express.js
database.csv	Sample CSV for read_file tasks	Pass as first argument to 2-main / 3-main

Doing the exercises yourself

1. 0: Export a function that calls console.log.
2. 1: Use process.stdin (or readline) to read name and print welcome message.
3. 2–3: Use fs.readFileSync or fs.promises.readFile; parse CSV and group by field; throw Error if file missing.
4. 4–5: Use http.createServer and respond with plain text or HTML.
5. 6–7: Use Express: app.get('/', ...) and optionally other routes.

Run: node <script>.js [args] (e.g. node 2-read_file.js database.csv).

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0x06 — Unittests in JS

Problem: Ensure functions and APIs behave correctly using tests. This project uses Mocha, Chai, and Sinon to write unit tests (assert, expect, spies, stubs) and simple integration tests.

Files and what they solve

File	Problem solved	How to run
0-calcul.js	calculateNumber(a, b) — round a and b, return sum	Implementation
0-calcul.test.js	Test calculateNumber with assert	npm test 0-calcul.test.js
1-calcul.js	calculateNumber(type, a, b) — SUM, SUBTRACT, DIVIDE (type)	Implementation
1-calcul.test.js	Test SUM, SUBTRACT, DIVIDE and edge cases (describe)	npm test 1-calcul.test.js
2-calcul_chai.js	Same as 1-calcul	Copy of 1-calcul
2-calcul_chai.test.js	Same tests using Chai expect	npm test 2-calcul_chai.test.js
utils.js	Utils.calculateNumber (same logic) — for payment tests	Used by 3–5
3-payment.js	sendPaymentRequestToApi(totalAmount, totalShipping); calls Utils.calculateNumber('SUM', ...), logs total	Implementation
3-payment.test.js	Spy on Utils.calculateNumber; verify called with SUM, 100, 20	npm test 3-payment.test.js
4-payment.js	Same as 3	Copy of 3
4-payment.test.js	Stub Utils.calculateNumber to return 10; spy on console.log; verify "The total is: 10"	npm test 4-payment.test.js
5-payment.js	Same as 4	Copy of 4
5-payment.test.js	Two tests; use beforeEach/afterEach to spy console.log; verify message and call count	npm test 5-payment.test.js
6-payment_token.js	getPaymentTokenFromAPI(success) → Promise with { data: '...' } if true	Implementation
6-payment_token.test.js	Async test with done(); test getPaymentTokenFromAPI(true)	npm test 6-payment_token.test.js
7-skip.test.js	Skip the failing test (it('1 is equal to 3')) with it.skip so suite passes	npm test 7-skip.test.js

Concepts

• Assert (Node): assert.strictEqual(actual, expected).
• Chai expect: expect(value).to.equal(...), expect(...).to.be.true.
• Sinon spy: sinon.spy(Utils, 'calculateNumber'); ... ; Utils.calculateNumber.restore();
• Sinon stub: sinon.stub(Utils, 'calculateNumber').returns(10);
• Async: Use done callback or return a Promise so Mocha waits.

Doing the exercises yourself

1. Implement calcul functions; write tests for normal and edge cases (e.g. DIVIDE by 0 → 'Error').
2. For payment: implement sendPaymentRequestToApi; in tests, spy or stub Utils.calculateNumber and optionally console.log.
3. For 6: implement getPaymentTokenFromAPI; in test, call with true and in .then (or async/await) assert on result and call done().
4. For 7: change the failing test to it.skip('1 is equal to 3', ...) so it is skipped.

Run: npm test or npm test <file>.test.js.

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Quick reference: run by project

# Promises (Babel + Node)
cd 0x01-ES6_promise && npm install && npm run dev 0-main.js

# Classes (Jest)
cd 0x02-ES6_classes && npm install && npm test

# Data manipulation (Jest)
cd 0x03-ES6_data_manipulation && npm install && npm test

# Node basic
cd 0x05-Node_JS_basic && node 0-main.js
cd 0x05-Node_JS_basic && node 2-main_0.js database.csv

# Unittests (Mocha)
cd 0x06-unittests_in_js && npm install && npm test 0-calcul.test.js

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Summary

• 0x01-ES6_promise: Create and consume Promises; then/catch, all/race/allSettled; throw/try/catch.
• 0x02-ES6_classes: ES6 classes, getters/setters, extends, super.
• 0x03-ES6_data_manipulation: map, filter, reduce, Set, Map, TypedArrays.
• 0x05-Node_JS_basic: Console, stdin, fs (sync/async), http, Express.
• 0x06-unittests_in_js: Mocha, Chai, Sinon (spy, stub), async tests, skip.

Each subfolder has its own README.md with detailed task descriptions; this file is the high-level map of what each file is for and how to run and redo the exercises.