TypeScript in Minutes: Types, Functions, Classes and Interfaces

mkdir ts-basics
cd ts-basics
npm init -y
npm install --save-dev typescript
npx tsc --init

let studentName: string = 'Mireille';
let age: number = 19;
let paid: boolean = false;

// TypeScript can infer the type too:
let city = 'Douala'; // string

type Student = {
  id: string;
  name: string;
  score?: number;
};

const students: Student[] = [
  { id: 's1', name: 'Mireille', score: 16 },
  { id: 's2', name: 'Joel' },
];

function percentage(score: number, total: number): number {
  if (total <= 0) throw new Error('total must be positive');
  return (score / total) * 100;
}

console.log(percentage(15, 20));

type Exam = 'GCE' | 'HND' | 'BTS';

type Learner = {
  name: string;
  exam: Exam;
};

const learner: Learner = { name: 'Amina', exam: 'HND' };

function formatId(id: string | number) {
  if (typeof id === 'number') {
    return id.toFixed(0);
  }
  return id.toUpperCase();
}

interface Product {
  id: string;
  name: string;
  price: number;
}

function label(product: Product) {
  return `${product.name}: ${product.price} FCFA`;
}

function first<T>(items: T[]): T | undefined {
  return items[0];
}

const firstScore = first([12, 14, 18]);      // number | undefined
const firstName = first(['A', 'B', 'C']);    // string | undefined

class Wallet {
  balance = 0;

  deposit(amount: number) {
    if (amount <= 0) throw new Error('amount must be positive');
    this.balance += amount;
  }
}

const wallet = new Wallet();
wallet.deposit(500);

npx tsc
node dist/index.js

type TaskStatus = 'todo' | 'done';

type StudyTask = {
  id: string;
  title: string;
  subject: string;
  status: TaskStatus;
};

const tasks: StudyTask[] = [];

function addTask(title: string, subject: string): StudyTask {
  const task: StudyTask = {
    id: crypto.randomUUID(),
    title,
    subject,
    status: 'todo',
  };
  tasks.push(task);
  return task;
}

function completeTask(id: string) {
  const task = tasks.find((item) => item.id === id);
  if (task) task.status = 'done';
}