TypeScript Introduction

TypeScript is a free, open-source programming language developed and maintained by Microsoft. It is a superset of JavaScript — every valid JavaScript program is also a valid TypeScript program — but TypeScript adds static type checking on top.

Think of it this way: JavaScript lets you write any code and hopes for the best at runtime. TypeScript catches your mistakes before the code even runs, in your editor or at compile time.

• Created by Anders Hejlsberg (also the creator of C# and Delphi).
• First released in 2012; now the most popular language for large frontend and backend Node.js codebases.
• Compiles to plain JavaScript — browsers never see .ts files.
• Used by Angular, Vue 3, SvelteKit, Next.js, Deno, and thousands of large companies.

Why TypeScript?

Here is a bug that JavaScript will not catch:

function add(a, b) {
  return a + b;
}
add("5", 3); // "53" — string concatenation, not addition

The function silently produces the wrong result. With TypeScript:

function add(a: number, b: number): number {
  return a + b;
}
add("5", 3);
// Error: Argument of type 'string' is not assignable to parameter of type 'number'.

The compiler catches the bug immediately — no runtime crash, no silent wrong answer.

What You Will Learn

This course covers the core TypeScript features you need for real-world projects:

Setting Up

You need Node.js (v18+) installed. Then:

# Install the TypeScript compiler globally
npm install -g typescript

# Check it works
tsc --version

# Create a new project
mkdir my-app && cd my-app
tsc --init          # generates tsconfig.json

The tsconfig.json file configures the compiler. Here is a sensible starting point:

{
  "compilerOptions": {
    "target": "ES2020",
    "module": "ESNext",
    "strict": true,
    "outDir": "./dist",
    "esModuleInterop": true,
    "skipLibCheck": true
  },
  "include": ["src"]
}

Compile and Run

# Compile once
tsc

# Watch mode — recompiles on every save
tsc --watch

# Run the output
node dist/main.js

Try It Yourself

TypeScript's official Playground lets you experiment with zero setup: typescriptlang.org/play.

Type any JavaScript, then hover over variables to see their inferred types. Add a type error and watch the red squiggly appear.

Key Terminology

What You Need

• A text editor (VS Code with the TypeScript extension is ideal)
• Node.js v18 or later
• A terminal

Next: the type system — TypeScript's core superpower.