🧠 Chapter 4 — Understanding TypeScript Types

You're now writing and running TypeScript — great job!
It's time to understand what makes TypeScript so powerful: its type system.

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🤔 What are types?

A type tells the computer what kind of value something is:

• Is it a number? a string? an object?
• Can it ever be undefined?
• Should it behave a certain way?

TypeScript helps you catch errors before your code runs, just by checking the types.

📘 Learn more:

• TypeScript Handbook – Basic Types

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🔤 Basic types

Type	Example
string	"hello"
number	42, 3.14
boolean	true, false
any	Can be anything (⚠️ avoid)
unknown	Safe "I don't know yet"
null / undefined	Empty values

Example:

let age: number = 30;
let name: string = "Alice";
let isStudent: boolean = true;

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🧮 Arrays and objects

Arrays

let numbers: number[] = [1, 2, 3];
let names: string[] = ["Alice", "Bob"];

📘 Array types in TS Handbook

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Objects (Interfaces)

interface User {
  name: string;
  age: number;
  isAdmin: boolean;
}

const user: User = {
  name: "Bob",
  age: 42,
  isAdmin: false,
};

📘 Interfaces vs types

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🧰 Functions with types

function greet(name: string): string {
  return `Hello, ${name}!`;
}

greet("World"); // ✅
greet(42);      // ❌ Type error

📘 Functions in TypeScript

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🧩 Special types: unknown and Partial

🔸 unknown: when you don’t know the type yet

let value: unknown;

value = 42;
value = "hello";
value = { name: "Bob" };

// ❌ You can't use it directly
// console.log(value.toUpperCase()); // Error

// ✅ You must check first
if (typeof value === "string") {
  console.log(value.toUpperCase());
}

Use unknown instead of any when you want safety + flexibility.

📘 Unknown vs any

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🔸 Partial<T>: make all properties optional

Sometimes you only want to update part of an object. Example:

interface User {
  name: string;
  age: number;
}

function updateUser(user: User, changes: Partial<User>) {
  return { ...user, ...changes };
}

const u = updateUser({ name: "Alice", age: 30 }, { age: 31 });

Without Partial<User>, you’d be forced to provide both name and age.

📘 Partial in utility types

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🧪 Try this

function add(a: number, b: number): number {
  return a + b;
}

const result = add(2, 3);
console.log(result);

add("hello", 4); // ❌ TypeScript will warn you here

Try running it with tsx or deno.

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🧠 Summary

Concept	Example
: type	let x: number = 10
[] arrays	let names: string[]
interface	Define an object’s structure
unknown	Value must be checked before use
Partial<T>	Makes all object props optional

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📚 Want to go deeper?

• Everyday Types
• Functions and unknown
• Utility types like Partial
• Playground to test types

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✅ What’s next?

Now that you know how to use types, let’s organize code in a clean way: with folders, files, and modules.

👉 Go to Chapter 5: Structuring a TypeScript Project