TypeScript Basics

TypeScript builds on JavaScript, introducing static types and robust tooling. Here’s what you need to know to get started mastering the basics.

Types & Type Annotations

TypeScript adds types to JavaScript. You can annotate types explicitly:

let name: string = 'Alice';
let age: number = 25;
let isActive: boolean = true;

Common types:

• string, number, boolean, null, undefined, any, unknown, never.
• For arrays: string[] or Array<string>
• For objects, you define shapes with interfaces or type aliases.

Type Inference

TypeScript usually infers types, so you don’t always need to write them:

let city = 'Nairobi'; // inferred as string
let scores = [90, 85, 70]; // inferred as number[]

Variables

You can use let, const, or (rarely) var (avoid using var):

const pi: number = 3.1416;
let counter: number = 0;

Functions

Function parameters and return types can have annotations:

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

• Functions can have optional (param?: type) and default parameters.
• Use arrow functions for concise syntax:

const add = (a: number, b: number): number => a + b;

Objects & Interfaces

Interfaces or type aliases describe object shapes:

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

const user: User = {
    name: 'Jane',
    age: 22
};

Arrays

TypeScript supports both syntaxes:

const users: User[] = [user];
const numbers: Array<number> = [1, 2, 3];

Enums

Use enums to define a set of named values:

enum Direction {
    Up,
    Down,
    Left,
    Right
}

let move: Direction = Direction.Up;

Summary Table

Concept	Example
Basic Type	let x: number = 5;
Array Type	let arr: string[] = ['a', 'b']
Object Type	let obj: {a: number} = {a: 1}
Interface	interface Car {model: string}
Enum	enum Color {Red, Blue}
Function Type	function f(a: number): void {}

For more examples, see the official handbook.