TypeScript Essentials

TypeScript Basic Types

Beginner Lesson 3 of 9

TypeScript Basic Types

TypeScript provides static typing through type annotations.

Primitive Types

// String
let firstName: string = "John";

// Number
let age: number = 25;
let price: number = 99.99;

// Boolean
let isActive: boolean = true;

// Null and Undefined
let nothing: null = null;
let notDefined: undefined = undefined;

// Symbol
let sym: symbol = Symbol("key");

// BigInt
let big: bigint = 100n;

Arrays

// Array of numbers
let numbers: number[] = [1, 2, 3];

// Generic syntax
let names: Array<string> = ["Alice", "Bob"];

// Mixed arrays (avoid if possible)
let mixed: (string | number)[] = [1, "two", 3];

// Readonly arrays
let readOnly: readonly number[] = [1, 2, 3];

Tuples

// Fixed-length array with specific types
let person: [string, number] = ["Alice", 25];

// Named tuples (TypeScript 4.0+)
let user: [name: string, age: number] = ["Bob", 30];

// Optional elements
let optional: [string, number?] = ["Alice"];

Enums

// Numeric enum
enum Direction {
  Up,    // 0
  Down,  // 1
  Left,  // 2
  Right  // 3
}

// String enum
enum Status {
  Active = "ACTIVE",
  Inactive = "INACTIVE",
  Pending = "PENDING"
}

// Usage
let dir: Direction = Direction.Up;

Special Types

// Any - avoid when possible
let anything: any = "string";
anything = 42;
anything = true;

// Unknown - safer than any
let value: unknown = "hello";
if (typeof value === "string") {
  console.log(value.toUpperCase());
}

// Void - no return value
function log(msg: string): void {
  console.log(msg);
}

// Never - never returns
function error(msg: string): never {
  throw new Error(msg);
}

Type Assertions

// Angle bracket syntax
let value: unknown = "hello";
let length: number = (<string>value).length;

// as syntax (preferred in JSX)
let length2: number = (value as string).length;

// Non-null assertion
let el = document.getElementById("app")!;

Next Steps

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