TypeScript Types #

TypeScript extends JavaScript by adding types to the language. Types provide a way to describe the shape of an object, providing better documentation and allowing TypeScript to validate that your code is working correctly.

Basic Types #

TypeScript supports several basic types including primitives and complex types:

// Primitives
let isDone: boolean = false;
let decimal: number = 6;
let color: string = "blue";
// Arrays
let list: number[] = [1, 2, 3];
let items: Array<number> = [1, 2, 3];
// Tuple
let tuple: [string, number] = ["hello", 10];
// Enum
enum Color { Red, Green, Blue }
let c: Color = Color.Green;
// Any - opts out of type checking
let notSure: any = 4;
notSure = "maybe a string";
// Void - absence of any type
function warn(): void {
 console.log("Warning!");
}
// Null and Undefined
let u: undefined = undefined;
let n: null = null;
// Never - represents values that never occur
function error(message: string): never {
 throw new Error(message);
}

Type Annotations #

You can add type annotations to variables, function parameters, and return types:

function greeter(person: string): string {
 return "Hello, " + person;
}
let user = "Jane";
console.log(greeter(user));

Type Inference #

TypeScript can automatically infer types based on the assigned value:

// Type is inferred as string
let message = "Hello World";
// Type is inferred as number
let count = 42;
// Type is inferred from return value
function add(a: number, b: number) {
 return a + b; // returns number
}

Union Types #

A union type describes a value that can be one of several types:

function printId(id: number | string) {
 console.log("Your ID is: " + id);
}
printId(101);
printId("202");
// Type narrowing
function welcomePeople(x: string[] | string) {
 if (Array.isArray(x)) {
 console.log("Hello, " + x.join(" and "));
 } else {
 console.log("Welcome lone traveler " + x);
 }
}

Type Aliases #

Create custom type names for any type:

type Point = {
 x: number;
 y: number;
};
type ID = number | string;
function printCoord(pt: Point) {
 console.log("x: " + pt.x + ", y: " + pt.y);
}
let myId: ID = "abc123";