TypeScript Basic Types

A type tells TypeScript what kind of value a variable can hold.

Type Annotations

Write a colon and the type after the variable name:

let studentName: string = "Ayesha";
let age: number = 20;
let isEnrolled: boolean = true;

console.log(studentName, age, isEnrolled);

Ayesha 20 true

If you assign the wrong kind of value, TypeScript shows an error:

let age: number = 20;
age = "twenty";

error TS2322: Type 'string' is not assignable to type 'number'.

The Primitive Types

Write type names in lowercase: string, not String.

Type Inference

When you give a value straight away, you can skip the annotation. TypeScript infers the type:

let shop = "Hassan Garments";   // inferred as string
let items = 12;                  // inferred as number

items = items + 1;   // fine
// shop = 5;         // Error: Type 'number' is not assignable to type 'string'

any

any turns type checking off for a variable. It can hold anything, and TypeScript will not complain about anything you do with it.

let data: any = "hello";
data = 10;
data.foo.bar();   // no error while compiling, but it crashes when it runs

Avoid any. It removes the safety TypeScript gives you.

unknown

unknown also accepts any value, but you must check its type before using it. It is the safe version of any.

let input: unknown = "0300-1234567";

if (typeof input === "string") {
  console.log(input.length);   // allowed: TypeScript knows it is a string here
}

12

void and never

• void is used for functions that return nothing
• never is used for functions that never finish, for example one that always throws an error

function logMessage(msg: string): void {
  console.log(msg);
}

function fail(reason: string): never {
  throw new Error(reason);
}

Enums

An enum gives friendly names to a fixed set of values:

enum OrderStatus {
  Pending = "PENDING",
  Shipped = "SHIPPED",
  Delivered = "DELIVERED"
}

let status: OrderStatus = OrderStatus.Shipped;
console.log(status);

SHIPPED

Many developers prefer union types (taught later) instead of enums, but you will see enums in many projects.

Exercise