Type Annotations — TypeScript Tutorial

Basic Type Annotations

Type annotations let you explicitly declare the type of a variable, parameter, or return value. You add a colon followed by the type after the variable name.

Example

// Variable annotations
let name: string = "Alice";
let age: number = 25;
let isActive: boolean = true;

// TypeScript can also infer types
let city = "New York"; // inferred as string
let count = 10;        // inferred as number

Function Annotations

You can annotate function parameters and return types to ensure functions are called correctly and return the expected type.

Example

// Parameter and return type annotations
function add(a: number, b: number): number {
  return a + b;
}

// Arrow function with types
const multiply = (x: number, y: number): number => x * y;

// Void return type
function logMessage(msg: string): void {
  console.log(msg);
}

add(5, 3);      // OK
add("5", 3);    // Error: string is not assignable to number

Object Type Annotations

You can describe the shape of objects using inline type annotations.

Example

// Object type annotation
function printUser(user: { name: string; age: number }) {
  console.log(`${user.name} is ${user.age} years old`);
}

printUser({ name: "Bob", age: 30 });

// Optional properties with ?
function greet(person: { first: string; last?: string }) {
  if (person.last) {
    console.log(`Hello, ${person.first} ${person.last}`);
  } else {
    console.log(`Hello, ${person.first}`);
  }
}