What are type annotations and how do you use them?

Type annotations are explicit types you write after a colon, as in let age: number. They tell TypeScript what type a variable, parameter or return value must have, and the compiler rejects any value that doesn't fit.

// Variable annotations
let username: string = "john";
let age: number = 25;
let isLoggedIn: boolean = false;

// Function parameter and return type annotations
function greet(name: string): string {
  return `Hello, ${name}!`;
}

// Array annotations
let numbers: number[] = [1, 2, 3];
let names: Array<string> = ["Alice", "Bob"];

Array types take either form, and string[] means the same as Array<string>.

In practice you annotate function parameters and often return types. You let type inference handle most local variables, because an annotation that repeats an obvious initial value adds nothing.