Type Annotations

String Annotations

Use the string type annotation, written as name: string, when a variable should hold text data. Once annotated this way, assigning anything other than a string to that variable becomes a compile-time error.

Example: String Annotations

let name: string = "Neel";
// name = 42; // rejected: number is not a string
console.log(name);

Number and Boolean Annotations

The number and boolean types describe numeric values and true-or-false values respectively, using the same simple name: type annotation syntax.

TypeScript uses a single number type for both integers and decimals, unlike languages with separate int and float types.

Example: Number and Boolean Annotations

let age: number = 30;
let isMember: boolean = true;
console.log(age, isMember);

Array Annotations

Array annotations, written as type[], specify the type every single element stored in that array must match, such as string[] for an array of text values.

This lets the compiler catch a mismatched item the moment it's added, not just when it's later used.

Example: Array Annotations

let tags: string[] = ["ts", "types"];
// tags.push(42); // rejected: number is not a string
console.log(tags);

Object Annotations

Object type annotations describe the properties an object must have and the type each property's value must match, either inline or through a named interface.

This documents an object's expected shape directly in the code, rather than leaving it implicit.

Example: Object Annotations

interface Book {
  title: string;
  pages: number;
}
let book: Book = { title: "TypeScript Basics", pages: 200 };
console.log(book);

Why Type Annotations Matter

Type annotations across all of these make the intended type of a piece of data explicit at the point of declaration, rather than something a reader has to infer.

This lets TypeScript detect an invalid value being assigned the moment it happens during development, instead of surfacing as a bug later.

Example: Why Type Annotations Matter

let quantity: number = 5;
// quantity = "five"; // caught immediately, not discovered later at runtime
console.log(quantity);