**Course Title:** Mastering TypeScript: From Basics to Advanced Applications **Section Title:** Introduction to TypeScript and Setup **Topic:** Basic syntax: variables, data types, and type annotations. As we dive deeper into the world of TypeScript, it's essential to understand the fundamental building blocks of the language. In this topic, we'll explore the basic syntax of TypeScript, including variables, data types, and type annotations. These concepts are crucial for writing efficient and maintainable code. ### Variables in TypeScript In TypeScript, variables are declared using the `let`, `const`, or `var` keywords. The main difference between these keywords is the scope and behavior of the variable. * `let`: The `let` keyword is used to declare block-scoped variables. This means that the variable is only accessible within the block it's declared in. * `const`: The `const` keyword is used to declare constant variables. Variables declared with `const` cannot be reassigned. * `var`: The `var` keyword is used to declare function-scoped variables. Variables declared with `var` are accessible throughout the function they're declared in. Here's an example: ```typescript // let declaration let name = 'John Doe'; // const declaration const PI = 3.14159; // var declaration var age = 25; ``` ### Data Types in TypeScript TypeScript supports the following data types: * **Number**: The `number` type represents a numeric value. It can be an integer or a floating-point number. * **String**: The `string` type represents a sequence of characters. Strings can be enclosed in single quotes or double quotes. * **Boolean**: The `boolean` type represents a boolean value. It can be either `true` or `false`. * **Array**: The `array` type represents a collection of values. * **Null**: The `null` type represents the absence of any object value. * **Undefined**: The `undefined` type represents an uninitialized variable or a symbol without a binding. * **Tuple**: The `tuple` type represents a fixed-length array. * **Enum**: The `enum` type represents a set of named values. Here's an example of each data type: ```typescript // number let age: number = 25; // string let name: string = 'John Doe'; // boolean let isAdmin: boolean = false; // array let numbers: number[] = [1, 2, 3]; // null let nullValue: null = null; // undefined let undefinedValue: undefined = undefined; // tuple let person: [string, number] = ['John Doe', 25]; // enum enum Color { Red, Green, Blue } let favoriteColor: Color = Color.Green; ``` ### Type Annotations Type annotations in TypeScript are used to assign a specific type to a variable, function parameter, or return value. Type annotations are denoted by the `:` symbol followed by the type. Here are a few examples: ```typescript // variable type annotation let name: string = 'John Doe'; // function parameter type annotation function greet(name: string): void { console.log(`Hello, ${name}!`); } // function return type annotation function add(x: number, y: number): number { return x + y; } ``` Best practices for type annotations: * Always include type annotations for variable declarations. * Include type annotations for function parameters and return values. * Use type annotations for properties in interfaces. **Practice Time!** 1. Declare a variable `username` with the type `string` and assign it the value `'johnDoe'`. 2. Declare a variable `age` with the type `number` and assign it the value `25`. 3. Create an array of numbers and assign it to a variable `numbers`. **Resources:** * [TypeScript Documentation - Basic Types](https://www.typescriptlang.org/docs/handbook/basic-types.html) * [TypeScript Documentation - Variable Declarations](https://www.typescriptlang.org/docs/handbook/variable-declarations.html) If you have any questions or need further clarification, please leave a comment below and I'll be happy to help. In the next topic, we'll explore how to compile TypeScript to JavaScript. This will include setting up the TypeScript compiler, understanding compiler options, and working with source maps.

// let declaration
let name = 'John Doe';

// const declaration
const PI = 3.14159;

// var declaration
var age = 25;

// number
let age: number = 25;

// string
let name: string = 'John Doe';

// boolean
let isAdmin: boolean = false;

// array
let numbers: number[] = [1, 2, 3];

// null
let nullValue: null = null;

// undefined
let undefinedValue: undefined = undefined;

// tuple
let person: [string, number] = ['John Doe', 25];

// enum
enum Color {
    Red,
    Green,
    Blue
}

let favoriteColor: Color = Color.Green;

// variable type annotation
let name: string = 'John Doe';

// function parameter type annotation
function greet(name: string): void {
    console.log(`Hello, ${name}!`);
}

// function return type annotation
function add(x: number, y: number): number {
    return x + y;
}