**Course Title:** Mastering TypeScript: From Basics to Advanced Applications **Section Title:** Control Structures and Functions **Topic:** Defining functions: function types, optional and default parameters. **Overview** In the previous topics, we have covered control structures such as conditional statements and loops. Now, we will dive into one of the fundamental building blocks of programming languages: functions. In TypeScript, functions are first-class citizens, meaning they can be assigned to variables, passed as arguments to other functions, and returned from functions. In this topic, we will explore how to define functions in TypeScript, covering function types, optional parameters, and default parameters. **Function Types** Functions in TypeScript can be categorized into several types based on their purpose and structure: * **Normal functions**: These are the most common type of function, which takes arguments, performs some operations, and returns a value. ```typescript function add(x: number, y: number): number { return x + y; } ``` * **Anonymous functions**: These are functions without a declared name. ```typescript const greet = function (name: string) { console.log(`Hello, ${name}!`); }; ``` * **Arrow functions**: These are concise alternatives to normal functions. ```typescript const sum = (x: number, y: number) => x + y; ``` * **Constructor functions**: These are used to create new objects. ```typescript class Person { private name: string; private age: number; constructor(name: string, age: number) { this.name = name; this.age = age; } } ``` **Optional Parameters** Optional parameters allow us to specify a parameter that can be omitted when calling a function. In TypeScript, we use the `?` operator to denote optional parameters. ```typescript function greet(name: string, age?: number) { console.log(`Hello, ${name}!`); if (age) { console.log(`You are ${age} years old.`); } } greet("John"); // Output: Hello, John! greet("Jane", 30); // Output: Hello, Jane! You are 30 years old. ``` **Default Parameters** Default parameters allow us to assign a default value to a parameter if no value is provided when calling a function. In TypeScript, we use the `=` operator to assign a default value. ```typescript function greet(name: string, age: number = 30) { console.log(`Hello, ${name}! You are ${age} years old.`); } greet("John"); // Output: Hello, John! You are 30 years old. greet("Jane", 25); // Output: Hello, Jane! You are 25 years old. ``` **Practical Takeaways** 1. **Use type annotations**: Always use type annotations when defining function parameters and return types to ensure type safety and make your code more readable. 2. **Use optional parameters**: Use optional parameters to make your functions more flexible and easy to use. 3. **Use default parameters**: Use default parameters to provide a fallback value for a parameter if no value is provided. **Practice Time!** Try defining your own functions using the concepts learned in this topic. Create a function that calculates the area and perimeter of a rectangle. Use optional parameters to make the function flexible. **External Resources** For more information on functions in TypeScript, refer to the official TypeScript documentation: [https://www.typescriptlang.org/docs/handbook/2/functions.html](https://www.typescriptlang.org/docs/handbook/2/functions.html) **What's Next?** In the next topic, we will explore **function overloading**, which allows us to define multiple functions with the same name but different parameters. **Leave a Comment/Ask for Help** Have questions or need help with the concepts covered in this topic? Leave a comment below, and I'll be happy to assist you.