**Course Title:** Modern JavaScript Programming: From Fundamentals to Full-Stack Development **Section Title:** Control Structures and Functions **Topic:** Default parameters and rest/spread operators. In this topic, we will explore two powerful features in JavaScript: default parameters and rest/spread operators. These features were introduced in ECMAScript 2015 (ES6) and have since become a fundamental part of modern JavaScript development. **Default Parameters** In JavaScript, it's common for functions to have optional parameters that may or may not be passed by the caller. Prior to ES6, we had to use complex logic to handle such cases. With the introduction of default parameters, we can now define a default value for a parameter, which will be used if the caller doesn't provide one. **Example:** ```javascript function greet(name = 'World') { console.log(`Hello, ${name}!`); } greet('John'); // Output: Hello, John! greet(); // Output: Hello, World! ``` In this example, the `greet` function takes an optional `name` parameter with a default value of `'World'`. If we call `greet` without passing any argument, the function will use the default value. **Key Concepts:** * Default parameters are defined using the `=` operator after the parameter name. * Default values are only applied if the parameter is not provided by the caller. * Default values can be expressions, not just literals. For more information on default parameters, check out the [MDN documentation](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Default_parameters). **Rest Parameters** Rest parameters allow us to represent an indefinite number of arguments as an array. We can use rest parameters to handle functions that take a variable number of arguments. **Example:** ```javascript function sum(...numbers) { return numbers.reduce((acc, current) => acc + current, 0); } console.log(sum(1, 2, 3, 4, 5)); // Output: 15 ``` In this example, the `sum` function uses rest parameters to represent the variable number of arguments as an array called `numbers`. We can then use the `reduce` method to calculate the sum of these numbers. **Key Concepts:** * Rest parameters are defined using the `...` operator before the parameter name. * Rest parameters can only be used at the end of a function's parameter list. * Rest parameters are useful when we don't know the number of arguments a function will receive. For more information on rest parameters, check out the [MDN documentation](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/rest_parameters). **Spread Operators** Spread operators are similar to rest parameters but are used in function calls, not definitions. They allow us to pass an array as individual arguments to a function. **Example:** ```javascript function greet(name, age) { console.log(`Hello, ${name}! You are ${age} years old.`); } const person = ['John', 30]; greet(...person); // Output: Hello, John! You are 30 years old. ``` In this example, the `greet` function takes two arguments, but we are passing an array to it. We use the spread operator to unpack the `person` array into individual arguments. **Key Concepts:** * Spread operators are defined using the `...` operator. * Spread operators can be used in function calls and in array literals. For more information on spread operators, check out the [MDN documentation](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Spread_syntax). **Practical Takeaways:** * Use default parameters to handle optional function parameters. * Use rest parameters to handle functions with a variable number of arguments. * Use spread operators to pass arrays as individual arguments to functions. We now have a solid understanding of default parameters, rest parameters, and spread operators in JavaScript. In the next topic, we will explore creating and working with objects and arrays, another fundamental aspect of JavaScript programming. **What's Next?** Please proceed to the next topic: **Creating and working with objects and arrays**, where we will delve into the world of JavaScript objects and arrays. **Comments and Help:** If you have any questions or need help with the concepts covered in this topic, feel free to leave a comment below. We'll be happy to help you clarify any doubts.