Functions become more useful when we can send them information and get something back. In JavaScript, we use parameters to pass data into functions, and we use the return keyword to send a result back. This makes functions more powerful and flexible, letting us reuse the same logic with different values.

Passing arguments to functions

You can pass values into a function using parameters. These are like placeholders for the values (called arguments) you give when calling the function.

          function greet(name) {
  console.log("Hello, " + name + "!");
}

greet("Alice"); // Output: Hello, Alice!
greet("Bob");   // Output: Hello, Bob!
        

Here, name is the parameter, and "Alice" is the argument passed into the function.

Returning values with return

Functions can also return a value using the return keyword. This lets you use the result somewhere else in your code.

          function add(a, b) {
  return a + b;
}

let result = add(5, 3); 
console.log(result); // Output: 8
        

Once a function hits a return statement, it stops running and sends the result back to where it was called.

Default parameters

You can set default values for parameters in case the caller doesn't provide them. This helps prevent errors and gives your function some fallback behavior.

          function greet(name = "friend") {
  console.log("Hello, " + name + "!");
}

greet();        // Output: Hello, friend!
greet("Sami");  // Output: Hello, Sami!
        

With default parameters, your functions become more flexible and less likely to break when no arguments are passed.