How do you define default parameters in JavaScript functions?

Defining default parameters in JavaScript functions allows you to specify default values to use when no argument is provided for a parameter during function invocation. This feature can be implemented directly in the function declaration by assigning a default value to the parameter. When the function is called without an argument for that parameter, the default value is used.

For example:

function greet(name = 'Guest') {
    console.log(`Hello, ${name}!`);
}

greet(); // Outputs: Hello, Guest!

In this case, if the greet function is called without an argument, the default value 'Guest' is used.

Initializing the parameter within the function body does not allow for default parameter functionality in a clean or efficient manner, as you'd have to check for undefined or null values each time. Using a default keyword is not valid syntax in JavaScript, and calling the function with a default argument simply invokes the function; it does not define a default parameter. Therefore, defining default parameters directly in the function declaration is the proper and most effective method to achieve this feature.