Javascript Certification Practice Test

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:

```javascript

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.