In JavaScript, functions can have zero or more parameters. Function parameters are variables used inside a function to perform specific operations. You define a function parameter by specifying a variable name inside the function's parentheses. Syntax:

function <function-name>(parameter1, parameter2, ...){
// function body
}

Here's an example of a function with two parameters:

function greetUser(firstName, lastName) {
console.log(Hello, ${firstName} ${lastName});
}

The greetUser() function has two parameters: firstName and lastName. Inside the function, these parameters act like normal variables to perform specific tasks. To call the function, pass values for each parameter, separated by commas. Here's an example calling greetUser() with two arguments:

greetUser('John', 'Doe');

In the above example, the values "John" and "Doe" are passed as arguments to the greetUser() function, and these values are assigned to the firstName and lastName parameters, respectively. It's important to note that if you pass fewer arguments than the number of parameters in a function, the missing parameters will have a value of undefined.

greetUser('John');

In the above example, only one argument is passed to the greetUser() function, so the lastName parameter will have a value of undefined. You can also pass more arguments than the number of parameters in a function, but those extra arguments will be ignored by the function.

greetUser('John', 'Doe', 'Smith');

In the above example, the function greetUser() takes only two parameters, but three arguments are passed to the function. The extra argument "Smith" will be ignored by the function. Overall, function parameters are a useful feature in JavaScript that allows you to create reusable functions that can perform specific operations based on the values passed as arguments.