Parameters (arguments object)

Another unique aspect of JavaScript functions is that you can pass any number of parameters to any function without causing an error. That’s because function parameters are actually stored as an array-like structure called arguments.

The arguments object is automatically available inside any function.
This means named parameters in a function exist mostly for convenience and don’t actually limit the number of arguments that a function can accept.
The arguments object is not an instance of Array and therefore doesn’t have the same methods as an array; Array.isArray(arguments) always returns false.

On the other hand, JavaScript doesn’t ignore the named parameters of a function either. The number of arguments a function expects is stored on the function’s length property. Remember, a function is actually just an object, so it can have properties. The length property indicates the function’s arity, or the number of parameters it expects. Knowing the function’s arity is important in JavaScript because functions won’t throw an error if you pass in too many or too few parameters

(see next code running here -> https://playcode.io/664865)

function reflect(value) {
 return value;
}

console.log(reflect("Hi!")); // "Hi!"
console.log(reflect("Hi!", 25)); // "Hi!"
console.log(reflect.length); // 1

reflect = function() {
 return arguments[0];
};

console.log(reflect("Hi!")); // "Hi!"
console.log(reflect("Hi!", 25)); // "Hi!"
console.log(reflect.length); // 0

The version that uses the arguments object can be confusing because there are no named arguments, and you must read the body of the function to determine if arguments are used. That is why many developers prefer to avoid using arguments unless necessary.

Sometimes, however, using arguments is actually more effective than naming parameters

function sum() {
    var result = 0, i = 0,  len = arguments.length;
    while (i < len) {
      result += arguments[i];
      i++;
    }
    return result;
}


console.log(sum(1, 2)); // 3
console.log(sum(3, 4, 5, 6)); // 18
console.log(sum(50)); // 50
console.log(sum()); //

Parameters (arguments object)

References