We can tell how many arguments a function expects by accessing its length property.

While that works for many cases it isn't generally true. For instance:

    function wtf() { return arguments[0]; }

While that may seem like a ridiculous example, the following idiom is quite common in JS programming:

    function accessor(value) {
        if (typeof value === 'undefined') {
            return getValue();
        }
        else {
            setValue(value):
        }
    }

In general it's not a good idea to expect any particular value for a function's length property.