In JavaScript a constructor is just an ordinary function. Any function which can be called as a constructor with 'new' can be called as an ordinary function. But functions will almost always be written as either a constructor or non-constructor function, and using it the other way is an error.
The built-in objects however are specifically designed so that you can use them both ways (as constructor and as ordinary functions) with two different results. For example new Date(1) creates a new Date object (as you would expect for a constructor) while Date(1) returns a string representation of the corresponding date, which is perhaps more surprising.
