One unique aspect of JavaScript is that nothing is sacred. By default, you can modify any object you can get your hands on. It doesn’t matter if the object is developer-defined or part of the default execution environment—it’s possible to change that object as long as you have access to it. This isn’t a problem in a one-developer project, in which exactly what is being modified is always known by the one person who’s in control of all code. On a multiple-developer project, however, the indiscriminate modification of objects is a big problem.