During my work on a reservation system I figured out, that it's useful to write Javascript libraries in one browser instance while testing the functionality in a different one. To clearly separate the two environments I preferably use two different browsers. Two windows of the same browser are not enough due to sessions being saved in a window-spanning manner. Alternatively, it is possible to open a second instance of the same browser in "private" mode. In doing so, a new session is opened for each window separately.