sterretje:
Note
It's not advisable to use String (capital S); they can leave holes in your memory and as a result you might run out of memory resulting in difficult to find problems.
Much less of a problem with esp8266, which has loads of ram (80K?), than with Uno etc (2K ram). I've monitored free space over extended periods with some of my code that uses String class, and it remained stable after a while. Perhaps there is some simple garbage collection going on, or at least a reasonably good recycling of heap space. I would not recommend using String on Uno either.