Well isn't this natural that they are unexpected to you if you did not read the document that specifies what is expected? I'm interested by the undefined behav

Well isn't this natural that they are unexpected to you if you did not read the document that specifies what is expected? I'm interested by the undefined behavior you mention. While JavaScript clearly has some counterintuitive mechanics they are pretty much... deterministic. Could you share an example of what is undefined? I assume you mean "C/C++"-like "undefinedness".