[geofft]

> If a bug causes an unexpected undefined value, it will lead your code to an undefined behavior.

Let's not confuse "undefined", a JS value that is basically like C's NULL, with "undefined behavior", the concept from e.g. the C language spec. Operations on the JS value "undefined" are perfectly well-defined in the C sense; you can reliably test for it and have a case to handle it. In particular, the well-defined behavior for Underscore/Lodash in response to mapping over "undefined" is to return an empty array. The programmer upthread is using the library's documented and well-defined behavior; there is nothing wrong with that.

It is just like how, in C, some functions (like time()) are well-defined if you pass them a NULL pointer, and some functions (like strlen()) are not, and result in undefined behavior. In this case, the functions in question are all well-defined if you pass them "undefined".