Question

The code is as follows

var mailArchive = {0: "Dear nephew, ... (mail number 1)",
                   1: "(mail number 2)",
                   2: "(mail number 3)"};

for (var current = 0; current in mailArchive; current++)
  print("Processing e-mail #", current, ": ", mailArchive[current]);

I would think current < mailArchive.length would be a better closure condition….

Does current in mailArchive accomplish the same thing, as it appears to in the interpreter (required for print function)? If not, what does it accomplish and how?

Answer 1

Yes, it works in the interpreter, I tested it with a node.js interactive session. mailArchive.length, however, will not work as mailArchive is an Object, not an Array, so doesn’t have a property named length.

current in mailArchive checks whether the object mailArchive has a property with the name given in the variable current.

As you may or may not know, however, this approach is not usual for JavaScript. Much more common is to make mailArchive an array instead and use the usual condition current < mailArchive.length