Unlike strongly typed languages, JavaScript does not enforce a return type on a function. This means that different paths through a function can return different types of values, which can be very confusing to the user and significantly harder to maintain.

In particular a function, in JavaScript, will return ``++undefined++`` in any of the following cases:

* It exits without a ``++return++`` statement.
* It executes a ``++return++`` with no value.

This rule verifies that return values are either always or never specified for each path through a function.