[dnautics]

Broadly speaking it's not bad if your null or undefined value has a different type than the shadowed type. The two examples in statically typed languages I can think of is rust and zig; in dynamically typed languages I think of elixir (which assigns it to an atom, and is a crash-fast language), and ruby (nil is it's own class)

Of the languages I have experience with, it's a major problem in C, java, and javascript.

For javascript specifically there is also the issue of falsey discipline; there are a ton of falsey values so you could get tripped up in ways you forgot about when doing a null check.

[z3t4]

In JavaScript it's only the value undefined that is interchangeable with null, and only if you use == (two equal signs vs explicit check with three equal signs). eg if(foo==null) so I would say it's extremely rare, but if you are feeling adventurous you could of course write if(foo) and it would match false, undefined, null, 0, empty string, and NaN. Which could be an issue if for example a variable is either a number (0) or null.