Question

Why does the following code not throw an error?

if false
  x = 0
end

x  #=> nil

Whereas the following does throw an error:

y  # NameError: undefined local variable or method `y' for main:Object

The same thing happens with unless & case statements.

Answer 1

It’s because of how the Ruby parser works. Variables are defined by the parser, which walks through the code line-by-line, regardless of whether it will actually be executed.

Once the parser sees x =, it defines the local variable x (with value nil) henceforth in the current scope. Since if/unless/case/for/while do not create a new scope, x is defined and available outside the code block. And since the inner block is never evaluated as the conditional is false, x is not assigned to (and is thus nil).

Here’s a similar example:

defined?(x) and x = 0
x  #=> nil

_{Note that this is a rather high-level overview of what happens, and isn’t necessarily exactly how the parser works.}