As an expression

The value of an if is the value of the last expression found in each of its branches:

a = if 2 > 1
      3
    else
      4
    end
a # => 3

If an if branch is empty, or it’s missing, it’s considered as if it had nil in it:

if 1 > 2
  3
end

# The above is the same as:
if 1 > 2
  3
else
  nil
end

# Another example:
if 1 > 2
else
  3
end

# The above is the same as:
if 1 > 2
  nil
else
  3
end

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https://crystal-lang.org/reference/syntax_and_semantics/as_an_expression.html