Undefined local variables and conditions in Ruby
- Name
- Dennis Paagman
- @djfpaagman
Ruby’s dynamic nature can sometimes be confusing. Consider this example, what do you expect to be returned?
# `active?` returns `false` for this example.
active = false if invoice.active?
{ active: active }
Since the condition is false, you might expect active to not be set and throw an error, like calling an undefined local variable normally would:
irb(main):001:0> active
NameError (undefined local variable or method `active' for main:Object)
But Ruby actually initializes the variable. It just doesn’t set a value because the condition returns false. This means active is nil in this case!
irb(main):004:0> { active: active }
=> {:active=>nil}
This can lead to unexpected behavior, for example when passing a hash into a Rails model. A better usage would be to use the boolean method directly inside the hash:
{ active: invoice.active? }
Side note: undefined instance variables always return nil.
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