Exploring Ruby’s `defined?` Keyword

Introduction

When working with Ruby, you may come across the `defined?` keyword. This keyword is used to determine whether a variable, method, or constant has been defined in the current scope. In this article, we will explore how to use the `defined?` keyword in Ruby and understand its various applications.

Using `defined?` with Variables

One common use case for the `defined?` keyword is to check if a variable has been defined. Let's take a look at an example:


name = "Alice"
puts defined?(name) # => "local-variable"

In this example, we have defined a variable `name` with the value "Alice". When we use the `defined?` keyword with the variable `name`, it returns "local-variable", indicating that the variable has been defined in the current scope.

Using `defined?` with Methods

The `defined?` keyword can also be used to check if a method has been defined. Here's an example:


def greet
  puts "Hello, World!"
end

puts defined?(greet) # => "method"

In this example, we have defined a method `greet` that prints "Hello, World!". When we use the `defined?` keyword with the method `greet`, it returns "method", indicating that the method has been defined in the current scope.

Using `defined?` with Constants

Another use case for the `defined?` keyword is to check if a constant has been defined. Let's see an example:


MY_CONSTANT = 42

puts defined?(MY_CONSTANT) # => "constant"

In this example, we have defined a constant `MY_CONSTANT` with the value 42. When we use the `defined?` keyword with the constant `MY_CONSTANT`, it returns "constant", indicating that the constant has been defined in the current scope.

Checking for Undefined Variables

If a variable, method, or constant has not been defined, the `defined?` keyword will return `nil`. Let's look at an example:


puts defined?(undefined_variable) # => nil

In this example, we are trying to check if the variable `undefined_variable` has been defined. Since this variable has not been defined, the `defined?` keyword returns `nil`.

Conclusion

In this article, we have explored the `defined?` keyword in Ruby and how it can be used to check if a variable, method, or constant has been defined in the current scope. By understanding how to use the `defined?` keyword, you can write more robust and error-free Ruby code.