MY_CONSTANT = "Hello, world" # constant
Constant = 'This is also constant' # constant
my_variable = "Hello, venus" # not constatn

Constant name start with capital letter. Everything that start with capital letter are considered as constant in Ruby. So class and module are also constant. Best practice is use all capital letter for declaring constant.

MY_CONSTANT = "Hello, world"
MY_CONSTANT = "Hullo, world"

The above code results in a warning, because you should be using variables if you want to change their values. However it is possible to change one letter at a time in a constant without a warning, like this:

MY_CONSTANT = "Hello, world"
MY_CONSTANT[1] = "u"

Now, after changing the second letter of MY_CONSTANT, it becomes "Hullo, world".

def say_hi
  MESSAGE = "Hello"
  puts MESSAGE
end

The above code results in an error: SyntaxError: (irb):2: dynamic constant assignment.

class Message
  DEFAULT_MESSAGE = "Hello, world"

  def speak(message = nil)
    if message
      puts message
    else
      puts DEFAULT_MESSAGE
    end
  end
end

The constant DEFAULT_MESSAGE can be changed with the following code:

Message::DEFAULT_MESSAGE = "Hullo, world"

Syntax

• MY_CONSTANT_NAME = "my value"

Remarks

Constants are useful in Ruby when you have values that you do not want to be mistakenly changed in a program, such as API keys.