What is the '===' operator in Ruby?

The === operator (triple equals) in Ruby is called the case equality operator.

It is used in case/when expressions and is defined by classes to check whether the right operand belongs to the "category" or "range"" represented by the left operand.

The behavior of the === operator depends on the class of the left operand.

Examples:

• Class Module: Checks if the right operand is an instance or subclass of the module/class.

  # Checks if the right operand is an instance of String or its subclass.
  String === "hello"
  # => true
  

• Class Range: Checks if the right operand is within the range.

  # Checks if 5 is within the range 1..10.
  (1..10) === 5
  # => true
  

• Class Regexp: Checks if the string matches the regular expression.

  # Checks if the string "/ruby/" matches the regex /ruby/.
  /ruby/ === "/ruby/"
  # => true
  

• Class Proc: Calls the block with the right operand as an argument.

  # Calls the block with argument 5.
  ->(x) { x > 3 } === 5
  # => true
  

• Class Symbol: Compares the symbol with another string or symbol.

  # Compares the symbol :test with the string "test".
  :test === "test"
  # => true
  

In most other classes, === behaves like ==.

Usage in case/when:

The expression case condition is equivalent to a series of === calls on each when value with condition as the right operand.

# Equivalent to String === value
# Then Integer === value
case value
when String
  # ...
when Integer
  # ...
end