Syntax

• are encapsulated in []
• Structure is
  [condition]
       syntax
  [else]
      syntax
  [end] or [global] is similar since V 12
• combining criteria with || (or) or/and && (and)
• negated with !
• Since Version 12 there can be constants
• conditions can not be nested but when using files with @import f. e.
• null-safe operator possible since Version 12:
  # Previously: [getTSFE() && getTSFE().id == 123]
  # Now: [getTSFE()?.id == 123]
• Conditions can not be nested within curly braces. The example below is invalid syntax and the backend modules mumble with "missing braces":
  # Invalid: Conditions must not be used within code blocks
  someIdentifier {
    someProperty = foo
    [frontend.user.isloggedIn]
       someProperty = bar
    [GLOBAL]
  }

 

Resource: https://docs.typo3.org/m/typo3/reference-typoscript/main/en-us/Syntax/Conditions/Index.html#typoscript-syntax-conditions