Preconditions are useful, especially in a dynamically-typed language.

require can check for a boolean expression along with an error message. In case of error, it throws an AssertionError.

function foo = |a| {
  require(a oftype String.class, "a must be a String")
  ...
}

You may also use requireNotNull that… well… checks that its argument is not null:

function foo = |a| {
  requireNotNull(a)
  ...
}