5.18 return

A return expression can come with or without a value expression:

return;
return expression;

It leaves the control-flow of the innermost function it is declared in, which has to be distinguished when local functions are involved:

function f1() {
  function f2() {
    return;
  }
  f2();
  expression;
}

The return leaves local function f2, but not f1, meaning expression is still evaluated.

If return is used without a value expression, the typer ensures that the return type of the function it returns from is of Void. If it has a value expression, the typer unifies its type with the return type (explicitly given or inferred by previous return expressions) of the function it returns from.