It looks like tail call optimization only works in simple situations.

    let rec foo i = if i < 0 then "foo" else bar (i - 1)
    and bar i = if i < 0 then "bar" else foo (i - 1)

    function foo(i) {
      if (i < 0) {
        return "foo";
      }
      else {
        return bar(i - 1 | 0);
      }
    }
    
    function bar(i) {
      if (i < 0) {
        return "bar";
      }
      else {
        return foo(i - 1 | 0);
      }
    }

It would be nice to get a warning when tail calls cannot be optimized.