cond_better_switch.sjs

macro _arms {
  rule {(default => $value:expr)} => {
    else {
      return $value;
    }
  }
  rule {(rule {$cond:expr} => $value:expr)} => {
    if($cond) {
      return $value;
    }
  }
  rule {(
    $(rule {$cond:expr} => $value:expr) $rest ...
      )} => {
     _arms (rule {$cond} => $value)
     _arms ($rest ...)
  }
}

macro cond {
  rule {{ $arms ... }} => {
    (function() {
      _arms($arms ...)
    })();
  }
}
 
var x = [];
var type = cond {
  rule {(x === null)} => "null"
  rule {Array.isArray(x)} => "array"
  rule {(typeof x === "object")} => "object"
  default => typeof x
}

This can also expand nicely to a nested ternary:

macro cond {
    rule { {case { } => $value:expr} } => {
        $value;
    }
    rule { {case {$cond:expr} => $value:expr $rest ...} } => {
        ($cond) ? $value : cond { $rest ... }
    }
}

var x2 = [], type2 = cond {
    case { x2 === null } => "null"
    case { Array.isArray(x2) } => "array"
    case { typeof x2 === "object" } => "object"
    case { } => typeof x2
}
// var x2 = [], type2 = x2 === null ? 'null' : Array.isArray(x2) ? 'array' : typeof x2 === 'object' ? 'object' : typeof x2;