sweet.js version of Groovy's Elvis operator

elvis.sjs

// Groovy's Elvis operator
//
//     lhs ?: rhs
//
//  is equivalent to:
//
//     lhs == null ? rhs : lhs
//
// or:
//
//     (function ($lhs) { return $lhs == null ? rhs : $lhs })(lhs)
//
// depending on whether lhs has (or might have) side-effects
// (e.g. getters/proxies/expressions)

binaryop (?:) 4 right {
    macro {
        rule { (null)      ($rhs:expr) } => { $rhs }
        rule { (undefined) ($rhs:expr) } => { $rhs }
        rule { ($lhs:lit)  ($rhs:expr) } => { $lhs }

        rule { ($lhs:ident) ($rhs:expr) } => {
            ($lhs == null ? $rhs : $lhs)
        }

        rule { ($lhs:expr) ($rhs:expr) } => {
            (function(lhs) { return lhs == null ? $rhs : lhs }($lhs))
        }
    }
}

var foo = bar ? baz : quux;
var foo = bar ? baz ?: 42 : quux ?: 42;

var foo = undefined ?: undefined ?: 42;
var foo = null ?: null ?: 42;
var foo = undefined ?: null ?: 42;
var foo = null ?: undefined ?: 42;

var foo = foo ?: bar ?: baz;

var foo = '' ?: 0 ?: bar;
var foo = 0  ?: '' ?: bar;

var foo = foo.bar ?: foo.baz ?: foo.quux;

var foo = undefined ?: null ?: 0 ?: '' ?: foo ?: foo.bar;
var foo = foo.bar ?: foo ?: 0 ?: '' ?: null ?: undefined;

var foo = ++bar ?: ++baz;