adding the notion of existence -- postfixing an expression with a question mark will check if to see if it's not null or undefined

lib/coffee_script/grammar.y

@@ -19,7 +19,7 @@ token INDENT OUTDENT
 
 # Declare order of operations.
 prechigh
-  nonassoc UMINUS SPLAT NOT '!' '!!' '~' '++' '--'
+  nonassoc UMINUS SPLAT NOT '!' '!!' '~' '++' '--' '?'
   left     '*' '/' '%'
   left     '+' '-'
   left     '<<' '>>' '>>>'
@@ -72,6 +72,7 @@ rule
   | Switch
   | Extends
   | Splat
+  | Existence
   | Comment
   ;
 
@@ -181,6 +182,10 @@ rule
   | Expression INSTANCEOF Expression  { result = OpNode.new(val[1], val[0], val[2]) }
   ;
 
+  Existence:
+    Expression '?'                    { result = ExistenceNode.new(val[0]) }
+  ;
+
   # Function definition.
   Code:
     ParamList "=>" Block              { result = CodeNode.new(val[0], val[2]) }

lib/coffee_script/nodes.rb

@@ -715,6 +715,19 @@ module CoffeeScript
     end
   end
 
+  # Check an expression for existence (meaning not null or undefined).
+  class ExistenceNode < Node
+    attr_reader :expression
+
+    def initialize(expression)
+      @expression = expression
+    end
+
+    def compile(o={})
+      write("(#{@expression.compile(super(o))} != undefined)")
+    end
+  end
+
   # An extra set of parentheses, supplied by the script source.
   # You can't wrap parentheses around bits that get compiled into JS statements,
   # unfortunately.