diff --git a/lib/coffee_script/grammar.y b/lib/coffee_script/grammar.y
index 799f04d4..0817762f 100644
--- a/lib/coffee_script/grammar.y
+++ b/lib/coffee_script/grammar.y
@@ -38,7 +38,7 @@ prechigh
   right    WHEN LEADING_WHEN IN OF BY
   right    THROW FOR NEW SUPER
   left     EXTENDS
-  left     ASSIGN '||=' '&&='
+  left     ASSIGN '||=' '&&=' '?='
   right    RETURN
   right    '=>' '==>' UNLESS IF ELSE WHILE
 preclow
@@ -187,6 +187,7 @@ rule
   | Expression '%=' Expression        { result = OpNode.new(val[1], val[0], val[2]) }
   | Expression '||=' Expression       { result = OpNode.new(val[1], val[0], val[2]) }
   | Expression '&&=' Expression       { result = OpNode.new(val[1], val[0], val[2]) }
+  | Expression '?=' Expression        { result = OpNode.new(val[1], val[0], val[2]) }
 
   | Expression INSTANCEOF Expression  { result = OpNode.new(val[1], val[0], val[2]) }
   | Expression IN Expression          { result = OpNode.new(val[1], val[0], val[2]) }
diff --git a/lib/coffee_script/lexer.rb b/lib/coffee_script/lexer.rb
index 1d0cb83d..dad8b3d7 100644
--- a/lib/coffee_script/lexer.rb
+++ b/lib/coffee_script/lexer.rb
@@ -25,7 +25,7 @@ module CoffeeScript
     STRING     = /\A(""|''|"(.*?)([^\\]|\\\\)"|'(.*?)([^\\]|\\\\)')/m
     HEREDOC    = /\A("{6}|'{6}|"{3}\n?(.*?)\n?(\s*)"{3}|'{3}\n?(.*?)\n?(\s*)'{3})/m
     JS         = /\A(``|`(.*?)([^\\]|\\\\)`)/m
-    OPERATOR   = /\A([+\*&|\/\-%=<>:!]+)/
+    OPERATOR   = /\A([+\*&|\/\-%=<>:!?]+)/
     WHITESPACE = /\A([ \t]+)/
     COMMENT    = /\A(((\n?[ \t]*)?#.*$)+)/
     CODE       = /\A(=?=>)/
diff --git a/lib/coffee_script/nodes.rb b/lib/coffee_script/nodes.rb
index e0db94ff..525e1af6 100644
--- a/lib/coffee_script/nodes.rb
+++ b/lib/coffee_script/nodes.rb
@@ -556,7 +556,7 @@ module CoffeeScript
       :not    => '!'
     }
     CHAINABLE        = [:<, :>, :>=, :<=, :===, :'!===']
-    CONDITIONALS     = [:'||=', :'&&=']
+    CONDITIONALS     = [:'||=', :'&&=', :'?=']
     PREFIX_OPERATORS = [:typeof, :delete]
 
     def initialize(operator, first, second=nil, flip=false)
@@ -593,7 +593,9 @@ module CoffeeScript
 
     def compile_conditional(o)
       first, second = @first.compile(o), @second.compile(o)
+      o[:scope].find(first) if @first.unwrap.is_a?(Value)
       sym = @operator[0..1]
+      return "#{first} = (typeof #{first} !== \"undefined\" && #{first} !== null) ? #{first} : #{second}" if @operator == '?='
       "#{first} = #{first} #{sym} #{second}"
     end
 
diff --git a/test/fixtures/execution/test_operations.coffee b/test/fixtures/execution/test_operations.coffee
index 56f57b6c..0b996da6 100644
--- a/test/fixtures/execution/test_operations.coffee
+++ b/test/fixtures/execution/test_operations.coffee
@@ -16,3 +16,13 @@ i: 0
 func: => i++
 
 print(1 > func() < 1)
+
+
+# The conditional assignment based on existence.
+
+a: 5
+a: null
+a ?= 10
+b ?= 10
+
+print(a is 10 and b is 10)
\ No newline at end of file