destructuring assignment with empty array in object

Author: zdenko

lib/coffeescript/nodes.js

@@ -2215,7 +2215,13 @@ exports.Assign = class Assign extends Base
         @variable.base.lhs = yes
         # Check if @variable contains Obj with splats.
         hasSplat = @variable.contains (node) -> node instanceof Obj and node.hasSplat()
-        return @compileDestructuring o if not @variable.isAssignable() or @variable.isArray() and hasSplat
+        # Check if `@variable` containes an empty array as a property, e.g. `{a:[]} = b`.
+        # An empty array is not assignable and would invoke `@compileDestructuring` method,
+        # which is not needed since this is valid ES syntax and can be compiled directly.
+        hasArrayProp = @variable.contains (node) ->
+            node instanceof Assign and node.context is 'object' and node.value.base instanceof Arr
+        if (not @variable.isAssignable() or @variable.isArray() and hasSplat) and not hasArrayProp
+          return @compileDestructuring o
         # Object destructuring. Can be removed once ES proposal hits Stage 4.
         objDestructAnswer = @compileObjectDestruct(o) if @variable.isObject() and hasSplat
         return objDestructAnswer if objDestructAnswer
@@ -2226,7 +2232,7 @@ exports.Assign = class Assign extends Base
 
     unless @context
       varBase = @variable.unwrapAll()
-      unless varBase.isAssignable()
+      unless varBase.isAssignable() or hasArrayProp
         @variable.error "'#{@variable.compile o}' can't be assigned"
 
       varBase.eachName (name) =>

src/nodes.coffee

@@ -985,3 +985,21 @@ test "#4878: Compile error when using destructuring with a splat or expansion in
         []
 
   arrayEq bar(arr), ['a', ['b', 'c', 'd']]
+
+test "destructuring assignment with empty array in object", ->
+  obj =
+    a1: [1, 2]
+    b1: 3
+
+  {a1:[], b1} = obj
+  eq 'undefined', typeof a1
+  eq b1, 3
+
+  obj =
+    a2:
+      b2: [1, 2]
+    c2: 3
+
+  {a2: {b2:[]}, c2} = obj
+  eq 'undefined', typeof b2
+  eq c2, 3