Ignore inherited object members in variable lookups

index.js

@@ -13,14 +13,20 @@ function variable (variables, node, str, name, opts, result) {
   if (isIgnore(node, name)) return str
 
   if (opts.only) {
-    if (typeof opts.only[name] !== 'undefined') {
+    if (
+      Object.prototype.hasOwnProperty.call(opts.only, name) &&
+      typeof opts.only[name] !== 'undefined'
+    ) {
       return opts.only[name]
     }
 
     return str
   }
 
-  if (typeof variables[name] !== 'undefined') {
+  if (
+    Object.prototype.hasOwnProperty.call(variables, name) &&
+    typeof variables[name] !== 'undefined'
+  ) {
     return variables[name]
   }
 

index.test.js

@@ -257,4 +257,27 @@ test('works inside function-like declarations', () => {
   )
 })
 
+test('rejects inherited object members as undefined variables', () => {
+  throws(() => {
+    postcss([plugin()]).process('a { color: $constructor }', {
+      from: '/test.css'
+    }).css
+  }, /Undefined variable/)
+})
+
+test('keeps inherited names unchanged in silent mode', () => {
+  run('a { color: $toString }', 'a { color: $toString }', { silent: true })
+})
+
+test('ignores inherited members of only', () => {
+  run('a { color: $constructor }', 'a { color: $constructor }', { only: {} })
+})
+
+test('allows explicitly defined object member names', () => {
+  run('$constructor: red; a { color: $constructor }', 'a { color: red }')
+  run('a { color: $toString }', 'a { color: blue }', {
+    only: { toString: 'blue' }
+  })
+})
+
 test.run()