even more aggressive checks for protocol pollution

index.js

@@ -68,13 +68,21 @@ module.exports = function (args, opts) {
 
     function setKey (obj, keys, value) {
         var o = obj;
-        keys.slice(0,-1).forEach(function (key) {
+        for (var i = 0; i < keys.length-1; i++) {
+            var key = keys[i];
+            if (key === '__proto__') return;
             if (o[key] === undefined) o[key] = {};
-            if (o[key] === {}.__proto__) o[key] = {};
+            if (o[key] === Object.prototype || o[key] === Number.prototype
+                || o[key] === String.prototype) o[key] = {};
+            if (o[key] === Array.prototype) o[key] = [];
             o = o[key];
-        });
+        }
 
         var key = keys[keys.length - 1];
+        if (key === '__proto__') return;
+        if (o === Object.prototype || o === Number.prototype
+            || o === String.prototype) o = {};
+        if (o === Array.prototype) o = [];
         if (o[key] === undefined || flags.bools[key] || typeof o[key] === 'boolean') {
             o[key] = value;
         }

test/proto.js

@@ -4,7 +4,7 @@ var test = require('tape');
 test('proto pollution', function (t) {
     var argv = parse(['--__proto__.x','123']);
     t.equal({}.x, undefined);
-    t.equal(argv.__proto__.x, 123);
+    t.equal(argv.__proto__.x, undefined);
     t.equal(argv.x, undefined);
     t.end();
 });
@@ -14,7 +14,7 @@ test('proto pollution (array)', function (t) {
     t.equal({}.z, undefined);
     t.deepEqual(argv.x, [4,5]);
     t.equal(argv.x.z, undefined);
-    t.equal(argv.x.__proto__.z, 789);
+    t.equal(argv.x.__proto__.z, undefined);
     t.end();
 });