diff --git a/package.json b/package.json
index 04e2296f..6dc68c95 100644
--- a/package.json
+++ b/package.json
@@ -72,7 +72,7 @@
     "build:dist": "npm run build:dist:dev && npm run build:dist:prod",
     "build": "npm run build:data && npm run build:lib && npm run build:dist",
     "lint": "eslint .",
-    "test": "cd tests && node sanity.js && node noderunner.js && node saucelabs.js",
+    "test": "cd tests && node polyfilling.js && node sanity.js && node noderunner.js && node saucelabs.js",
     "pretest": "npm run lint",
     "preversion": "npm run clean && npm run build && npm run test",
     "prepublish": "npm run clean && npm run build"
diff --git a/tests/polyfilling.js b/tests/polyfilling.js
new file mode 100644
index 00000000..b7813ff6
--- /dev/null
+++ b/tests/polyfilling.js
@@ -0,0 +1,34 @@
+const vm = require('vm');
+
+function assert(value, expected, message) {
+    console.log(message);
+    if (value !== expected) {
+        console.error(' > ERROR: expected value ' + expected + ' but the actual value is ' + value);
+        process.exit(1);
+    } else {
+        console.log(' > PASSED');
+    }
+}
+
+const context = new vm.createContext();
+var script = new vm.Script('this');
+const window = script.runInContext(context);
+
+const fs = require('fs');
+const code = fs.readFileSync(__dirname + '/../dist/Intl.js', 'utf8');
+
+// first evaluation
+window.window = window;  // circular, in case the polyfill uses window
+var originalIntl = window.Intl;
+var script = new vm.Script(code);
+script.runInContext(context);
+assert(typeof global.Intl, 'object', 'for this test to function, global.Intl is required');
+assert(typeof window.IntlPolyfill, 'object', 'polyfill should always add the custom global IntlPolyfill');
+assert(window.Intl, originalIntl, 'validating that the polyfilling process does not touch the original Intl value');
+
+// second evaluation
+window.window = window;  // circular, in case the polyfill uses window
+window.Intl = undefined; // disabling Intl
+var script = new vm.Script(code);
+script.runInContext(context);
+assert(window.Intl, window.IntlPolyfill, 'validating that the polyfilling process does patch Intl if it does not exist');
diff --git a/tests/sanity.js b/tests/sanity.js
index 07737c8a..bbd0d42d 100644
--- a/tests/sanity.js
+++ b/tests/sanity.js
@@ -1,24 +1,27 @@
 var IntlPolyfill = require('../');
 
-function assert(value, expected) {
+function assert(value, expected, message) {
+    console.log(message);
     if (value !== expected) {
-        console.log('expected value ' + expected + ' but the actual value is ' + value);
+        console.error(' > ERROR: expected value ' + expected + ' but the actual value is ' + value);
         process.exit(1);
+    } else {
+        console.log(' > PASSED');
     }
 }
 
 assert(new IntlPolyfill.NumberFormat('de-DE', {
     minimumFractionDigits: 2,
     maximumFractionDigits: 2
-}).format(0.015), "0,02");
+}).format(0.015), "0,02", 'fractional digits');
 
 assert(new IntlPolyfill.NumberFormat('en-US', {
     style: 'currency',
     currency: 'GBP',
     minimumFractionDigits: 2,
-}).format(59.88), '£59.88');
+}).format(59.88), '£59.88', 'currency with fragtional digits');
 
 assert(new IntlPolyfill.DateTimeFormat('en', {
     month:'numeric',
     day: 'numeric'
-}).format(new Date('2016/05/16')), '5/16');
+}).format(new Date('2016/05/16')), '5/16', 'month and day');