diff --git a/js/src/frontend/TokenStream.cpp b/js/src/frontend/TokenStream.cpp
index 0c21ba6f7fa0..248c32ada93d 100644
--- a/js/src/frontend/TokenStream.cpp
+++ b/js/src/frontend/TokenStream.cpp
@@ -1509,9 +1509,11 @@ TokenStream::getTokenInternal(TokenKind* ttp, Modifier modifier)
         goto out;
 
       case '*':
+#ifdef JS_HAS_EXPONENTIATION
         if (matchChar('*'))
             tp->type = matchChar('=') ? TOK_POWASSIGN : TOK_POW;
         else
+#endif
             tp->type = matchChar('=') ? TOK_MULASSIGN : TOK_MUL;
         goto out;
 
@@ -1823,7 +1825,7 @@ TokenStream::getStringOrTemplateToken(int untilChar, Token** tp)
                     }
 
                     c = char16_t(val);
-                } 
+                }
                 break;
             }
         } else if (TokenBuf::isRawEOLChar(c)) {
diff --git a/js/src/jsversion.h b/js/src/jsversion.h
index bc2f833c1353..cf93a6aa6e5d 100644
--- a/js/src/jsversion.h
+++ b/js/src/jsversion.h
@@ -41,9 +41,14 @@
  */
 #define JS_OLD_GETTER_SETTER_METHODS    1
 
-/* Support for ES6 Classes. */
 #ifdef NIGHTLY_BUILD
+
+/* Support for ES6 Classes. */
 #define JS_HAS_CLASSES 1
-#endif
+
+/* Support for ES7 Exponentiation proposal. */
+//#define JS_HAS_EXPONENTIATION 1
+
+#endif // NIGHTLY_BUILD
 
 #endif /* jsversion_h */
diff --git a/js/src/tests/ecma_7/Math/Pow.js b/js/src/tests/ecma_7/Math/Pow.js
index 31c64770100f..c7cba3391aef 100644
--- a/js/src/tests/ecma_7/Math/Pow.js
+++ b/js/src/tests/ecma_7/Math/Pow.js
@@ -9,6 +9,8 @@ var summary = "Implement the exponentiation operator";
 
 print(BUGNUMBER + ": " + summary);
 
+var test = `
+
 // Constant folding
 assertEq(2 ** 2 ** 3, 256);
 assertEq(1 ** 1 ** 4, 1);
@@ -99,6 +101,18 @@ function assertTrue(v) {
 function assertFalse(v) {
     assertEq(v, false);
 }
+`;
+
+function exponentiationEnabled() {
+    try {
+        Function("1 ** 1");
+        return true;
+    } catch (e if e instanceof SyntaxError) { }
+    return false;
+}
+
+if (exponentiationEnabled())
+    eval(test);
 
 if (typeof reportCompare === "function")
     reportCompare(true, true);