diff --git a/js/public/HashTable.h b/js/public/HashTable.h
index f60014766c3b..918093c7e53c 100644
--- a/js/public/HashTable.h
+++ b/js/public/HashTable.h
@@ -295,8 +295,13 @@ class HashTable : private AllocPolicy
     uint64       mutationCount;
 #endif
 
-    static const unsigned sMinSizeLog2  = 4;
+    /* The default initial capacity is 16, but you can ask for as small as 4. */
+    static const unsigned sMinSizeLog2  = 2;
     static const unsigned sMinSize      = 1 << sMinSizeLog2;
+    static const unsigned sDefaultInitSizeLog2 = 4;
+  public:
+    static const unsigned sDefaultInitSize = 1 << sDefaultInitSizeLog2;
+  private:
     static const unsigned sMaxInit      = JS_BIT(23);
     static const unsigned sMaxCapacity  = JS_BIT(24);
     static const unsigned sHashBits     = tl::BitSize<HashNumber>::result;
@@ -986,7 +991,7 @@ class HashMap
      * init after constructing a HashMap and check the return value.
      */
     HashMap(AllocPolicy a = AllocPolicy()) : impl(a) {}
-    bool init(uint32 len = 0)                         { return impl.init(len); }
+    bool init(uint32 len = Impl::sDefaultInitSize)    { return impl.init(len); }
     bool initialized() const                          { return impl.initialized(); }
 
     /*
@@ -1214,7 +1219,7 @@ class HashSet
      * init after constructing a HashSet and check the return value.
      */
     HashSet(AllocPolicy a = AllocPolicy()) : impl(a) {}
-    bool init(uint32 len = 0)                         { return impl.init(len); }
+    bool init(uint32 len = Impl::sDefaultInitSize)    { return impl.init(len); }
     bool initialized() const                          { return impl.initialized(); }
 
     /*