diff --git a/src/lib/es2015.iterable.d.ts b/src/lib/es2015.iterable.d.ts
index 07b7322dd1a12..b986bc06a3969 100644
--- a/src/lib/es2015.iterable.d.ts
+++ b/src/lib/es2015.iterable.d.ts
@@ -168,7 +168,7 @@ interface MapConstructor {
 interface WeakMap<K extends WeakKey, V> {}
 
 interface WeakMapConstructor {
-    new <K extends WeakKey, V>(iterable: Iterable<readonly [K, V]>): WeakMap<K, V>;
+    new <K extends WeakKey = WeakKey, V = any>(iterable?: Iterable<readonly [K, V]> | null): WeakMap<K, V>;
 }
 
 interface SetIterator<T> extends IteratorObject<T, BuiltinIteratorReturn, unknown> {
diff --git a/tests/baselines/reference/dissallowSymbolAsWeakType.errors.txt b/tests/baselines/reference/dissallowSymbolAsWeakType.errors.txt
index 8e1b7bb3ff954..bca1e6d3df2e3 100644
--- a/tests/baselines/reference/dissallowSymbolAsWeakType.errors.txt
+++ b/tests/baselines/reference/dissallowSymbolAsWeakType.errors.txt
@@ -12,7 +12,7 @@ dissallowSymbolAsWeakType.ts(4,8): error TS2345: Argument of type 'symbol' is no
 dissallowSymbolAsWeakType.ts(5,8): error TS2345: Argument of type 'symbol' is not assignable to parameter of type 'object'.
 dissallowSymbolAsWeakType.ts(6,11): error TS2345: Argument of type 'symbol' is not assignable to parameter of type 'object'.
 dissallowSymbolAsWeakType.ts(8,16): error TS2769: No overload matches this call.
-  Overload 1 of 2, '(iterable: Iterable<readonly [object, boolean]>): WeakMap<object, boolean>', gave the following error.
+  Overload 1 of 2, '(iterable?: Iterable<readonly [object, boolean]> | null | undefined): WeakMap<object, boolean>', gave the following error.
     Argument of type '[symbol, false][]' is not assignable to parameter of type 'Iterable<readonly [object, boolean]>'.
       The types returned by '[Symbol.iterator]().next(...)' are incompatible between these types.
         Type 'IteratorResult<[symbol, false], undefined>' is not assignable to type 'IteratorResult<readonly [object, boolean], any>'.
@@ -60,7 +60,7 @@ dissallowSymbolAsWeakType.ts(19,14): error TS2345: Argument of type 'symbol' is
     const wm = new WeakMap([[s, false]]);
                    ~~~~~~~
 !!! error TS2769: No overload matches this call.
-!!! error TS2769:   Overload 1 of 2, '(iterable: Iterable<readonly [object, boolean]>): WeakMap<object, boolean>', gave the following error.
+!!! error TS2769:   Overload 1 of 2, '(iterable?: Iterable<readonly [object, boolean]> | null | undefined): WeakMap<object, boolean>', gave the following error.
 !!! error TS2769:     Argument of type '[symbol, false][]' is not assignable to parameter of type 'Iterable<readonly [object, boolean]>'.
 !!! error TS2769:       The types returned by '[Symbol.iterator]().next(...)' are incompatible between these types.
 !!! error TS2769:         Type 'IteratorResult<[symbol, false], undefined>' is not assignable to type 'IteratorResult<readonly [object, boolean], any>'.
diff --git a/tests/baselines/reference/extendingSetWithCheckJs.symbols b/tests/baselines/reference/extendingCollectionsWithCheckJs.symbols
similarity index 78%
rename from tests/baselines/reference/extendingSetWithCheckJs.symbols
rename to tests/baselines/reference/extendingCollectionsWithCheckJs.symbols
index 3d5eec300e8ff..1adf6491d1e1c 100644
--- a/tests/baselines/reference/extendingSetWithCheckJs.symbols
+++ b/tests/baselines/reference/extendingCollectionsWithCheckJs.symbols
@@ -1,8 +1,8 @@
-//// [tests/cases/compiler/extendingSetWithCheckJs.ts] ////
+//// [tests/cases/compiler/extendingCollectionsWithCheckJs.ts] ////
 
-=== extendingSetWithCheckJs.ts ===
+=== extendingCollectionsWithCheckJs.js ===
 class MySet extends Set {
->MySet : Symbol(MySet, Decl(extendingSetWithCheckJs.ts, 0, 0))
+>MySet : Symbol(MySet, Decl(extendingCollectionsWithCheckJs.js, 0, 0))
 >Set : Symbol(Set, Decl(lib.es2015.collection.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2015.symbol.wellknown.d.ts, --, --))
 
     constructor() {
@@ -12,7 +12,7 @@ class MySet extends Set {
 }
 
 class MyWeakSet extends WeakSet {
->MyWeakSet : Symbol(MyWeakSet, Decl(extendingSetWithCheckJs.ts, 4, 1))
+>MyWeakSet : Symbol(MyWeakSet, Decl(extendingCollectionsWithCheckJs.js, 4, 1))
 >WeakSet : Symbol(WeakSet, Decl(lib.es2015.collection.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2015.symbol.wellknown.d.ts, --, --))
 
     constructor() {
@@ -22,7 +22,7 @@ class MyWeakSet extends WeakSet {
 }
 
 class MyMap extends Map {
->MyMap : Symbol(MyMap, Decl(extendingSetWithCheckJs.ts, 10, 1))
+>MyMap : Symbol(MyMap, Decl(extendingCollectionsWithCheckJs.js, 10, 1))
 >Map : Symbol(Map, Decl(lib.es2015.collection.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2015.symbol.wellknown.d.ts, --, --))
 
     constructor() {
@@ -32,7 +32,7 @@ class MyMap extends Map {
 }
 
 class MyWeakMap extends WeakMap {
->MyWeakMap : Symbol(MyWeakMap, Decl(extendingSetWithCheckJs.ts, 16, 1))
+>MyWeakMap : Symbol(MyWeakMap, Decl(extendingCollectionsWithCheckJs.js, 16, 1))
 >WeakMap : Symbol(WeakMap, Decl(lib.es2015.collection.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2015.symbol.wellknown.d.ts, --, --))
 
     constructor() {
@@ -40,3 +40,4 @@ class MyWeakMap extends WeakMap {
 >super : Symbol(WeakMapConstructor, Decl(lib.es2015.collection.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --))
     }
 }
+
diff --git a/tests/baselines/reference/extendingSetWithCheckJs.types b/tests/baselines/reference/extendingCollectionsWithCheckJs.types
similarity index 84%
rename from tests/baselines/reference/extendingSetWithCheckJs.types
rename to tests/baselines/reference/extendingCollectionsWithCheckJs.types
index 8e2fa141e55de..002f13c4bf855 100644
--- a/tests/baselines/reference/extendingSetWithCheckJs.types
+++ b/tests/baselines/reference/extendingCollectionsWithCheckJs.types
@@ -1,6 +1,6 @@
-//// [tests/cases/compiler/extendingSetWithCheckJs.ts] ////
+//// [tests/cases/compiler/extendingCollectionsWithCheckJs.ts] ////
 
-=== extendingSetWithCheckJs.ts ===
+=== extendingCollectionsWithCheckJs.js ===
 class MySet extends Set {
 >MySet : MySet
 >      : ^^^^^
@@ -60,3 +60,4 @@ class MyWeakMap extends WeakMap {
 >      : ^^^^^^^^^^^^^^^^^^
     }
 }
+
diff --git a/tests/baselines/reference/newMap.errors.txt b/tests/baselines/reference/newMap.errors.txt
index 5b24f2962ce9c..ac5cce1f5ed80 100644
--- a/tests/baselines/reference/newMap.errors.txt
+++ b/tests/baselines/reference/newMap.errors.txt
@@ -5,4 +5,5 @@ newMap.ts(1,9): error TS2743: No overload expects 1 type arguments, but overload
     new Map<string>();
             ~~~~~~
 !!! error TS2743: No overload expects 1 type arguments, but overloads do exist that expect either 0 or 2 type arguments.
+    new WeakMap<object>();
     
\ No newline at end of file
diff --git a/tests/baselines/reference/newMap.js b/tests/baselines/reference/newMap.js
index b279f97b68ebf..027d6e291ac5a 100644
--- a/tests/baselines/reference/newMap.js
+++ b/tests/baselines/reference/newMap.js
@@ -2,8 +2,10 @@
 
 //// [newMap.ts]
 new Map<string>();
+new WeakMap<object>();
 
 
 //// [newMap.js]
 "use strict";
 new Map();
+new WeakMap();
diff --git a/tests/baselines/reference/newMap.symbols b/tests/baselines/reference/newMap.symbols
index d36bcb439d787..a18449151bd19 100644
--- a/tests/baselines/reference/newMap.symbols
+++ b/tests/baselines/reference/newMap.symbols
@@ -4,3 +4,6 @@
 new Map<string>();
 >Map : Symbol(Map, Decl(lib.es2015.collection.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2015.symbol.wellknown.d.ts, --, --))
 
+new WeakMap<object>();
+>WeakMap : Symbol(WeakMap, Decl(lib.es2015.collection.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2015.symbol.wellknown.d.ts, --, --))
+
diff --git a/tests/baselines/reference/newMap.types b/tests/baselines/reference/newMap.types
index 5ee1100b302f3..d04382cd361de 100644
--- a/tests/baselines/reference/newMap.types
+++ b/tests/baselines/reference/newMap.types
@@ -7,3 +7,9 @@ new Map<string>();
 >Map : MapConstructor
 >    : ^^^^^^^^^^^^^^
 
+new WeakMap<object>();
+>new WeakMap<object>() : WeakMap<object, any>
+>                      : ^^^^^^^^^^^^^^^^^^^^
+>WeakMap : WeakMapConstructor
+>        : ^^^^^^^^^^^^^^^^^^
+
diff --git a/tests/cases/compiler/extendingSetWithCheckJs.ts b/tests/cases/compiler/extendingCollectionsWithCheckJs.ts
similarity index 82%
rename from tests/cases/compiler/extendingSetWithCheckJs.ts
rename to tests/cases/compiler/extendingCollectionsWithCheckJs.ts
index 2a62243e25630..32345344abf3a 100644
--- a/tests/cases/compiler/extendingSetWithCheckJs.ts
+++ b/tests/cases/compiler/extendingCollectionsWithCheckJs.ts
@@ -4,6 +4,7 @@
 // @noEmit: true
 // @lib: es2017
 
+// @filename: extendingCollectionsWithCheckJs.js
 class MySet extends Set {
     constructor() {
         super();
@@ -26,4 +27,4 @@ class MyWeakMap extends WeakMap {
     constructor() {
         super();
     }
-}
\ No newline at end of file
+}
diff --git a/tests/cases/compiler/newMap.ts b/tests/cases/compiler/newMap.ts
index a72cd4a5afbea..14854f5d49f06 100644
--- a/tests/cases/compiler/newMap.ts
+++ b/tests/cases/compiler/newMap.ts
@@ -1,3 +1,4 @@
 // @target: es2015
 // @lib: es6
 new Map<string>();
+new WeakMap<object>();