use null instead of undefined as an empty placeholder in some cases

packages/core-js/internals/collection-strong.js

@@ -23,8 +23,8 @@ module.exports = {
       setInternalState(that, {
         type: CONSTRUCTOR_NAME,
         index: create(null),
-        first: undefined,
-        last: undefined,
+        first: null,
+        last: null,
         size: 0
       });
       if (!DESCRIPTORS) that.size = 0;
@@ -49,7 +49,7 @@ module.exports = {
           key: key,
           value: value,
           previous: previous = state.last,
-          next: undefined,
+          next: null,
           removed: false
         };
         if (!state.first) state.first = entry;
@@ -83,10 +83,10 @@ module.exports = {
         var entry = state.first;
         while (entry) {
           entry.removed = true;
-          if (entry.previous) entry.previous = entry.previous.next = undefined;
+          if (entry.previous) entry.previous = entry.previous.next = null;
           entry = entry.next;
         }
-        state.first = state.last = undefined;
+        state.first = state.last = null;
         state.index = create(null);
         if (DESCRIPTORS) state.size = 0;
         else that.size = 0;
@@ -178,7 +178,7 @@ module.exports = {
         target: iterated,
         state: getInternalCollectionState(iterated),
         kind: kind,
-        last: undefined
+        last: null
       });
     }, function () {
       var state = getInternalIteratorState(this);
@@ -189,7 +189,7 @@ module.exports = {
       // get next entry
       if (!state.target || !(state.last = entry = entry ? entry.next : state.state.first)) {
         // or finish the iteration
-        state.target = undefined;
+        state.target = null;
         return createIterResultObject(undefined, true);
       }
       // return step by kind

packages/core-js/internals/collection-weak.js

@@ -62,7 +62,7 @@ module.exports = {
       setInternalState(that, {
         type: CONSTRUCTOR_NAME,
         id: id++,
-        frozen: undefined
+        frozen: null
       });
       if (!isNullOrUndefined(iterable)) iterate(iterable, that[ADDER], { that: that, AS_ENTRIES: IS_MAP });
     });
@@ -110,7 +110,7 @@ module.exports = {
         if (isObject(key)) {
           var data = getWeakData(key);
           if (data === true) return uncaughtFrozenStore(state).get(key);
-          return data ? data[state.id] : undefined;
+          if (data) return data[state.id];
         }
       },
       // `WeakMap.prototype.set(key, value)` method

packages/core-js/modules/es.array.iterator.js

@@ -37,7 +37,7 @@ module.exports = defineIterator(Array, 'Array', function (iterated, kind) {
   var target = state.target;
   var index = state.index++;
   if (!target || index >= target.length) {
-    state.target = undefined;
+    state.target = null;
     return createIterResultObject(undefined, true);
   }
   switch (state.kind) {

packages/core-js/modules/es.promise.constructor.js

@@ -220,7 +220,7 @@ if (FORCED_PROMISE_CONSTRUCTOR) {
       reactions: new Queue(),
       rejection: false,
       state: PENDING,
-      value: undefined
+      value: null
     });
   };
 

packages/core-js/modules/esnext.async-disposable-stack.constructor.js

@@ -72,14 +72,14 @@ defineBuiltIns(AsyncDisposableStackPrototype, {
       var loop = function () {
         if (i) {
           var disposeMethod = stack[--i];
-          stack[i] = undefined;
+          stack[i] = null;
           try {
             Promise.resolve(disposeMethod()).then(loop, handleError);
           } catch (error) {
             handleError(error);
           }
         } else {
-          internalState.stack = undefined;
+          internalState.stack = null;
           thrown ? reject(suppressed) : resolve(undefined);
         }
       };

packages/core-js/modules/esnext.disposable-stack.constructor.js

@@ -56,7 +56,7 @@ defineBuiltIns(DisposableStackPrototype, {
     var suppressed;
     while (i) {
       var disposeMethod = stack[--i];
-      stack[i] = undefined;
+      stack[i] = null;
       try {
         disposeMethod();
       } catch (errorResult) {
@@ -68,7 +68,7 @@ defineBuiltIns(DisposableStackPrototype, {
         }
       }
     }
-    internalState.stack = undefined;
+    internalState.stack = null;
     if (thrown) throw suppressed;
   },
   use: function use(value) {

packages/core-js/modules/esnext.observable.constructor.js

@@ -30,16 +30,16 @@ var getSubscriptionObserverInternalState = getterFor(SUBSCRIPTION_OBSERVER);
 
 var SubscriptionState = function (observer) {
   this.observer = anObject(observer);
-  this.cleanup = undefined;
-  this.subscriptionObserver = undefined;
+  this.cleanup = null;
+  this.subscriptionObserver = null;
 };
 
 SubscriptionState.prototype = {
   type: SUBSCRIPTION,
   clean: function () {
     var cleanup = this.cleanup;
     if (cleanup) {
-      this.cleanup = undefined;
+      this.cleanup = null;
       try {
         cleanup();
       } catch (error) {
@@ -53,10 +53,10 @@ SubscriptionState.prototype = {
       var subscriptionObserver = this.subscriptionObserver;
       subscription.closed = true;
       if (subscriptionObserver) subscriptionObserver.closed = true;
-    } this.observer = undefined;
+    } this.observer = null;
   },
   isClosed: function () {
-    return this.observer === undefined;
+    return this.observer === null;
   }
 };
 

packages/core-js/modules/web.url-search-params.constructor.js

@@ -115,7 +115,7 @@ var URLSearchParamsIterator = createIteratorConstructor(function Iterator(params
   var target = state.target;
   var index = state.index++;
   if (!target || index >= target.length) {
-    state.target = undefined;
+    state.target = null;
     return createIterResultObject(undefined, true);
   }
   var entry = target[index];