Fix null values being treated as missing in 'put' method of a specialized string-keyed map

js/js.libraries/src/core/collections/InternalStringMap.kt

@@ -23,6 +23,10 @@ import kotlin.collections.MutableMap.MutableEntry
 
 /**
  * A simple wrapper around JavaScript Map for key type is string.
+ *
+ * Though this map is instantiated only with K=String, the K type is not fixed to String statically,
+ * because we want to have it erased to Any? in order not to generate type-safe override bridges for
+ * [get], [contains], [remove] etc, if they ever are generated.
  */
 internal class InternalStringMap<K, V>(override val equality: EqualityComparator) : InternalMap<K, V> {
 
@@ -55,7 +59,7 @@ internal class InternalStringMap<K, V>(override val equality: EqualityComparator
         val oldValue = backingMap[key]
         backingMap[key] = value
 
-        if (oldValue == undefined) {
+        if (oldValue === undefined) {
             size++
 //            structureChanged(host)
             return null
@@ -99,10 +103,12 @@ internal class InternalStringMap<K, V>(override val equality: EqualityComparator
             override fun next(): MutableEntry<K, V> {
                 val key = iterator.next()
                 lastKey = key
+                @Suppress("UNCHECKED_CAST")
                 return newMapEntry(key as K)
             }
 
             override fun remove() {
+                @Suppress("UNCHECKED_CAST")
                 this@InternalStringMap.remove(checkNotNull(lastKey) as K)
             }
         }

libraries/stdlib/test/js/MapJsTest.kt

@@ -45,7 +45,7 @@ class PrimitiveMapJsTest : MapJsTest() {
     override fun emptyMutableMapWithNullableKeyValue(): MutableMap<String?, Int?> = HashMap()
 
     @Test fun compareBehavior() {
-        val specialJsStringMap = HashMap<String, Any>()
+        val specialJsStringMap = stringMapOf<Any>()
         specialJsStringMap.put("k1", "v1")
         compare(genericHashMapOf("k1" to "v1"), specialJsStringMap) { mapBehavior() }
 
@@ -53,6 +53,20 @@ class PrimitiveMapJsTest : MapJsTest() {
         specialJsNumberMap.put(5, "v5")
         compare(genericHashMapOf(5 to "v5"), specialJsNumberMap) { mapBehavior() }
     }
+
+    @Test fun putNull() {
+        val map = stringMapOf("k" to null)
+        assertEquals(1, map.size)
+
+        map.put("k", null)
+        assertEquals(1, map.size)
+
+        map["k"] = null
+        assertEquals(1, map.size)
+
+        map.remove("k")
+        assertEquals(0, map.size)
+    }
 }
 
 class LinkedHashMapJsTest : MapJsTest() {