diff --git a/libraries/stdlib/js/src/org.w3c/org.w3c.dom.kt b/libraries/stdlib/js/src/org.w3c/org.w3c.dom.kt
index 70cbd4e7c5e..db78e113095 100644
--- a/libraries/stdlib/js/src/org.w3c/org.w3c.dom.kt
+++ b/libraries/stdlib/js/src/org.w3c/org.w3c.dom.kt
@@ -1793,7 +1793,7 @@ public external interface ElementDefinitionOptions {
 }
 
 @kotlin.internal.InlineOnly
-public inline fun ElementDefinitionOptions(extends: String? = null): ElementDefinitionOptions {
+public inline fun ElementDefinitionOptions(extends: String? = undefined): ElementDefinitionOptions {
     val o = js("({})")
 
     o["extends"] = extends
@@ -2109,7 +2109,7 @@ public external interface PromiseRejectionEventInit : EventInit {
 }
 
 @kotlin.internal.InlineOnly
-public inline fun PromiseRejectionEventInit(promise: Promise<Any?>?, reason: Any? = null, bubbles: Boolean? = false, cancelable: Boolean? = false, composed: Boolean? = false): PromiseRejectionEventInit {
+public inline fun PromiseRejectionEventInit(promise: Promise<Any?>?, reason: Any? = undefined, bubbles: Boolean? = false, cancelable: Boolean? = false, composed: Boolean? = false): PromiseRejectionEventInit {
     val o = js("({})")
 
     o["promise"] = promise
@@ -2365,7 +2365,7 @@ public external interface ImageBitmapOptions {
 }
 
 @kotlin.internal.InlineOnly
-public inline fun ImageBitmapOptions(imageOrientation: ImageOrientation? = ImageOrientation.NONE, premultiplyAlpha: PremultiplyAlpha? = PremultiplyAlpha.DEFAULT, colorSpaceConversion: ColorSpaceConversion? = ColorSpaceConversion.DEFAULT, resizeWidth: Int? = null, resizeHeight: Int? = null, resizeQuality: ResizeQuality? = ResizeQuality.LOW): ImageBitmapOptions {
+public inline fun ImageBitmapOptions(imageOrientation: ImageOrientation? = ImageOrientation.NONE, premultiplyAlpha: PremultiplyAlpha? = PremultiplyAlpha.DEFAULT, colorSpaceConversion: ColorSpaceConversion? = ColorSpaceConversion.DEFAULT, resizeWidth: Int? = undefined, resizeHeight: Int? = undefined, resizeQuality: ResizeQuality? = ResizeQuality.LOW): ImageBitmapOptions {
     val o = js("({})")
 
     o["imageOrientation"] = imageOrientation
@@ -2416,9 +2416,7 @@ public inline fun MessageEventInit(data: Any? = null, origin: String? = "", last
     o["origin"] = origin
     o["lastEventId"] = lastEventId
     o["source"] = source
-    if (ports != null) {
-        o["ports"] = ports
-    }
+    o["ports"] = ports
     o["bubbles"] = bubbles
     o["cancelable"] = cancelable
     o["composed"] = composed
@@ -2998,7 +2996,7 @@ public external interface MutationObserverInit {
 }
 
 @kotlin.internal.InlineOnly
-public inline fun MutationObserverInit(childList: Boolean? = false, attributes: Boolean? = null, characterData: Boolean? = null, subtree: Boolean? = false, attributeOldValue: Boolean? = null, characterDataOldValue: Boolean? = null, attributeFilter: Array<String>? = null): MutationObserverInit {
+public inline fun MutationObserverInit(childList: Boolean? = false, attributes: Boolean? = undefined, characterData: Boolean? = undefined, subtree: Boolean? = false, attributeOldValue: Boolean? = undefined, characterDataOldValue: Boolean? = undefined, attributeFilter: Array<String>? = undefined): MutationObserverInit {
     val o = js("({})")
 
     o["childList"] = childList
@@ -3007,9 +3005,7 @@ public inline fun MutationObserverInit(childList: Boolean? = false, attributes:
     o["subtree"] = subtree
     o["attributeOldValue"] = attributeOldValue
     o["characterDataOldValue"] = characterDataOldValue
-    if (attributeFilter != null) {
-        o["attributeFilter"] = attributeFilter
-    }
+    o["attributeFilter"] = attributeFilter
 
     return o
 }
@@ -3122,7 +3118,7 @@ public external interface ElementCreationOptions {
 }
 
 @kotlin.internal.InlineOnly
-public inline fun ElementCreationOptions(is_: String? = null): ElementCreationOptions {
+public inline fun ElementCreationOptions(is_: String? = undefined): ElementCreationOptions {
     val o = js("({})")
 
     o["is"] = is_
@@ -3706,7 +3702,7 @@ public external interface ScrollToOptions : ScrollOptions {
 }
 
 @kotlin.internal.InlineOnly
-public inline fun ScrollToOptions(left: Double? = null, top: Double? = null, behavior: ScrollBehavior? = ScrollBehavior.AUTO): ScrollToOptions {
+public inline fun ScrollToOptions(left: Double? = undefined, top: Double? = undefined, behavior: ScrollBehavior? = ScrollBehavior.AUTO): ScrollToOptions {
     val o = js("({})")
 
     o["left"] = left
@@ -3810,7 +3806,7 @@ public external interface BoxQuadOptions {
 }
 
 @kotlin.internal.InlineOnly
-public inline fun BoxQuadOptions(box: CSSBoxType? = CSSBoxType.BORDER, relativeTo: dynamic = null): BoxQuadOptions {
+public inline fun BoxQuadOptions(box: CSSBoxType? = CSSBoxType.BORDER, relativeTo: dynamic = undefined): BoxQuadOptions {
     val o = js("({})")
 
     o["box"] = box
diff --git a/libraries/stdlib/js/src/org.w3c/org.w3c.fetch.kt b/libraries/stdlib/js/src/org.w3c/org.w3c.fetch.kt
index 7a01e68b0b6..f047eca71ae 100644
--- a/libraries/stdlib/js/src/org.w3c/org.w3c.fetch.kt
+++ b/libraries/stdlib/js/src/org.w3c/org.w3c.fetch.kt
@@ -112,7 +112,7 @@ public external interface RequestInit {
 }
 
 @kotlin.internal.InlineOnly
-public inline fun RequestInit(method: String? = null, headers: dynamic = null, body: dynamic = null, referrer: String? = null, referrerPolicy: dynamic = null, mode: RequestMode? = null, credentials: RequestCredentials? = null, cache: RequestCache? = null, redirect: RequestRedirect? = null, integrity: String? = null, keepalive: Boolean? = null, window: Any? = null): RequestInit {
+public inline fun RequestInit(method: String? = undefined, headers: dynamic = undefined, body: dynamic = undefined, referrer: String? = undefined, referrerPolicy: dynamic = undefined, mode: RequestMode? = undefined, credentials: RequestCredentials? = undefined, cache: RequestCache? = undefined, redirect: RequestRedirect? = undefined, integrity: String? = undefined, keepalive: Boolean? = undefined, window: Any? = undefined): RequestInit {
     val o = js("({})")
 
     o["method"] = method
@@ -171,7 +171,7 @@ public external interface ResponseInit {
 }
 
 @kotlin.internal.InlineOnly
-public inline fun ResponseInit(status: Short? = 200, statusText: String? = "OK", headers: dynamic = null): ResponseInit {
+public inline fun ResponseInit(status: Short? = 200, statusText: String? = "OK", headers: dynamic = undefined): ResponseInit {
     val o = js("({})")
 
     o["status"] = status
diff --git a/libraries/stdlib/js/src/org.w3c/org.w3c.files.kt b/libraries/stdlib/js/src/org.w3c/org.w3c.files.kt
index 2647be2622e..f174194d0f1 100644
--- a/libraries/stdlib/js/src/org.w3c/org.w3c.files.kt
+++ b/libraries/stdlib/js/src/org.w3c/org.w3c.files.kt
@@ -64,7 +64,7 @@ public external interface FilePropertyBag : BlobPropertyBag {
 }
 
 @kotlin.internal.InlineOnly
-public inline fun FilePropertyBag(lastModified: Int? = null, type: String? = ""): FilePropertyBag {
+public inline fun FilePropertyBag(lastModified: Int? = undefined, type: String? = ""): FilePropertyBag {
     val o = js("({})")
 
     o["lastModified"] = lastModified
diff --git a/libraries/stdlib/js/src/org.w3c/org.w3c.notifications.kt b/libraries/stdlib/js/src/org.w3c/org.w3c.notifications.kt
index cbd0e001dcd..6f66cda7460 100644
--- a/libraries/stdlib/js/src/org.w3c/org.w3c.notifications.kt
+++ b/libraries/stdlib/js/src/org.w3c/org.w3c.notifications.kt
@@ -111,7 +111,7 @@ public external interface NotificationOptions {
 }
 
 @kotlin.internal.InlineOnly
-public inline fun NotificationOptions(dir: NotificationDirection? = NotificationDirection.AUTO, lang: String? = "", body: String? = "", tag: String? = "", image: String? = null, icon: String? = null, badge: String? = null, sound: String? = null, vibrate: dynamic = null, timestamp: Number? = null, renotify: Boolean? = false, silent: Boolean? = false, noscreen: Boolean? = false, requireInteraction: Boolean? = false, sticky: Boolean? = false, data: Any? = null, actions: Array<NotificationAction>? = arrayOf()): NotificationOptions {
+public inline fun NotificationOptions(dir: NotificationDirection? = NotificationDirection.AUTO, lang: String? = "", body: String? = "", tag: String? = "", image: String? = undefined, icon: String? = undefined, badge: String? = undefined, sound: String? = undefined, vibrate: dynamic = undefined, timestamp: Number? = undefined, renotify: Boolean? = false, silent: Boolean? = false, noscreen: Boolean? = false, requireInteraction: Boolean? = false, sticky: Boolean? = false, data: Any? = null, actions: Array<NotificationAction>? = arrayOf()): NotificationOptions {
     val o = js("({})")
 
     o["dir"] = dir
@@ -130,9 +130,7 @@ public inline fun NotificationOptions(dir: NotificationDirection? = Notification
     o["requireInteraction"] = requireInteraction
     o["sticky"] = sticky
     o["data"] = data
-    if (actions != null) {
-        o["actions"] = actions
-    }
+    o["actions"] = actions
 
     return o
 }
@@ -150,7 +148,7 @@ public external interface NotificationAction {
 }
 
 @kotlin.internal.InlineOnly
-public inline fun NotificationAction(action: String?, title: String?, icon: String? = null): NotificationAction {
+public inline fun NotificationAction(action: String?, title: String?, icon: String? = undefined): NotificationAction {
     val o = js("({})")
 
     o["action"] = action
diff --git a/libraries/stdlib/js/src/org.w3c/org.w3c.workers.kt b/libraries/stdlib/js/src/org.w3c/org.w3c.workers.kt
index e1554889e9a..4619c2d75e3 100644
--- a/libraries/stdlib/js/src/org.w3c/org.w3c.workers.kt
+++ b/libraries/stdlib/js/src/org.w3c/org.w3c.workers.kt
@@ -91,7 +91,7 @@ public external interface RegistrationOptions {
 }
 
 @kotlin.internal.InlineOnly
-public inline fun RegistrationOptions(scope: String? = null, type: WorkerType? = WorkerType.CLASSIC): RegistrationOptions {
+public inline fun RegistrationOptions(scope: String? = undefined, type: WorkerType? = WorkerType.CLASSIC): RegistrationOptions {
     val o = js("({})")
 
     o["scope"] = scope
@@ -130,16 +130,14 @@ public external interface ServiceWorkerMessageEventInit : EventInit {
 }
 
 @kotlin.internal.InlineOnly
-public inline fun ServiceWorkerMessageEventInit(data: Any? = null, origin: String? = null, lastEventId: String? = null, source: UnionMessagePortOrServiceWorker? = null, ports: Array<MessagePort>? = null, bubbles: Boolean? = false, cancelable: Boolean? = false, composed: Boolean? = false): ServiceWorkerMessageEventInit {
+public inline fun ServiceWorkerMessageEventInit(data: Any? = undefined, origin: String? = undefined, lastEventId: String? = undefined, source: UnionMessagePortOrServiceWorker? = undefined, ports: Array<MessagePort>? = undefined, bubbles: Boolean? = false, cancelable: Boolean? = false, composed: Boolean? = false): ServiceWorkerMessageEventInit {
     val o = js("({})")
 
     o["data"] = data
     o["origin"] = origin
     o["lastEventId"] = lastEventId
     o["source"] = source
-    if (ports != null) {
-        o["ports"] = ports
-    }
+    o["ports"] = ports
     o["bubbles"] = bubbles
     o["cancelable"] = cancelable
     o["composed"] = composed
@@ -237,12 +235,8 @@ public external interface ForeignFetchOptions {
 public inline fun ForeignFetchOptions(scopes: Array<String>?, origins: Array<String>?): ForeignFetchOptions {
     val o = js("({})")
 
-    if (scopes != null) {
-        o["scopes"] = scopes
-    }
-    if (origins != null) {
-        o["origins"] = origins
-    }
+    o["scopes"] = scopes
+    o["origins"] = origins
 
     return o
 }
@@ -324,14 +318,12 @@ public external interface ForeignFetchResponse {
 }
 
 @kotlin.internal.InlineOnly
-public inline fun ForeignFetchResponse(response: Response?, origin: String? = null, headers: Array<String>? = null): ForeignFetchResponse {
+public inline fun ForeignFetchResponse(response: Response?, origin: String? = undefined, headers: Array<String>? = undefined): ForeignFetchResponse {
     val o = js("({})")
 
     o["response"] = response
     o["origin"] = origin
-    if (headers != null) {
-        o["headers"] = headers
-    }
+    o["headers"] = headers
 
     return o
 }
@@ -366,16 +358,14 @@ public external interface ExtendableMessageEventInit : ExtendableEventInit {
 }
 
 @kotlin.internal.InlineOnly
-public inline fun ExtendableMessageEventInit(data: Any? = null, origin: String? = null, lastEventId: String? = null, source: UnionClientOrMessagePortOrServiceWorker? = null, ports: Array<MessagePort>? = null, bubbles: Boolean? = false, cancelable: Boolean? = false, composed: Boolean? = false): ExtendableMessageEventInit {
+public inline fun ExtendableMessageEventInit(data: Any? = undefined, origin: String? = undefined, lastEventId: String? = undefined, source: UnionClientOrMessagePortOrServiceWorker? = undefined, ports: Array<MessagePort>? = undefined, bubbles: Boolean? = false, cancelable: Boolean? = false, composed: Boolean? = false): ExtendableMessageEventInit {
     val o = js("({})")
 
     o["data"] = data
     o["origin"] = origin
     o["lastEventId"] = lastEventId
     o["source"] = source
-    if (ports != null) {
-        o["ports"] = ports
-    }
+    o["ports"] = ports
     o["bubbles"] = bubbles
     o["cancelable"] = cancelable
     o["composed"] = composed
@@ -412,7 +402,7 @@ public external interface CacheQueryOptions {
 }
 
 @kotlin.internal.InlineOnly
-public inline fun CacheQueryOptions(ignoreSearch: Boolean? = false, ignoreMethod: Boolean? = false, ignoreVary: Boolean? = false, cacheName: String? = null): CacheQueryOptions {
+public inline fun CacheQueryOptions(ignoreSearch: Boolean? = false, ignoreMethod: Boolean? = false, ignoreVary: Boolean? = false, cacheName: String? = undefined): CacheQueryOptions {
     val o = js("({})")
 
     o["ignoreSearch"] = ignoreSearch
@@ -439,7 +429,7 @@ public external interface CacheBatchOperation {
 }
 
 @kotlin.internal.InlineOnly
-public inline fun CacheBatchOperation(type: String? = null, request: Request? = null, response: Response? = null, options: CacheQueryOptions? = null): CacheBatchOperation {
+public inline fun CacheBatchOperation(type: String? = undefined, request: Request? = undefined, response: Response? = undefined, options: CacheQueryOptions? = undefined): CacheBatchOperation {
     val o = js("({})")
 
     o["type"] = type
diff --git a/libraries/tools/idl2k/src/main/kotlin/render.kt b/libraries/tools/idl2k/src/main/kotlin/render.kt
index 49955276509..6d1879f7527 100644
--- a/libraries/tools/idl2k/src/main/kotlin/render.kt
+++ b/libraries/tools/idl2k/src/main/kotlin/render.kt
@@ -17,7 +17,6 @@
 package org.jetbrains.idl2k
 
 import org.jetbrains.idl2k.util.mapEnumConstant
-import java.io.*
 import java.math.BigInteger
 
 private fun <O : Appendable> O.indent(commented: Boolean = false, level: Int) {
@@ -88,7 +87,7 @@ private val keywords = setOf("interface", "is", "as")
 
 private fun String.parse() = if (this.startsWith("0x")) BigInteger(this.substring(2), 16) else BigInteger(this)
 private fun String.replaceWrongConstants(type: Type) = when {
-    this == "null" && type.nullable -> "null"
+    this == "undefined" && type.nullable -> "undefined"
     this == "definedExternally" || type is SimpleType && type.type == "Int" && parse() > BigInteger.valueOf(Int.MAX_VALUE.toLong()) -> "definedExternally"
     type is SimpleType && type.type == "Double" && this.matches("[0-9]+".toRegex()) -> "${this}.0"
     else -> this
@@ -286,12 +285,15 @@ private fun GenerateAttribute.kindNotChanged(superAttributesByName: Map<String,
 
 private fun GenerateAttribute.hasSuperImplementation(allSuperTypes: List<GenerateTraitOrClass>) = allSuperTypes.any { st -> st.kind != GenerateDefinitionKind.INTERFACE && st.memberAttributes.any { it.signature == signature } }
 
+private fun GenerateAttribute.hasNoDefaultValue() =
+     this.initializer == null && (this.type.nullable || this.type == DynamicType) && !this.required
+
 fun Appendable.renderBuilderFunction(dictionary: GenerateTraitOrClass, allSuperTypes: List<GenerateTraitOrClass>, allTypes: Set<String>) {
     val fields = (dictionary.memberAttributes + allSuperTypes.flatMap { it.memberAttributes })
             .distinctBy { it.signature }
             .map { it.copy(kind = AttributeKind.ARGUMENT) }
             .dynamicIfUnknownType(allTypes)
-            .map { if (it.initializer == null && (it.type.nullable || it.type == DynamicType) && !it.required) it.copy(initializer = "null") else it }
+            .map { if (it.hasNoDefaultValue()) it.copy(initializer = "undefined") else it }
 
     appendln("@kotlin.internal.InlineOnly")
     append("public inline fun ${dictionary.name}")
@@ -306,10 +308,10 @@ fun Appendable.renderBuilderFunction(dictionary: GenerateTraitOrClass, allSuperT
         indent(level = 1)
 
         val escapedFieldName = field.name.replaceKeywords()
-        val nullGuardedAssignment = field.type is ArrayType
+        val nullGuardedAssignment = field.hasNoDefaultValue()
 
         if (nullGuardedAssignment) {
-            appendln("if ($escapedFieldName != null) {")
+            appendln("if ($escapedFieldName !== undefined) {")
             indent(level = 2)
         }