diff --git a/idea/testData/completion/basic/common/ExtensionForProperty.kt b/idea/testData/completion/basic/common/ExtensionForProperty.kt
index 2cc6f43a8ad..5f04657be34 100644
--- a/idea/testData/completion/basic/common/ExtensionForProperty.kt
+++ b/idea/testData/completion/basic/common/ExtensionForProperty.kt
@@ -5,7 +5,7 @@ class Test {
 // INVOCATION_COUNT: 1
 // EXIST: String~(jet)
 // EXIST: IllegalStateException
-// EXIST_JAVA_ONLY: StringBuilder
+// EXIST: StringBuilder
 // EXIST_JAVA_ONLY: StringBuffer
 // ABSENT: HTMLStyleElement
 // ABSENT: Statement@Statement~(java.sql)
diff --git a/idea/testData/completion/basic/common/ExtensionInsideFunction.kt b/idea/testData/completion/basic/common/ExtensionInsideFunction.kt
index 61f36bf7114..9fee7a1d3c2 100644
--- a/idea/testData/completion/basic/common/ExtensionInsideFunction.kt
+++ b/idea/testData/completion/basic/common/ExtensionInsideFunction.kt
@@ -17,6 +17,6 @@ class StrMore {
 // EXIST: StrSome
 // EXIST: StrMore
 // EXIST: StrInFun
-// EXIST_JAVA_ONLY: StringBuilder
+// EXIST: StringBuilder
 // EXIST_JAVA_ONLY: StringBuffer
 // ABSENT: StrAbsent
\ No newline at end of file
diff --git a/idea/testData/completion/basic/common/InParametersTypes.kt b/idea/testData/completion/basic/common/InParametersTypes.kt
index 103ad8e798c..8303fe7da07 100644
--- a/idea/testData/completion/basic/common/InParametersTypes.kt
+++ b/idea/testData/completion/basic/common/InParametersTypes.kt
@@ -8,7 +8,7 @@ class SomeClass {
 // EXIST: SomeClass, SomeInternal
 // EXIST: String~(jet)
 // EXIST: IllegalStateException
-// EXIST_JAVA_ONLY: StringBuilder
+// EXIST: StringBuilder
 // EXIST_JAVA_ONLY: StringBuffer
 // ABSENT: HTMLStyleElement
 // ABSENT: Statement@Statement~(java.sql)
\ No newline at end of file
diff --git a/js/js.libraries/src/core/javalang.kt b/js/js.libraries/src/core/javalang.kt
index 44ebefb712a..6ebcda3acb1 100644
--- a/js/js.libraries/src/core/javalang.kt
+++ b/js/js.libraries/src/core/javalang.kt
@@ -39,7 +39,17 @@ public trait Comparable<T> {
 
 library
 public trait Appendable {
-    public open fun append(csq: CharSequence?): Appendable?
-    public open fun append(csq: CharSequence?, start: Int, end: Int): Appendable?
-    public open fun append(c: Char): Appendable?
+    public open fun append(csq: CharSequence?): Appendable
+    //public open fun append(csq: CharSequence?, start: Int, end: Int): Appendable
+    public open fun append(c: Char): Appendable
+}
+
+library
+public class StringBuilder() : Appendable {
+    override fun append(c: Char): StringBuilder = js.noImpl
+    override fun append(csq: CharSequence?): StringBuilder = js.noImpl
+    //TODO
+    //override fun append(csq: CharSequence?, start: Int, end: Int): StringBuilder = js.noImpl
+    public fun append(obj: Any?): StringBuilder = js.noImpl
+    public fun toString(): String = js.noImpl
 }
diff --git a/js/js.libraries/src/core/javautil.kt b/js/js.libraries/src/core/javautil.kt
index e6f87ebe9c7..166b72411b8 100644
--- a/js/js.libraries/src/core/javautil.kt
+++ b/js/js.libraries/src/core/javautil.kt
@@ -103,15 +103,6 @@ public open class HashMap<K, V>(capacity: Int = 0) : MutableMap<K, V> {
     public override fun entrySet(): MutableSet<MutableMap.MutableEntry<K, V>> = js.noImpl
 }
 
-library
-public class StringBuilder() : Appendable {
-    override fun append(c: Char): Appendable? = js.noImpl
-    override fun append(csq: CharSequence?): Appendable? = js.noImpl
-    override fun append(csq: CharSequence?, start: Int, end: Int): Appendable? = js.noImpl
-    public fun append(obj: Any?): StringBuilder = js.noImpl
-    public fun toString(): String = js.noImpl
-}
-
 library
 public class NoSuchElementException() : Exception() {
 }
diff --git a/js/js.translator/testFiles/standardClasses/cases/stringBuilder.kt b/js/js.translator/testFiles/standardClasses/cases/stringBuilder.kt
index 6fa8db32f82..8c138a1e1d0 100644
--- a/js/js.translator/testFiles/standardClasses/cases/stringBuilder.kt
+++ b/js/js.translator/testFiles/standardClasses/cases/stringBuilder.kt
@@ -1,12 +1,11 @@
 package foo
 
-import java.util.StringBuilder
 
 fun box(): String {
     val s = StringBuilder()
     s.append("a")
-    s.append("b")?.append("c")
-    s.append("d")!!.append("e")
+    s.append("b").append("c")
+    s.append('d').append("e")
 
     if (s.toString() != "abcde") return s.toString()
     return "OK"