diff --git a/js/js.libraries/src/core/char.kt b/js/js.libraries/src/core/char.kt
new file mode 100644
index 00000000000..22f7c70d9ee
--- /dev/null
+++ b/js/js.libraries/src/core/char.kt
@@ -0,0 +1,20 @@
+/*
+ * Copyright 2010-2015 JetBrains s.r.o.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package kotlin.js
+
+// actually \s is enough to match all whitespace, but \xA0 added because of different regexp behavior of Rhino used in Selenium tests
+public fun Char.isWhitespace(): Boolean = toString().matches("[\\s\\xA0]")
diff --git a/js/js.libraries/src/core/javalang.kt b/js/js.libraries/src/core/javalang.kt
index 96dc7f2ddfe..b7db2beaeec 100644
--- a/js/js.libraries/src/core/javalang.kt
+++ b/js/js.libraries/src/core/javalang.kt
@@ -37,7 +37,7 @@ public trait Appendable {
 }
 
 library
-public class StringBuilder() : Appendable {
+public class StringBuilder(capacity: Int? = null) : Appendable {
     override fun append(c: Char): StringBuilder = noImpl
     override fun append(csq: CharSequence?): StringBuilder = noImpl
     override fun append(csq: CharSequence?, start: Int, end: Int): StringBuilder = noImpl
diff --git a/js/js.libraries/src/core/string.kt b/js/js.libraries/src/core/string.kt
index 363a284cc3e..74b2e9aaeeb 100644
--- a/js/js.libraries/src/core/string.kt
+++ b/js/js.libraries/src/core/string.kt
@@ -31,7 +31,8 @@ native public fun String.concat(str : String) : String = noImpl
 
 native public fun String.match(regex : String) : Array<String> = noImpl
 
-native public fun String.trim() : String = noImpl
+//native public fun String.trim() : String = noImpl
+//TODO: String.replace to implement effective trimLeading and trimTrailing
 
 native("length")
 public val CharSequence.size: Int get() = noImpl
diff --git a/libraries/stdlib/test/text/StringTest.kt b/libraries/stdlib/test/text/StringTest.kt
index 2b6025b7af0..387d6c29007 100644
--- a/libraries/stdlib/test/text/StringTest.kt
+++ b/libraries/stdlib/test/text/StringTest.kt
@@ -169,7 +169,7 @@ class StringTest {
         assertEquals("a", "\na".trimLeading())
 
         assertEquals("a=", "-=-=a=".trimLeading('-','='))
-        assertEquals("123a", "ab123a".trimLeading { !it.isDigit() })
+        assertEquals("123a", "ab123a".trimLeading { it < '0' || it > '9' }) // TODO: Use !it.isDigit when available in JS
     }
 
     test fun trimTrailing() {
@@ -188,7 +188,7 @@ class StringTest {
         assertEquals("a", "a\n".trimTrailing())
 
         assertEquals("=a", "=a=-=-".trimTrailing('-','='))
-        assertEquals("ab123", "ab123a".trimTrailing { !it.isDigit() })
+        assertEquals("ab123", "ab123a".trimTrailing { it < '0' || it > '9' }) // TODO: Use !it.isDigit when available in JS
     }
 
     test fun trimTrailingAndLeading() {
@@ -215,7 +215,7 @@ class StringTest {
         )
 
         val trimChars = charArray('-','=')
-        val trimPredicate = { (it: Char) -> !it.isDigit() }
+        val trimPredicate = { (it: Char) -> it < '0' || it > '9' } // TODO: Use !it.isDigit when available in JS
         for (example in examplesForPredicate) {
             assertEquals(example.trimLeading(*trimChars).trimTrailing(*trimChars), example.trim(*trimChars))
             assertEquals(example.trimLeading(trimPredicate).trimTrailing(trimPredicate), example.trim(trimPredicate))