diff --git a/analysis/analysis-api-fir/src/org/jetbrains/kotlin/analysis/api/fir/diagnostics/KtFirDataClassConverters.kt b/analysis/analysis-api-fir/src/org/jetbrains/kotlin/analysis/api/fir/diagnostics/KtFirDataClassConverters.kt
index cdd71ccbd85..9c9a07669ae 100644
--- a/analysis/analysis-api-fir/src/org/jetbrains/kotlin/analysis/api/fir/diagnostics/KtFirDataClassConverters.kt
+++ b/analysis/analysis-api-fir/src/org/jetbrains/kotlin/analysis/api/fir/diagnostics/KtFirDataClassConverters.kt
@@ -5742,6 +5742,12 @@ internal val KT_DIAGNOSTIC_CONVERTER = KtDiagnosticConverterBuilder.buildConvert
             token,
         )
     }
+    add(FirJsErrors.NAMED_COMPANION_IN_EXPORTED_INTERFACE) { firDiagnostic ->
+        NamedCompanionInExportedInterfaceImpl(
+            firDiagnostic as KtPsiDiagnostic,
+            token,
+        )
+    }
     add(FirWebCommonErrors.NESTED_JS_EXPORT) { firDiagnostic ->
         NestedJsExportImpl(
             firDiagnostic as KtPsiDiagnostic,
@@ -5881,6 +5887,12 @@ internal val KT_DIAGNOSTIC_CONVERTER = KtDiagnosticConverterBuilder.buildConvert
             token,
         )
     }
+    add(FirWebCommonErrors.NAMED_COMPANION_IN_EXTERNAL_INTERFACE) { firDiagnostic ->
+        NamedCompanionInExternalInterfaceImpl(
+            firDiagnostic as KtPsiDiagnostic,
+            token,
+        )
+    }
     add(FirWebCommonErrors.JSCODE_ARGUMENT_NON_CONST_EXPRESSION) { firDiagnostic ->
         JscodeArgumentNonConstExpressionImpl(
             firDiagnostic as KtPsiDiagnostic,
diff --git a/analysis/analysis-api-fir/src/org/jetbrains/kotlin/analysis/api/fir/diagnostics/KtFirDiagnostics.kt b/analysis/analysis-api-fir/src/org/jetbrains/kotlin/analysis/api/fir/diagnostics/KtFirDiagnostics.kt
index a77b51c4c79..683f13e9781 100644
--- a/analysis/analysis-api-fir/src/org/jetbrains/kotlin/analysis/api/fir/diagnostics/KtFirDiagnostics.kt
+++ b/analysis/analysis-api-fir/src/org/jetbrains/kotlin/analysis/api/fir/diagnostics/KtFirDiagnostics.kt
@@ -3999,6 +3999,10 @@ sealed interface KtFirDiagnostic<PSI : PsiElement> : KtDiagnosticWithPsi<PSI> {
         val name: String
     }
 
+    interface NamedCompanionInExportedInterface : KtFirDiagnostic<KtElement> {
+        override val diagnosticClass get() = NamedCompanionInExportedInterface::class
+    }
+
     interface NestedJsExport : KtFirDiagnostic<KtElement> {
         override val diagnosticClass get() = NestedJsExport::class
     }
@@ -4094,6 +4098,10 @@ sealed interface KtFirDiagnostic<PSI : PsiElement> : KtDiagnosticWithPsi<PSI> {
         val typeArgument: KtType
     }
 
+    interface NamedCompanionInExternalInterface : KtFirDiagnostic<KtElement> {
+        override val diagnosticClass get() = NamedCompanionInExternalInterface::class
+    }
+
     interface JscodeArgumentNonConstExpression : KtFirDiagnostic<KtElement> {
         override val diagnosticClass get() = JscodeArgumentNonConstExpression::class
     }
diff --git a/analysis/analysis-api-fir/src/org/jetbrains/kotlin/analysis/api/fir/diagnostics/KtFirDiagnosticsImpl.kt b/analysis/analysis-api-fir/src/org/jetbrains/kotlin/analysis/api/fir/diagnostics/KtFirDiagnosticsImpl.kt
index 7276c72f72a..b7b8ee1752d 100644
--- a/analysis/analysis-api-fir/src/org/jetbrains/kotlin/analysis/api/fir/diagnostics/KtFirDiagnosticsImpl.kt
+++ b/analysis/analysis-api-fir/src/org/jetbrains/kotlin/analysis/api/fir/diagnostics/KtFirDiagnosticsImpl.kt
@@ -4828,6 +4828,11 @@ internal class NonConsumableExportedIdentifierImpl(
     token: KtLifetimeToken,
 ) : KtAbstractFirDiagnostic<KtElement>(firDiagnostic, token), KtFirDiagnostic.NonConsumableExportedIdentifier
 
+internal class NamedCompanionInExportedInterfaceImpl(
+    firDiagnostic: KtPsiDiagnostic,
+    token: KtLifetimeToken,
+) : KtAbstractFirDiagnostic<KtElement>(firDiagnostic, token), KtFirDiagnostic.NamedCompanionInExportedInterface
+
 internal class NestedJsExportImpl(
     firDiagnostic: KtPsiDiagnostic,
     token: KtLifetimeToken,
@@ -4945,6 +4950,11 @@ internal class ExternalInterfaceAsReifiedTypeArgumentImpl(
     token: KtLifetimeToken,
 ) : KtAbstractFirDiagnostic<KtElement>(firDiagnostic, token), KtFirDiagnostic.ExternalInterfaceAsReifiedTypeArgument
 
+internal class NamedCompanionInExternalInterfaceImpl(
+    firDiagnostic: KtPsiDiagnostic,
+    token: KtLifetimeToken,
+) : KtAbstractFirDiagnostic<KtElement>(firDiagnostic, token), KtFirDiagnostic.NamedCompanionInExternalInterface
+
 internal class JscodeArgumentNonConstExpressionImpl(
     firDiagnostic: KtPsiDiagnostic,
     token: KtLifetimeToken,
diff --git a/compiler/fir/checkers/checkers-component-generator/src/org/jetbrains/kotlin/fir/checkers/generator/diagnostics/FirJsDiagnosticsList.kt b/compiler/fir/checkers/checkers-component-generator/src/org/jetbrains/kotlin/fir/checkers/generator/diagnostics/FirJsDiagnosticsList.kt
index 5a77b01f955..04844d0bd08 100644
--- a/compiler/fir/checkers/checkers-component-generator/src/org/jetbrains/kotlin/fir/checkers/generator/diagnostics/FirJsDiagnosticsList.kt
+++ b/compiler/fir/checkers/checkers-component-generator/src/org/jetbrains/kotlin/fir/checkers/generator/diagnostics/FirJsDiagnosticsList.kt
@@ -113,6 +113,7 @@ object JS_DIAGNOSTICS_LIST : DiagnosticList("FirJsErrors") {
         val NON_CONSUMABLE_EXPORTED_IDENTIFIER by warning<KtElement>(PositioningStrategy.DEFAULT) {
             parameter<String>("name")
         }
+        val NAMED_COMPANION_IN_EXPORTED_INTERFACE by error<KtElement>(PositioningStrategy.DECLARATION_SIGNATURE_OR_DEFAULT)
     }
 
     val DYNAMICS by object : DiagnosticGroup("Dynamics") {
diff --git a/compiler/fir/checkers/checkers-component-generator/src/org/jetbrains/kotlin/fir/checkers/generator/diagnostics/FirWebCommonDiagnosticList.kt b/compiler/fir/checkers/checkers-component-generator/src/org/jetbrains/kotlin/fir/checkers/generator/diagnostics/FirWebCommonDiagnosticList.kt
index 322e7f9a6b9..4bcf0b10f30 100644
--- a/compiler/fir/checkers/checkers-component-generator/src/org/jetbrains/kotlin/fir/checkers/generator/diagnostics/FirWebCommonDiagnosticList.kt
+++ b/compiler/fir/checkers/checkers-component-generator/src/org/jetbrains/kotlin/fir/checkers/generator/diagnostics/FirWebCommonDiagnosticList.kt
@@ -44,6 +44,7 @@ object WEB_COMMON_DIAGNOSTICS_LIST : DiagnosticList("FirWebCommonErrors") {
         val EXTERNAL_INTERFACE_AS_REIFIED_TYPE_ARGUMENT by error<KtElement>(PositioningStrategy.DECLARATION_SIGNATURE_OR_DEFAULT) {
             parameter<ConeKotlinType>("typeArgument")
         }
+        val NAMED_COMPANION_IN_EXTERNAL_INTERFACE by error<KtElement>(PositioningStrategy.DECLARATION_SIGNATURE_OR_DEFAULT)
     }
 
     val EXPORT by object : DiagnosticGroup("Export") {
diff --git a/compiler/fir/checkers/checkers.js/gen/org/jetbrains/kotlin/fir/analysis/diagnostics/js/FirJsErrors.kt b/compiler/fir/checkers/checkers.js/gen/org/jetbrains/kotlin/fir/analysis/diagnostics/js/FirJsErrors.kt
index 3ef72e81fe4..c863bae1573 100644
--- a/compiler/fir/checkers/checkers.js/gen/org/jetbrains/kotlin/fir/analysis/diagnostics/js/FirJsErrors.kt
+++ b/compiler/fir/checkers/checkers.js/gen/org/jetbrains/kotlin/fir/analysis/diagnostics/js/FirJsErrors.kt
@@ -78,6 +78,7 @@ object FirJsErrors {
     val WRONG_EXPORTED_DECLARATION: KtDiagnosticFactory1<String> by error1<KtElement, String>(SourceElementPositioningStrategies.DECLARATION_SIGNATURE_OR_DEFAULT)
     val NON_EXPORTABLE_TYPE: KtDiagnosticFactory2<String, ConeKotlinType> by warning2<KtElement, String, ConeKotlinType>(SourceElementPositioningStrategies.DECLARATION_SIGNATURE_OR_DEFAULT)
     val NON_CONSUMABLE_EXPORTED_IDENTIFIER: KtDiagnosticFactory1<String> by warning1<KtElement, String>()
+    val NAMED_COMPANION_IN_EXPORTED_INTERFACE: KtDiagnosticFactory0 by error0<KtElement>(SourceElementPositioningStrategies.DECLARATION_SIGNATURE_OR_DEFAULT)
 
     // Dynamics
     val DELEGATION_BY_DYNAMIC: KtDiagnosticFactory0 by error0<KtElement>()
diff --git a/compiler/fir/checkers/checkers.js/src/org/jetbrains/kotlin/fir/analysis/diagnostics/js/FirJsErrorsDefaultMessages.kt b/compiler/fir/checkers/checkers.js/src/org/jetbrains/kotlin/fir/analysis/diagnostics/js/FirJsErrorsDefaultMessages.kt
index a8a74a36a7b..0d16bd1b18a 100644
--- a/compiler/fir/checkers/checkers.js/src/org/jetbrains/kotlin/fir/analysis/diagnostics/js/FirJsErrorsDefaultMessages.kt
+++ b/compiler/fir/checkers/checkers.js/src/org/jetbrains/kotlin/fir/analysis/diagnostics/js/FirJsErrorsDefaultMessages.kt
@@ -35,6 +35,7 @@ import org.jetbrains.kotlin.fir.analysis.diagnostics.js.FirJsErrors.JS_NAME_ON_P
 import org.jetbrains.kotlin.fir.analysis.diagnostics.js.FirJsErrors.JS_NAME_PROHIBITED_FOR_EXTENSION_PROPERTY
 import org.jetbrains.kotlin.fir.analysis.diagnostics.js.FirJsErrors.JS_NAME_PROHIBITED_FOR_NAMED_NATIVE
 import org.jetbrains.kotlin.fir.analysis.diagnostics.js.FirJsErrors.JS_NAME_PROHIBITED_FOR_OVERRIDE
+import org.jetbrains.kotlin.fir.analysis.diagnostics.js.FirJsErrors.NAMED_COMPANION_IN_EXPORTED_INTERFACE
 import org.jetbrains.kotlin.fir.analysis.diagnostics.js.FirJsErrors.NAME_CONTAINS_ILLEGAL_CHARS
 import org.jetbrains.kotlin.fir.analysis.diagnostics.js.FirJsErrors.NATIVE_ANNOTATIONS_ALLOWED_ONLY_ON_MEMBER_OR_EXTENSION_FUN
 import org.jetbrains.kotlin.fir.analysis.diagnostics.js.FirJsErrors.NATIVE_GETTER_RETURN_TYPE_SHOULD_BE_NULLABLE
@@ -189,5 +190,6 @@ object FirJsErrorsDefaultMessages : BaseDiagnosticRendererFactory() {
             "Exported declaration contains non-consumable identifier ''{0}'', which cannot be represented inside TS definitions and ESM.",
             CommonRenderers.STRING,
         )
+        map.put(NAMED_COMPANION_IN_EXPORTED_INTERFACE, "Named companions are not allowed inside exported interfaces.")
     }
 }
diff --git a/compiler/fir/checkers/checkers.js/src/org/jetbrains/kotlin/fir/analysis/js/checkers/declaration/FirJsExportDeclarationChecker.kt b/compiler/fir/checkers/checkers.js/src/org/jetbrains/kotlin/fir/analysis/js/checkers/declaration/FirJsExportDeclarationChecker.kt
index f7a09055f9a..41ef5209f86 100644
--- a/compiler/fir/checkers/checkers.js/src/org/jetbrains/kotlin/fir/analysis/js/checkers/declaration/FirJsExportDeclarationChecker.kt
+++ b/compiler/fir/checkers/checkers.js/src/org/jetbrains/kotlin/fir/analysis/js/checkers/declaration/FirJsExportDeclarationChecker.kt
@@ -30,6 +30,7 @@ import org.jetbrains.kotlin.fir.types.*
 import org.jetbrains.kotlin.js.common.RESERVED_KEYWORDS
 import org.jetbrains.kotlin.js.common.SPECIAL_KEYWORDS
 import org.jetbrains.kotlin.name.JsStandardClassIds
+import org.jetbrains.kotlin.name.SpecialNames.DEFAULT_NAME_FOR_COMPANION_OBJECT
 
 object FirJsExportDeclarationChecker : FirBasicDeclarationChecker(MppCheckerKind.Common) {
     override fun check(declaration: FirDeclaration, context: CheckerContext, reporter: DiagnosticReporter) {
@@ -135,11 +136,15 @@ object FirJsExportDeclarationChecker : FirBasicDeclarationChecker(MppCheckerKind
                         declaration.isInline -> "value class"
                         else -> null
                     }
-                    else -> if (context.isInsideInterface) {
-                        "${if (declaration.status.isCompanion) "companion object" else "nested/inner declaration"} inside exported interface"
+                    else -> if (context.isInsideInterface && !declaration.status.isCompanion) {
+                        "nested/inner declaration inside exported interface"
                     } else null
                 }
 
+                if (context.isInsideInterface && declaration.status.isCompanion && declaration.nameOrSpecialName != DEFAULT_NAME_FOR_COMPANION_OBJECT) {
+                    reporter.reportOn(declaration.source, FirJsErrors.NAMED_COMPANION_IN_EXPORTED_INTERFACE, context)
+                }
+
                 if (wrongDeclaration != null) {
                     reportWrongExportedDeclaration(wrongDeclaration)
                 }
diff --git a/compiler/fir/checkers/checkers.js/src/org/jetbrains/kotlin/fir/analysis/js/checkers/declaration/FirJsExternalChecker.kt b/compiler/fir/checkers/checkers.js/src/org/jetbrains/kotlin/fir/analysis/js/checkers/declaration/FirJsExternalChecker.kt
index 80de3c130b9..1e2570bbd5c 100644
--- a/compiler/fir/checkers/checkers.js/src/org/jetbrains/kotlin/fir/analysis/js/checkers/declaration/FirJsExternalChecker.kt
+++ b/compiler/fir/checkers/checkers.js/src/org/jetbrains/kotlin/fir/analysis/js/checkers/declaration/FirJsExternalChecker.kt
@@ -31,7 +31,7 @@ import org.jetbrains.kotlin.name.JsStandardClassIds
 import org.jetbrains.kotlin.name.JsStandardClassIds.Annotations.JsNative
 import org.jetbrains.kotlin.psi.KtParameter
 
-object FirJsExternalChecker : FirWebCommonExternalChecker() {
+object FirJsExternalChecker : FirWebCommonExternalChecker(allowCompanionInInterface = true) {
     override fun isNativeOrEffectivelyExternal(symbol: FirBasedSymbol<*>, session: FirSession): Boolean {
         return symbol.isNativeObject(session)
     }
diff --git a/compiler/fir/checkers/checkers.wasm/src/org/jetbrains/kotlin/fir/analysis/wasm/checkers/declaration/FirWasmExternalChecker.kt b/compiler/fir/checkers/checkers.wasm/src/org/jetbrains/kotlin/fir/analysis/wasm/checkers/declaration/FirWasmExternalChecker.kt
index 74f3eb38066..369a1ce9821 100644
--- a/compiler/fir/checkers/checkers.wasm/src/org/jetbrains/kotlin/fir/analysis/wasm/checkers/declaration/FirWasmExternalChecker.kt
+++ b/compiler/fir/checkers/checkers.wasm/src/org/jetbrains/kotlin/fir/analysis/wasm/checkers/declaration/FirWasmExternalChecker.kt
@@ -17,7 +17,7 @@ import org.jetbrains.kotlin.fir.symbols.FirBasedSymbol
 import org.jetbrains.kotlin.name.CallableId
 import org.jetbrains.kotlin.name.WebCommonStandardClassIds
 
-object FirWasmExternalChecker : FirWebCommonExternalChecker() {
+object FirWasmExternalChecker : FirWebCommonExternalChecker(allowCompanionInInterface = false) {
     override fun isNativeOrEffectivelyExternal(symbol: FirBasedSymbol<*>, session: FirSession): Boolean {
         return symbol.isEffectivelyExternal(session)
     }
diff --git a/compiler/fir/checkers/checkers.web.common/gen/org/jetbrains/kotlin/fir/analysis/diagnostics/web/common/FirWebCommonErrors.kt b/compiler/fir/checkers/checkers.web.common/gen/org/jetbrains/kotlin/fir/analysis/diagnostics/web/common/FirWebCommonErrors.kt
index 69acc38ffca..960e2017bea 100644
--- a/compiler/fir/checkers/checkers.web.common/gen/org/jetbrains/kotlin/fir/analysis/diagnostics/web/common/FirWebCommonErrors.kt
+++ b/compiler/fir/checkers/checkers.web.common/gen/org/jetbrains/kotlin/fir/analysis/diagnostics/web/common/FirWebCommonErrors.kt
@@ -43,6 +43,7 @@ object FirWebCommonErrors {
     val UNCHECKED_CAST_TO_EXTERNAL_INTERFACE: KtDiagnosticFactory2<ConeKotlinType, ConeKotlinType> by warning2<KtElement, ConeKotlinType, ConeKotlinType>()
     val EXTERNAL_INTERFACE_AS_CLASS_LITERAL: KtDiagnosticFactory0 by error0<KtElement>()
     val EXTERNAL_INTERFACE_AS_REIFIED_TYPE_ARGUMENT: KtDiagnosticFactory1<ConeKotlinType> by error1<KtElement, ConeKotlinType>(SourceElementPositioningStrategies.DECLARATION_SIGNATURE_OR_DEFAULT)
+    val NAMED_COMPANION_IN_EXTERNAL_INTERFACE: KtDiagnosticFactory0 by error0<KtElement>(SourceElementPositioningStrategies.DECLARATION_SIGNATURE_OR_DEFAULT)
 
     // Export
     val NESTED_JS_EXPORT: KtDiagnosticFactory0 by error0<KtElement>()
diff --git a/compiler/fir/checkers/checkers.web.common/src/org/jetbrains/kotlin/fir/analysis/diagnostics/web/common/FirWebCommonErrorsDefaultMessages.kt b/compiler/fir/checkers/checkers.web.common/src/org/jetbrains/kotlin/fir/analysis/diagnostics/web/common/FirWebCommonErrorsDefaultMessages.kt
index b5f726b4948..cfed3cd0406 100644
--- a/compiler/fir/checkers/checkers.web.common/src/org/jetbrains/kotlin/fir/analysis/diagnostics/web/common/FirWebCommonErrorsDefaultMessages.kt
+++ b/compiler/fir/checkers/checkers.web.common/src/org/jetbrains/kotlin/fir/analysis/diagnostics/web/common/FirWebCommonErrorsDefaultMessages.kt
@@ -18,6 +18,7 @@ import org.jetbrains.kotlin.fir.analysis.diagnostics.web.common.FirWebCommonErro
 import org.jetbrains.kotlin.fir.analysis.diagnostics.web.common.FirWebCommonErrors.EXTERNAL_INTERFACE_AS_REIFIED_TYPE_ARGUMENT
 import org.jetbrains.kotlin.fir.analysis.diagnostics.web.common.FirWebCommonErrors.INLINE_EXTERNAL_DECLARATION
 import org.jetbrains.kotlin.fir.analysis.diagnostics.web.common.FirWebCommonErrors.JSCODE_ARGUMENT_NON_CONST_EXPRESSION
+import org.jetbrains.kotlin.fir.analysis.diagnostics.web.common.FirWebCommonErrors.NAMED_COMPANION_IN_EXTERNAL_INTERFACE
 import org.jetbrains.kotlin.fir.analysis.diagnostics.web.common.FirWebCommonErrors.NESTED_CLASS_IN_EXTERNAL_INTERFACE
 import org.jetbrains.kotlin.fir.analysis.diagnostics.web.common.FirWebCommonErrors.NESTED_EXTERNAL_DECLARATION
 import org.jetbrains.kotlin.fir.analysis.diagnostics.web.common.FirWebCommonErrors.NESTED_JS_EXPORT
@@ -76,5 +77,6 @@ object FirWebCommonErrorsDefaultMessages : BaseDiagnosticRendererFactory() {
         map.put(NESTED_JS_EXPORT, "'@JsExport' is only allowed on files and top-level declarations.")
 
         map.put(JSCODE_ARGUMENT_NON_CONST_EXPRESSION, "An argument for the 'js()' function must be a constant string expression.")
+        map.put(NAMED_COMPANION_IN_EXTERNAL_INTERFACE, "Named companions are not allowed inside external interfaces.")
     }
 }
\ No newline at end of file
diff --git a/compiler/fir/checkers/checkers.web.common/src/org/jetbrains/kotlin/fir/analysis/web/common/checkers/declaration/FirWebCommonExternalChecker.kt b/compiler/fir/checkers/checkers.web.common/src/org/jetbrains/kotlin/fir/analysis/web/common/checkers/declaration/FirWebCommonExternalChecker.kt
index 4312c759070..7f9ee60148c 100644
--- a/compiler/fir/checkers/checkers.web.common/src/org/jetbrains/kotlin/fir/analysis/web/common/checkers/declaration/FirWebCommonExternalChecker.kt
+++ b/compiler/fir/checkers/checkers.web.common/src/org/jetbrains/kotlin/fir/analysis/web/common/checkers/declaration/FirWebCommonExternalChecker.kt
@@ -6,9 +6,7 @@
 package org.jetbrains.kotlin.fir.analysis.web.common.checkers.declaration
 
 import org.jetbrains.kotlin.*
-import org.jetbrains.kotlin.descriptors.ClassKind
-import org.jetbrains.kotlin.descriptors.Modality
-import org.jetbrains.kotlin.descriptors.Visibilities
+import org.jetbrains.kotlin.descriptors.*
 import org.jetbrains.kotlin.diagnostics.DiagnosticReporter
 import org.jetbrains.kotlin.diagnostics.reportOn
 import org.jetbrains.kotlin.fir.FirElement
@@ -30,9 +28,10 @@ import org.jetbrains.kotlin.fir.symbols.impl.FirClassSymbol
 import org.jetbrains.kotlin.fir.types.*
 import org.jetbrains.kotlin.lexer.KtTokens
 import org.jetbrains.kotlin.name.CallableId
+import org.jetbrains.kotlin.name.SpecialNames.DEFAULT_NAME_FOR_COMPANION_OBJECT
 import org.jetbrains.kotlin.utils.addToStdlib.firstIsInstanceOrNull
 
-abstract class FirWebCommonExternalChecker : FirBasicDeclarationChecker(MppCheckerKind.Common) {
+abstract class FirWebCommonExternalChecker(private val allowCompanionInInterface: Boolean) : FirBasicDeclarationChecker(MppCheckerKind.Common) {
     abstract fun isNativeOrEffectivelyExternal(symbol: FirBasedSymbol<*>, session: FirSession): Boolean
 
     abstract fun reportExternalEnum(declaration: FirDeclaration, context: CheckerContext, reporter: DiagnosticReporter)
@@ -87,12 +86,24 @@ abstract class FirWebCommonExternalChecker : FirBasicDeclarationChecker(MppCheck
 
         if (
             declaration is FirClass &&
-            declaration.classKind != ClassKind.INTERFACE &&
-            container is FirClass && container.classKind == ClassKind.INTERFACE
+            !declaration.classKind.isInterface && (!allowCompanionInInterface || !declaration.status.isCompanion) &&
+            container is FirClass && container.classKind.isInterface
         ) {
             reporter.reportOn(declaration.source, FirWebCommonErrors.NESTED_CLASS_IN_EXTERNAL_INTERFACE, context)
         }
 
+        if (
+            allowCompanionInInterface &&
+            declaration is FirClass &&
+            declaration.status.isCompanion &&
+            container is FirClass &&
+            container.isInterface &&
+            declaration.nameOrSpecialName != DEFAULT_NAME_FOR_COMPANION_OBJECT
+        ) {
+            reporter.reportOn(declaration.source, FirWebCommonErrors.NAMED_COMPANION_IN_EXTERNAL_INTERFACE, context)
+        }
+
+
         if (declaration !is FirPropertyAccessor && declaration is FirCallableDeclaration && declaration.isExtension) {
             val target = when (declaration) {
                 is FirFunction -> "extension function"
diff --git a/compiler/fir/checkers/gen/org/jetbrains/kotlin/fir/analysis/diagnostics/FirNonSuppressibleErrorNames.kt b/compiler/fir/checkers/gen/org/jetbrains/kotlin/fir/analysis/diagnostics/FirNonSuppressibleErrorNames.kt
index c580dddaf3f..12962cd574d 100644
--- a/compiler/fir/checkers/gen/org/jetbrains/kotlin/fir/analysis/diagnostics/FirNonSuppressibleErrorNames.kt
+++ b/compiler/fir/checkers/gen/org/jetbrains/kotlin/fir/analysis/diagnostics/FirNonSuppressibleErrorNames.kt
@@ -645,6 +645,7 @@ val FIR_NON_SUPPRESSIBLE_ERROR_NAMES: Set<String> = setOf(
     "JS_EXTERNAL_INHERITORS_ONLY",
     "JS_EXTERNAL_ARGUMENT",
     "WRONG_EXPORTED_DECLARATION",
+    "NAMED_COMPANION_IN_EXPORTED_INTERFACE",
     "NESTED_JS_EXPORT",
     "DELEGATION_BY_DYNAMIC",
     "PROPERTY_DELEGATION_BY_DYNAMIC",
@@ -700,6 +701,7 @@ val FIR_NON_SUPPRESSIBLE_ERROR_NAMES: Set<String> = setOf(
     "CANNOT_CHECK_FOR_EXTERNAL_INTERFACE",
     "EXTERNAL_INTERFACE_AS_CLASS_LITERAL",
     "EXTERNAL_INTERFACE_AS_REIFIED_TYPE_ARGUMENT",
+    "NAMED_COMPANION_IN_EXTERNAL_INTERFACE",
     "JSCODE_ARGUMENT_NON_CONST_EXPRESSION",
     "SYNTAX",
 )
diff --git a/compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/export/ExportModelGenerator.kt b/compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/export/ExportModelGenerator.kt
index 44b9eaeee92..f732862bdc5 100644
--- a/compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/export/ExportModelGenerator.kt
+++ b/compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/export/ExportModelGenerator.kt
@@ -23,6 +23,7 @@ import org.jetbrains.kotlin.ir.symbols.IrClassifierSymbol
 import org.jetbrains.kotlin.ir.symbols.IrTypeParameterSymbol
 import org.jetbrains.kotlin.ir.types.*
 import org.jetbrains.kotlin.ir.util.*
+import org.jetbrains.kotlin.name.SpecialNames
 import org.jetbrains.kotlin.serialization.js.ModuleKind
 import org.jetbrains.kotlin.utils.*
 import org.jetbrains.kotlin.utils.addToStdlib.runIf
@@ -328,12 +329,15 @@ class ExportModelGenerator(val context: JsIrBackendContext, val generateNamespac
                     members.addIfNotNull(exportProperty(candidate)?.withAttributesFor(candidate))
 
                 is IrClass -> {
-                    if (klass.isInterface) continue
-                    val ec = exportClass(candidate)?.withAttributesFor(candidate)
-                    if (ec is ExportedClass) {
-                        nestedClasses.add(ec)
+                    if (klass.isInterface) {
+                        nestedClasses.addIfNotNull(klass.companionObject()?.let { exportClass(it) as? ExportedClass }?.withAttributesFor(candidate))
                     } else {
-                        members.addIfNotNull(ec)
+                        val ec = exportClass(candidate)?.withAttributesFor(candidate)
+                        if (ec is ExportedClass) {
+                            nestedClasses.add(ec)
+                        } else {
+                            members.addIfNotNull(ec)
+                        }
                     }
                 }
 
@@ -441,7 +445,7 @@ class ExportModelGenerator(val context: JsIrBackendContext, val generateNamespac
             .map { exportType(it, false) }
             .memoryOptimizedFilter { it !is ExportedType.ErrorType }
 
-        val name = klass.getExportedIdentifier()
+        val name = klass.getExportedIdentifierForClass()
 
         return if (klass.kind == ClassKind.OBJECT) {
             return ExportedObject(
@@ -862,12 +866,19 @@ val strictModeReservedWords = setOf(
 
 private val allReservedWords = reservedWords + strictModeReservedWords
 
-fun ExportedDeclaration.withAttributesFor(declaration: IrDeclaration): ExportedDeclaration {
+fun <T : ExportedDeclaration> T.withAttributesFor(declaration: IrDeclaration): T {
     declaration.getDeprecated()?.let { attributes.add(ExportedAttribute.DeprecatedAttribute(it)) }
 
     return this
 }
 
+fun IrClass.getExportedIdentifierForClass(): String {
+    val parentClass = parentClassOrNull
+    return if (parentClass != null && isCompanion && parentClass.isInterface) {
+        parentClass.getExportedIdentifierForClass()
+    } else getExportedIdentifier()
+}
+
 fun IrDeclarationWithName.getExportedIdentifier(): String =
     with(getJsNameOrKotlinName()) {
         if (isSpecial)
diff --git a/compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/export/ExportModelToJsStatements.kt b/compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/export/ExportModelToJsStatements.kt
index b7badfa5cc6..fb3716bcacf 100644
--- a/compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/export/ExportModelToJsStatements.kt
+++ b/compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/export/ExportModelToJsStatements.kt
@@ -33,7 +33,7 @@ class ExportModelToJsStatements(
     fun generateModuleExport(
         module: ExportedModule,
         internalModuleName: JsName?,
-        esModules: Boolean
+        esModules: Boolean,
     ): List<JsStatement> {
         return module.declarations.flatMap {
             generateDeclarationExport(it, internalModuleName?.makeRef(), esModules)
@@ -44,7 +44,7 @@ class ExportModelToJsStatements(
         declaration: ExportedDeclaration,
         namespace: JsExpression?,
         esModules: Boolean,
-        parentClass: IrClass? = null
+        parentClass: IrClass? = null,
     ): List<JsStatement> {
         return when (declaration) {
             is ExportedNamespace -> {
@@ -155,7 +155,9 @@ class ExportModelToJsStatements(
             }
 
             is ExportedRegularClass -> {
-                if (declaration.isInterface) return emptyList()
+                if (declaration.isInterface) {
+                    return declaration.nestedClasses.flatMap { generateDeclarationExport(it, namespace, esModules, parentClass) }
+                }
                 val (name, classInitialization) = declaration.getNameAndInitialization()
                 val newNameSpace = when {
                     namespace != null -> jsElementAccess(declaration.name, namespace)
diff --git a/compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/export/ExportModelToTsDeclarations.kt b/compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/export/ExportModelToTsDeclarations.kt
index 6a9d0b165a9..3c018f5a5c5 100644
--- a/compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/export/ExportModelToTsDeclarations.kt
+++ b/compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/export/ExportModelToTsDeclarations.kt
@@ -14,10 +14,7 @@ import org.jetbrains.kotlin.ir.backend.js.utils.getJsNameOrKotlinName
 import org.jetbrains.kotlin.ir.backend.js.utils.sanitizeName
 import org.jetbrains.kotlin.ir.declarations.IrClass
 import org.jetbrains.kotlin.ir.declarations.IrFunction
-import org.jetbrains.kotlin.ir.util.hasAnnotation
-import org.jetbrains.kotlin.ir.util.isObject
-import org.jetbrains.kotlin.ir.util.parentAsClass
-import org.jetbrains.kotlin.ir.util.primaryConstructor
+import org.jetbrains.kotlin.ir.util.*
 import org.jetbrains.kotlin.js.common.isValidES5Identifier
 import org.jetbrains.kotlin.serialization.js.ModuleKind
 import org.jetbrains.kotlin.utils.addToStdlib.firstIsInstanceOrNull
@@ -106,7 +103,7 @@ class ExportModelToTsDeclarations {
             is ExportedConstructSignature -> generateTypeScriptString(indent)
             is ExportedNamespace -> generateTypeScriptString(indent, prefix)
             is ExportedFunction -> generateTypeScriptString(indent, prefix)
-            is ExportedRegularClass -> generateTypeScriptString(indent, prefix)
+            is ExportedRegularClass -> generateTypeScriptString(indent, prefix, esModules)
             is ExportedProperty -> generateTypeScriptString(indent, prefix, esModules)
             is ExportedObject -> generateTypeScriptString(indent, prefix, esModules)
         }
@@ -235,7 +232,7 @@ class ExportModelToTsDeclarations {
             t = ExportedType.IntersectionType(t, ExportedType.InlineInterfaceType(listOf(constructor)))
         }
 
-        val maybeParentClass = ir.parent as? IrClass
+        val maybeParentClass = (ir.parent as? IrClass)?.takeIf { !it.isInterface }
 
         val propertyName = ir
             .takeIf { shouldRenderSeparatedAbstractClass }
@@ -275,21 +272,22 @@ class ExportModelToTsDeclarations {
 
             if (esModules && !property.isMember) {
                 property.copy(type = ExportedType.TypeOf(className), name = name)
-                    .generateTypeScriptString(indent, prefix, esModules) + "\n${classForRender.generateTypeScriptString(indent, declare)}"
+                    .generateTypeScriptString(indent, prefix, esModules) + "\n${classForRender.generateTypeScriptString(indent, declare, esModules)}"
             } else {
-                classForRender.generateTypeScriptString(indent, prefix)
+                classForRender.generateTypeScriptString(indent, prefix, esModules)
             }
         }
     }
 
-    private fun ExportedRegularClass.generateTypeScriptString(indent: String, prefix: String): String {
+    private fun ExportedRegularClass.generateTypeScriptString(indent: String, prefix: String, esModules: Boolean): String {
         val keyword = if (isInterface) "interface" else "class"
+        val (interfaceCompanions, allNestedClasses) = nestedClasses.partition { isInterface && it.ir.isCompanion }
         val superInterfacesKeyword = if (isInterface) "extends" else "implements"
 
         val superClassClause = superClasses.toExtendsClause(indent)
         val superInterfacesClause = superInterfaces.toImplementsClause(superInterfacesKeyword, indent)
 
-        val (memberObjects, nestedDeclarations) = nestedClasses.partition { it.couldBeProperty() }
+        val (memberObjects, nestedDeclarations) = allNestedClasses.partition { it.couldBeProperty() }
 
         val members = members.map {
             if (!ir.isInner || it !is ExportedFunction || !it.isStatic) {
@@ -321,7 +319,7 @@ class ExportModelToTsDeclarations {
 
         val bodyString = privateCtorString + membersString + indent
 
-        val nestedClasses = nonInnerClasses + innerClasses.map { it.withProtectedConstructors() }
+        val realNestedClasses = nonInnerClasses + innerClasses.map { it.withProtectedConstructors() }
         val tsIgnoreForPrivateConstructorInheritance = if (hasSuperClassWithPrivateConstructor()) {
             tsIgnore("extends class with private primary constructor") + "\n$indent"
         } else ""
@@ -329,9 +327,17 @@ class ExportModelToTsDeclarations {
         val klassExport =
             "$prefix$modifiers$keyword $name$renderedTypeParameters$superClassClause$superInterfacesClause {\n$bodyString}"
         val staticsExport =
-            if (nestedClasses.isNotEmpty()) "\n" + ExportedNamespace(name, nestedClasses).toTypeScript(indent, prefix) else ""
+            if (realNestedClasses.isNotEmpty()) "\n" + ExportedNamespace(name, realNestedClasses).toTypeScript(indent, prefix) else ""
 
-        return if (name.isValidES5Identifier()) tsIgnoreForPrivateConstructorInheritance + klassExport + staticsExport else ""
+        val interfaceCompanionsString = if (interfaceCompanions.isNotEmpty()) "\n" + interfaceCompanions.joinToString("\n") {
+            it.toTypeScript(
+                indent,
+                prefix,
+                esModules
+            )
+        } else ""
+
+        return if (name.isValidES5Identifier()) tsIgnoreForPrivateConstructorInheritance + klassExport + staticsExport + interfaceCompanionsString else ""
     }
 
     private fun ExportedRegularClass.hasSuperClassWithPrivateConstructor(): Boolean {
diff --git a/compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/transformers/irToJs/IrElementToJsExpressionTransformer.kt b/compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/transformers/irToJs/IrElementToJsExpressionTransformer.kt
index bb6941b3416..3bea90648a9 100644
--- a/compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/transformers/irToJs/IrElementToJsExpressionTransformer.kt
+++ b/compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/transformers/irToJs/IrElementToJsExpressionTransformer.kt
@@ -16,6 +16,7 @@ import org.jetbrains.kotlin.ir.types.isUnit
 import org.jetbrains.kotlin.ir.util.*
 import org.jetbrains.kotlin.js.backend.ast.*
 import org.jetbrains.kotlin.js.backend.ast.metadata.SideEffectKind
+import org.jetbrains.kotlin.js.backend.ast.metadata.constant
 import org.jetbrains.kotlin.js.backend.ast.metadata.sideEffects
 import org.jetbrains.kotlin.js.backend.ast.metadata.synthetic
 import org.jetbrains.kotlin.utils.memoryOptimizedMap
@@ -148,9 +149,8 @@ class IrElementToJsExpressionTransformer : BaseIrElementToJsNodeTransformer<JsEx
         assert(obj.kind == ClassKind.OBJECT)
         assert(obj.isEffectivelyExternal()) { "Non external IrGetObjectValue must be lowered" }
 
-        return when {
-            obj.isCompanion && obj.parentAsClass.let { it.isInterface && it.isExternal } -> JsNullLiteral()
-            else -> context.getRefForExternalClass(obj).withSource(expression, context)
+        return context.getRefForExternalClass(obj).withSource(expression, context).apply {
+            sideEffects = SideEffectKind.PURE
         }
     }
 
diff --git a/compiler/testData/diagnostics/testsWithJsStdLib/export/wrongExportedDeclaration.fir.kt b/compiler/testData/diagnostics/testsWithJsStdLib/export/wrongExportedDeclaration.fir.kt
index c132e0c9d9a..1806f77efea 100644
--- a/compiler/testData/diagnostics/testsWithJsStdLib/export/wrongExportedDeclaration.fir.kt
+++ b/compiler/testData/diagnostics/testsWithJsStdLib/export/wrongExportedDeclaration.fir.kt
@@ -25,7 +25,14 @@ external interface GoodInterface
 
 @JsExport
 interface InterfaceWithCompanion {
-    companion <!WRONG_EXPORTED_DECLARATION("companion object inside exported interface")!>object<!> {
+    companion object {
+        fun foo() = 42
+    }
+}
+
+@JsExport
+interface InterfaceWithNamedCompanion {
+    companion <!NAMED_COMPANION_IN_EXPORTED_INTERFACE!>object Named<!> {
         fun foo() = 42
     }
 }
diff --git a/compiler/testData/diagnostics/testsWithJsStdLib/export/wrongExportedDeclaration.kt b/compiler/testData/diagnostics/testsWithJsStdLib/export/wrongExportedDeclaration.kt
index dcb4984bd86..485ff351d5b 100644
--- a/compiler/testData/diagnostics/testsWithJsStdLib/export/wrongExportedDeclaration.kt
+++ b/compiler/testData/diagnostics/testsWithJsStdLib/export/wrongExportedDeclaration.kt
@@ -25,7 +25,14 @@ external interface GoodInterface
 
 @JsExport
 interface InterfaceWithCompanion {
-    companion <!WRONG_EXPORTED_DECLARATION("companion object inside exported interface")!>object<!> {
+    companion object {
+        fun foo() = 42
+    }
+}
+
+@JsExport
+interface InterfaceWithNamedCompanion {
+    companion <!NAMED_COMPANION_IN_EXPORTED_INTERFACE!>object Named<!> {
         fun foo() = 42
     }
 }
diff --git a/compiler/testData/diagnostics/testsWithJsStdLib/export/wrongExportedDeclaration.txt b/compiler/testData/diagnostics/testsWithJsStdLib/export/wrongExportedDeclaration.txt
index 052f63134e4..361d9ab3071 100644
--- a/compiler/testData/diagnostics/testsWithJsStdLib/export/wrongExportedDeclaration.txt
+++ b/compiler/testData/diagnostics/testsWithJsStdLib/export/wrongExportedDeclaration.txt
@@ -64,6 +64,20 @@ package foo {
         }
     }
 
+    @kotlin.js.JsExport public interface InterfaceWithNamedCompanion {
+        public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+
+        public companion object Named {
+            private constructor Named()
+            public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+            public final fun foo(): kotlin.Int
+            public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+            public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+        }
+    }
+
     @kotlin.js.JsExport public interface OuterInterface {
         public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
         public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
diff --git a/compiler/testData/diagnostics/testsWithJsStdLib/native/externalInterfaceNested.kt b/compiler/testData/diagnostics/testsWithJsStdLib/native/externalInterfaceNested.kt
index ce697540767..561cd133fee 100644
--- a/compiler/testData/diagnostics/testsWithJsStdLib/native/externalInterfaceNested.kt
+++ b/compiler/testData/diagnostics/testsWithJsStdLib/native/externalInterfaceNested.kt
@@ -9,5 +9,9 @@ external interface I {
 
     enum class <!ENUM_CLASS_IN_EXTERNAL_DECLARATION_WARNING, NESTED_CLASS_IN_EXTERNAL_INTERFACE!>E<!>
 
-    companion <!NESTED_CLASS_IN_EXTERNAL_INTERFACE!>object<!>
-}
\ No newline at end of file
+    companion object
+}
+
+external interface I2 {
+    companion <!NAMED_COMPANION_IN_EXTERNAL_INTERFACE!>object Named<!>
+}
diff --git a/compiler/testData/diagnostics/testsWithJsStdLib/native/externalInterfaceNested.txt b/compiler/testData/diagnostics/testsWithJsStdLib/native/externalInterfaceNested.txt
index a2f926d91b0..28b4298c9d7 100644
--- a/compiler/testData/diagnostics/testsWithJsStdLib/native/externalInterfaceNested.txt
+++ b/compiler/testData/diagnostics/testsWithJsStdLib/native/externalInterfaceNested.txt
@@ -47,3 +47,17 @@ public external interface I {
         public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
     }
 }
+
+public external interface I2 {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+
+    public companion object Named {
+        private constructor Named()
+        public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    }
+}
+
diff --git a/js/js.frontend/src/org/jetbrains/kotlin/js/resolve/JsPlatformConfigurator.kt b/js/js.frontend/src/org/jetbrains/kotlin/js/resolve/JsPlatformConfigurator.kt
index bc74e7ba978..e8709bd7370 100644
--- a/js/js.frontend/src/org/jetbrains/kotlin/js/resolve/JsPlatformConfigurator.kt
+++ b/js/js.frontend/src/org/jetbrains/kotlin/js/resolve/JsPlatformConfigurator.kt
@@ -22,7 +22,7 @@ object JsPlatformConfigurator : PlatformConfiguratorBase(
     additionalDeclarationCheckers = listOf(
         NativeInvokeChecker(), NativeGetterChecker(), NativeSetterChecker(),
         JsNameChecker, JsModuleChecker, JsExternalFileChecker,
-        JsExternalChecker, JsInheritanceChecker, JsMultipleInheritanceChecker,
+        JsInheritanceChecker, JsMultipleInheritanceChecker,
         JsExternalInheritorOnlyChecker,
         JsRuntimeAnnotationChecker,
         JsDynamicDeclarationChecker,
@@ -51,7 +51,8 @@ object JsPlatformConfigurator : PlatformConfiguratorBase(
         container.useInstance(ExtensionFunctionToExternalIsInlinable)
         container.useInstance(JsQualifierChecker)
         container.useInstance(JsNativeDiagnosticSuppressor)
-        container.useInstance(JsExportDeclarationChecker(includeUnsignedNumbers = false))
+        container.useInstance(JsExternalChecker(allowCompanionInInterface = true))
+        container.useInstance(JsExportDeclarationChecker(allowCompanionInInterface = true, includeUnsignedNumbers = false))
     }
 
     override fun configureModuleDependentCheckers(container: StorageComponentContainer) {
diff --git a/js/js.frontend/src/org/jetbrains/kotlin/js/resolve/diagnostics/DefaultErrorMessagesJs.kt b/js/js.frontend/src/org/jetbrains/kotlin/js/resolve/diagnostics/DefaultErrorMessagesJs.kt
index 0412442b46a..949a60f97cf 100644
--- a/js/js.frontend/src/org/jetbrains/kotlin/js/resolve/diagnostics/DefaultErrorMessagesJs.kt
+++ b/js/js.frontend/src/org/jetbrains/kotlin/js/resolve/diagnostics/DefaultErrorMessagesJs.kt
@@ -34,6 +34,9 @@ private val DIAGNOSTIC_FACTORY_TO_RENDERER by lazy {
         put(ErrorsJs.INLINE_CLASS_IN_EXTERNAL_DECLARATION_WARNING, "Using value classes as parameter type or return type of external declarations is experimental")
         put(ErrorsJs.ENUM_CLASS_IN_EXTERNAL_DECLARATION_WARNING, "Using enum classes with an `external` qualifier becomes deprecated and will be an error in future releases")
 
+        put(ErrorsJs.NAMED_COMPANION_IN_EXTERNAL_INTERFACE, "Named companions are not allowed inside external interfaces")
+        put(ErrorsJs.NAMED_COMPANION_IN_EXPORTED_INTERFACE, "Named companions are not allowed inside exported interfaces")
+
         put(ErrorsJs.JS_NAME_CLASH, "JavaScript name ({0}) generated for this declaration clashes with another declaration: {1}",
             STRING, Renderers.COMPACT)
         put(ErrorsJs.JS_FAKE_NAME_CLASH, "JavaScript name {0} is generated for different inherited members: {1} and {2}",
diff --git a/js/js.frontend/src/org/jetbrains/kotlin/js/resolve/diagnostics/ErrorsJs.java b/js/js.frontend/src/org/jetbrains/kotlin/js/resolve/diagnostics/ErrorsJs.java
index 13ca6de58f4..5af2dea25de 100644
--- a/js/js.frontend/src/org/jetbrains/kotlin/js/resolve/diagnostics/ErrorsJs.java
+++ b/js/js.frontend/src/org/jetbrains/kotlin/js/resolve/diagnostics/ErrorsJs.java
@@ -36,6 +36,8 @@ public interface ErrorsJs {
     DiagnosticFactory0<KtElement> INLINE_CLASS_IN_EXTERNAL_DECLARATION = DiagnosticFactory0.create(ERROR, DECLARATION_SIGNATURE_OR_DEFAULT);
     DiagnosticFactory0<KtElement> ENUM_CLASS_IN_EXTERNAL_DECLARATION_WARNING = DiagnosticFactory0.create(WARNING, DECLARATION_SIGNATURE_OR_DEFAULT);
     DiagnosticFactory0<KtElement> INLINE_CLASS_IN_EXTERNAL_DECLARATION_WARNING = DiagnosticFactory0.create(WARNING, DECLARATION_SIGNATURE_OR_DEFAULT);
+    DiagnosticFactory0<KtElement> NAMED_COMPANION_IN_EXTERNAL_INTERFACE = DiagnosticFactory0.create(ERROR, DECLARATION_SIGNATURE_OR_DEFAULT);
+    DiagnosticFactory0<KtElement> NAMED_COMPANION_IN_EXPORTED_INTERFACE = DiagnosticFactory0.create(ERROR, DECLARATION_SIGNATURE_OR_DEFAULT);
 
     DiagnosticFactory2<KtElement, String, DeclarationDescriptor> JS_NAME_CLASH = DiagnosticFactory2.create(
             ERROR, DECLARATION_SIGNATURE_OR_DEFAULT);
diff --git a/js/js.frontend/src/org/jetbrains/kotlin/js/resolve/diagnostics/JsDefinedExternallyCallChecker.kt b/js/js.frontend/src/org/jetbrains/kotlin/js/resolve/diagnostics/JsDefinedExternallyCallChecker.kt
index 45975cf90b1..342b5955a85 100644
--- a/js/js.frontend/src/org/jetbrains/kotlin/js/resolve/diagnostics/JsDefinedExternallyCallChecker.kt
+++ b/js/js.frontend/src/org/jetbrains/kotlin/js/resolve/diagnostics/JsDefinedExternallyCallChecker.kt
@@ -17,7 +17,7 @@
 package org.jetbrains.kotlin.js.resolve.diagnostics
 
 import com.intellij.psi.PsiElement
-import org.jetbrains.kotlin.js.resolve.diagnostics.JsExternalChecker.DEFINED_EXTERNALLY_PROPERTY_NAMES
+import org.jetbrains.kotlin.js.resolve.diagnostics.JsExternalChecker.Companion.DEFINED_EXTERNALLY_PROPERTY_NAMES
 import org.jetbrains.kotlin.js.translate.utils.AnnotationsUtils
 import org.jetbrains.kotlin.resolve.calls.checkers.CallChecker
 import org.jetbrains.kotlin.resolve.calls.checkers.CallCheckerContext
diff --git a/js/js.frontend/src/org/jetbrains/kotlin/js/resolve/diagnostics/JsExportDeclarationChecker.kt b/js/js.frontend/src/org/jetbrains/kotlin/js/resolve/diagnostics/JsExportDeclarationChecker.kt
index 73e25407b28..c90b36805d0 100644
--- a/js/js.frontend/src/org/jetbrains/kotlin/js/resolve/diagnostics/JsExportDeclarationChecker.kt
+++ b/js/js.frontend/src/org/jetbrains/kotlin/js/resolve/diagnostics/JsExportDeclarationChecker.kt
@@ -16,6 +16,7 @@ import org.jetbrains.kotlin.js.common.SPECIAL_KEYWORDS
 import org.jetbrains.kotlin.js.naming.NameSuggestion
 import org.jetbrains.kotlin.js.translate.utils.AnnotationsUtils
 import org.jetbrains.kotlin.name.FqName
+import org.jetbrains.kotlin.name.SpecialNames.DEFAULT_NAME_FOR_COMPANION_OBJECT
 import org.jetbrains.kotlin.psi.KtAnnotationEntry
 import org.jetbrains.kotlin.psi.KtDeclaration
 import org.jetbrains.kotlin.psi.KtNamedDeclaration
@@ -35,7 +36,10 @@ import org.jetbrains.kotlin.types.KotlinType
 import org.jetbrains.kotlin.types.isDynamic
 import org.jetbrains.kotlin.types.typeUtil.*
 
-class JsExportDeclarationChecker(private val includeUnsignedNumbers: Boolean) : DeclarationChecker {
+class JsExportDeclarationChecker(
+    private val includeUnsignedNumbers: Boolean,
+    private val allowCompanionInInterface: Boolean
+) : DeclarationChecker {
     override fun check(declaration: KtDeclaration, descriptor: DeclarationDescriptor, context: DeclarationCheckerContext) {
         val trace = context.trace
         val bindingContext = trace.bindingContext
@@ -130,11 +134,15 @@ class JsExportDeclarationChecker(private val includeUnsignedNumbers: Boolean) :
                         descriptor.isInlineClass() -> "${if (descriptor.isInline) "inline " else ""}${if (descriptor.isValue) "value " else ""}class"
                         else -> null
                     }
-                    else -> if (descriptor.isInsideInterface) {
+                    else -> if (descriptor.isInsideInterface && (!allowCompanionInInterface || !descriptor.isCompanionObject)) {
                         "${if (descriptor.isCompanionObject) "companion object" else "nested/inner declaration"} inside exported interface"
                     } else null
                 }
 
+                if (allowCompanionInInterface && descriptor.isCompanionObject && descriptor.isInsideInterface && descriptor.name != DEFAULT_NAME_FOR_COMPANION_OBJECT) {
+                    trace.report(ErrorsJs.NAMED_COMPANION_IN_EXPORTED_INTERFACE.on(declaration))
+                }
+
                 if (wrongDeclaration != null) {
                     reportWrongExportedDeclaration(wrongDeclaration)
                     return
diff --git a/js/js.frontend/src/org/jetbrains/kotlin/js/resolve/diagnostics/JsExternalChecker.kt b/js/js.frontend/src/org/jetbrains/kotlin/js/resolve/diagnostics/JsExternalChecker.kt
index 3aa4391f145..af5555a5621 100644
--- a/js/js.frontend/src/org/jetbrains/kotlin/js/resolve/diagnostics/JsExternalChecker.kt
+++ b/js/js.frontend/src/org/jetbrains/kotlin/js/resolve/diagnostics/JsExternalChecker.kt
@@ -15,6 +15,7 @@ import org.jetbrains.kotlin.js.PredefinedAnnotation
 import org.jetbrains.kotlin.js.translate.utils.AnnotationsUtils
 import org.jetbrains.kotlin.lexer.KtTokens
 import org.jetbrains.kotlin.name.JsStandardClassIds
+import org.jetbrains.kotlin.name.SpecialNames.DEFAULT_NAME_FOR_COMPANION_OBJECT
 import org.jetbrains.kotlin.platform.isWasm
 import org.jetbrains.kotlin.psi.*
 import org.jetbrains.kotlin.resolve.BindingContext
@@ -28,9 +29,11 @@ import org.jetbrains.kotlin.resolve.source.getPsi
 import org.jetbrains.kotlin.types.KotlinType
 import org.jetbrains.kotlin.types.TypeUtils
 
-object JsExternalChecker : DeclarationChecker {
-    val DEFINED_EXTERNALLY_PROPERTY_NAMES = JsStandardClassIds.Callables.definedExternallyPropertyNames
-        .map { it.asSingleFqName().toUnsafe() }
+class JsExternalChecker(private val allowCompanionInInterface: Boolean) : DeclarationChecker {
+    companion object {
+        val DEFINED_EXTERNALLY_PROPERTY_NAMES = JsStandardClassIds.Callables.definedExternallyPropertyNames
+            .map { it.asSingleFqName().toUnsafe() }
+    }
 
     override fun check(declaration: KtDeclaration, descriptor: DeclarationDescriptor, context: DeclarationCheckerContext) {
         if (!AnnotationsUtils.isNativeObject(descriptor)) return
@@ -68,12 +71,16 @@ object JsExternalChecker : DeclarationChecker {
             trace.report(ErrorsJs.WRONG_EXTERNAL_DECLARATION.on(declaration, "private member of class"))
         }
 
-        if (descriptor is ClassDescriptor && descriptor.kind != ClassKind.INTERFACE &&
-            descriptor.containingDeclaration.let { it is ClassDescriptor && it.kind == ClassKind.INTERFACE }
-        ) {
+        val containingDeclarationsIsInterface = descriptor.containingDeclaration.let { it is ClassDescriptor && it.kind == ClassKind.INTERFACE }
+
+        if (descriptor is ClassDescriptor && descriptor.kind != ClassKind.INTERFACE && (!allowCompanionInInterface || !descriptor.isCompanionObject) && containingDeclarationsIsInterface) {
             trace.report(ErrorsJs.NESTED_CLASS_IN_EXTERNAL_INTERFACE.on(declaration))
         }
 
+        if (allowCompanionInInterface && descriptor.isCompanionObject() && containingDeclarationsIsInterface && descriptor.name != DEFAULT_NAME_FOR_COMPANION_OBJECT) {
+            trace.report(ErrorsJs.NAMED_COMPANION_IN_EXTERNAL_INTERFACE.on(declaration))
+        }
+
         if (descriptor !is PropertyAccessorDescriptor && descriptor.isExtension) {
             val target = when (descriptor) {
                 is FunctionDescriptor -> "extension function"
diff --git a/js/js.inliner/src/org/jetbrains/kotlin/js/inline/clean/RedundantVariableDeclarationElimination.kt b/js/js.inliner/src/org/jetbrains/kotlin/js/inline/clean/RedundantVariableDeclarationElimination.kt
index d560acc904e..36fc47d63ab 100644
--- a/js/js.inliner/src/org/jetbrains/kotlin/js/inline/clean/RedundantVariableDeclarationElimination.kt
+++ b/js/js.inliner/src/org/jetbrains/kotlin/js/inline/clean/RedundantVariableDeclarationElimination.kt
@@ -17,6 +17,9 @@
 package org.jetbrains.kotlin.js.inline.clean
 
 import org.jetbrains.kotlin.js.backend.ast.*
+import org.jetbrains.kotlin.js.backend.ast.metadata.SideEffectKind
+import org.jetbrains.kotlin.js.backend.ast.metadata.constant
+import org.jetbrains.kotlin.js.backend.ast.metadata.sideEffects
 import org.jetbrains.kotlin.js.backend.ast.metadata.synthetic
 import org.jetbrains.kotlin.js.inline.util.collectFreeVariables
 
@@ -55,7 +58,8 @@ internal class RedundantVariableDeclarationElimination(private val root: JsState
         object : JsVisitorWithContextImpl() {
             override fun endVisit(x: JsVars, ctx: JsContext<*>) {
                 if (x.synthetic) {
-                    if (x.vars.removeAll { it.initExpression == null && it.name !in usages }) {
+                    if (
+                        x.vars.removeAll { it.initExpression.isPure && it.name !in usages }) {
                         hasChanges = true
                     }
                     if (x.vars.isEmpty()) {
@@ -65,6 +69,9 @@ internal class RedundantVariableDeclarationElimination(private val root: JsState
                 super.endVisit(x, ctx)
             }
 
+            private val JsExpression?.isPure: Boolean
+                get() = this == null || constant || sideEffects == SideEffectKind.PURE
+
             override fun visit(x: JsFunction, ctx: JsContext<*>) = false
         }.accept(root)
     }
diff --git a/js/js.tests/tests-gen/org/jetbrains/kotlin/js/test/fir/FirJsES6BoxTestGenerated.java b/js/js.tests/tests-gen/org/jetbrains/kotlin/js/test/fir/FirJsES6BoxTestGenerated.java
index c0771a59680..9804efcf26d 100644
--- a/js/js.tests/tests-gen/org/jetbrains/kotlin/js/test/fir/FirJsES6BoxTestGenerated.java
+++ b/js/js.tests/tests-gen/org/jetbrains/kotlin/js/test/fir/FirJsES6BoxTestGenerated.java
@@ -2605,6 +2605,12 @@ public class FirJsES6BoxTestGenerated extends AbstractFirJsES6BoxTest {
         runTest("js/js.translator/testData/box/esModules/jsExport/exportedDefaultStub.kt");
       }
 
+      @Test
+      @TestMetadata("interfaceWithCompanion.kt")
+      public void testInterfaceWithCompanion() {
+        runTest("js/js.translator/testData/box/esModules/jsExport/interfaceWithCompanion.kt");
+      }
+
       @Test
       @TestMetadata("jsExportInClass.kt")
       public void testJsExportInClass() {
@@ -2699,6 +2705,12 @@ public class FirJsES6BoxTestGenerated extends AbstractFirJsES6BoxTest {
         runTest("js/js.translator/testData/box/esModules/jsModule/interfaces.kt");
       }
 
+      @Test
+      @TestMetadata("interfacesWithCompanion.kt")
+      public void testInterfacesWithCompanion() {
+        runTest("js/js.translator/testData/box/esModules/jsModule/interfacesWithCompanion.kt");
+      }
+
       @Test
       @TestMetadata("jsExternalInheritorsOnly.kt")
       public void testJsExternalInheritorsOnly() {
@@ -7524,6 +7536,12 @@ public class FirJsES6BoxTestGenerated extends AbstractFirJsES6BoxTest {
       runTest("js/js.translator/testData/box/jsExport/exportedDefaultStub.kt");
     }
 
+    @Test
+    @TestMetadata("interfaceWithCompanion.kt")
+    public void testInterfaceWithCompanion() {
+      runTest("js/js.translator/testData/box/jsExport/interfaceWithCompanion.kt");
+    }
+
     @Test
     @TestMetadata("jsExportInClass.kt")
     public void testJsExportInClass() {
@@ -7660,6 +7678,12 @@ public class FirJsES6BoxTestGenerated extends AbstractFirJsES6BoxTest {
       runTest("js/js.translator/testData/box/jsModule/interfaces.kt");
     }
 
+    @Test
+    @TestMetadata("interfacesWithCompanion.kt")
+    public void testInterfacesWithCompanion() {
+      runTest("js/js.translator/testData/box/jsModule/interfacesWithCompanion.kt");
+    }
+
     @Test
     @TestMetadata("kt39378.kt")
     public void testKt39378() {
@@ -7836,6 +7860,12 @@ public class FirJsES6BoxTestGenerated extends AbstractFirJsES6BoxTest {
       runTest("js/js.translator/testData/box/jsQualifier/interfaces.kt");
     }
 
+    @Test
+    @TestMetadata("interfacesWithCompanion.kt")
+    public void testInterfacesWithCompanion() {
+      runTest("js/js.translator/testData/box/jsQualifier/interfacesWithCompanion.kt");
+    }
+
     @Test
     @TestMetadata("simple.kt")
     public void testSimple() {
diff --git a/js/js.tests/tests-gen/org/jetbrains/kotlin/js/test/fir/FirLightTreeJsBoxTestGenerated.java b/js/js.tests/tests-gen/org/jetbrains/kotlin/js/test/fir/FirLightTreeJsBoxTestGenerated.java
index 020ae58dce5..f0d3d34ca61 100644
--- a/js/js.tests/tests-gen/org/jetbrains/kotlin/js/test/fir/FirLightTreeJsBoxTestGenerated.java
+++ b/js/js.tests/tests-gen/org/jetbrains/kotlin/js/test/fir/FirLightTreeJsBoxTestGenerated.java
@@ -2499,6 +2499,12 @@ public class FirLightTreeJsBoxTestGenerated extends AbstractFirLightTreeJsBoxTes
         runTest("js/js.translator/testData/box/esModules/jsExport/exportedDefaultStub.kt");
       }
 
+      @Test
+      @TestMetadata("interfaceWithCompanion.kt")
+      public void testInterfaceWithCompanion() {
+        runTest("js/js.translator/testData/box/esModules/jsExport/interfaceWithCompanion.kt");
+      }
+
       @Test
       @TestMetadata("jsExportInClass.kt")
       public void testJsExportInClass() {
@@ -2593,6 +2599,12 @@ public class FirLightTreeJsBoxTestGenerated extends AbstractFirLightTreeJsBoxTes
         runTest("js/js.translator/testData/box/esModules/jsModule/interfaces.kt");
       }
 
+      @Test
+      @TestMetadata("interfacesWithCompanion.kt")
+      public void testInterfacesWithCompanion() {
+        runTest("js/js.translator/testData/box/esModules/jsModule/interfacesWithCompanion.kt");
+      }
+
       @Test
       @TestMetadata("jsExternalInheritorsOnly.kt")
       public void testJsExternalInheritorsOnly() {
@@ -7418,6 +7430,12 @@ public class FirLightTreeJsBoxTestGenerated extends AbstractFirLightTreeJsBoxTes
       runTest("js/js.translator/testData/box/jsExport/exportedDefaultStub.kt");
     }
 
+    @Test
+    @TestMetadata("interfaceWithCompanion.kt")
+    public void testInterfaceWithCompanion() {
+      runTest("js/js.translator/testData/box/jsExport/interfaceWithCompanion.kt");
+    }
+
     @Test
     @TestMetadata("jsExportInClass.kt")
     public void testJsExportInClass() {
@@ -7554,6 +7572,12 @@ public class FirLightTreeJsBoxTestGenerated extends AbstractFirLightTreeJsBoxTes
       runTest("js/js.translator/testData/box/jsModule/interfaces.kt");
     }
 
+    @Test
+    @TestMetadata("interfacesWithCompanion.kt")
+    public void testInterfacesWithCompanion() {
+      runTest("js/js.translator/testData/box/jsModule/interfacesWithCompanion.kt");
+    }
+
     @Test
     @TestMetadata("kt39378.kt")
     public void testKt39378() {
@@ -7730,6 +7754,12 @@ public class FirLightTreeJsBoxTestGenerated extends AbstractFirLightTreeJsBoxTes
       runTest("js/js.translator/testData/box/jsQualifier/interfaces.kt");
     }
 
+    @Test
+    @TestMetadata("interfacesWithCompanion.kt")
+    public void testInterfacesWithCompanion() {
+      runTest("js/js.translator/testData/box/jsQualifier/interfacesWithCompanion.kt");
+    }
+
     @Test
     @TestMetadata("simple.kt")
     public void testSimple() {
diff --git a/js/js.tests/tests-gen/org/jetbrains/kotlin/js/test/fir/FirPsiJsBoxTestGenerated.java b/js/js.tests/tests-gen/org/jetbrains/kotlin/js/test/fir/FirPsiJsBoxTestGenerated.java
index 29652c8faf4..75c3de0d541 100644
--- a/js/js.tests/tests-gen/org/jetbrains/kotlin/js/test/fir/FirPsiJsBoxTestGenerated.java
+++ b/js/js.tests/tests-gen/org/jetbrains/kotlin/js/test/fir/FirPsiJsBoxTestGenerated.java
@@ -2499,6 +2499,12 @@ public class FirPsiJsBoxTestGenerated extends AbstractFirPsiJsBoxTest {
         runTest("js/js.translator/testData/box/esModules/jsExport/exportedDefaultStub.kt");
       }
 
+      @Test
+      @TestMetadata("interfaceWithCompanion.kt")
+      public void testInterfaceWithCompanion() {
+        runTest("js/js.translator/testData/box/esModules/jsExport/interfaceWithCompanion.kt");
+      }
+
       @Test
       @TestMetadata("jsExportInClass.kt")
       public void testJsExportInClass() {
@@ -2593,6 +2599,12 @@ public class FirPsiJsBoxTestGenerated extends AbstractFirPsiJsBoxTest {
         runTest("js/js.translator/testData/box/esModules/jsModule/interfaces.kt");
       }
 
+      @Test
+      @TestMetadata("interfacesWithCompanion.kt")
+      public void testInterfacesWithCompanion() {
+        runTest("js/js.translator/testData/box/esModules/jsModule/interfacesWithCompanion.kt");
+      }
+
       @Test
       @TestMetadata("jsExternalInheritorsOnly.kt")
       public void testJsExternalInheritorsOnly() {
@@ -7418,6 +7430,12 @@ public class FirPsiJsBoxTestGenerated extends AbstractFirPsiJsBoxTest {
       runTest("js/js.translator/testData/box/jsExport/exportedDefaultStub.kt");
     }
 
+    @Test
+    @TestMetadata("interfaceWithCompanion.kt")
+    public void testInterfaceWithCompanion() {
+      runTest("js/js.translator/testData/box/jsExport/interfaceWithCompanion.kt");
+    }
+
     @Test
     @TestMetadata("jsExportInClass.kt")
     public void testJsExportInClass() {
@@ -7554,6 +7572,12 @@ public class FirPsiJsBoxTestGenerated extends AbstractFirPsiJsBoxTest {
       runTest("js/js.translator/testData/box/jsModule/interfaces.kt");
     }
 
+    @Test
+    @TestMetadata("interfacesWithCompanion.kt")
+    public void testInterfacesWithCompanion() {
+      runTest("js/js.translator/testData/box/jsModule/interfacesWithCompanion.kt");
+    }
+
     @Test
     @TestMetadata("kt39378.kt")
     public void testKt39378() {
@@ -7730,6 +7754,12 @@ public class FirPsiJsBoxTestGenerated extends AbstractFirPsiJsBoxTest {
       runTest("js/js.translator/testData/box/jsQualifier/interfaces.kt");
     }
 
+    @Test
+    @TestMetadata("interfacesWithCompanion.kt")
+    public void testInterfacesWithCompanion() {
+      runTest("js/js.translator/testData/box/jsQualifier/interfacesWithCompanion.kt");
+    }
+
     @Test
     @TestMetadata("simple.kt")
     public void testSimple() {
diff --git a/js/js.tests/tests-gen/org/jetbrains/kotlin/js/test/ir/IrBoxJsES6TestGenerated.java b/js/js.tests/tests-gen/org/jetbrains/kotlin/js/test/ir/IrBoxJsES6TestGenerated.java
index e68f33f6171..68765e024de 100644
--- a/js/js.tests/tests-gen/org/jetbrains/kotlin/js/test/ir/IrBoxJsES6TestGenerated.java
+++ b/js/js.tests/tests-gen/org/jetbrains/kotlin/js/test/ir/IrBoxJsES6TestGenerated.java
@@ -2605,6 +2605,12 @@ public class IrBoxJsES6TestGenerated extends AbstractIrBoxJsES6Test {
         runTest("js/js.translator/testData/box/esModules/jsExport/exportedDefaultStub.kt");
       }
 
+      @Test
+      @TestMetadata("interfaceWithCompanion.kt")
+      public void testInterfaceWithCompanion() {
+        runTest("js/js.translator/testData/box/esModules/jsExport/interfaceWithCompanion.kt");
+      }
+
       @Test
       @TestMetadata("jsExportInClass.kt")
       public void testJsExportInClass() {
@@ -2699,6 +2705,12 @@ public class IrBoxJsES6TestGenerated extends AbstractIrBoxJsES6Test {
         runTest("js/js.translator/testData/box/esModules/jsModule/interfaces.kt");
       }
 
+      @Test
+      @TestMetadata("interfacesWithCompanion.kt")
+      public void testInterfacesWithCompanion() {
+        runTest("js/js.translator/testData/box/esModules/jsModule/interfacesWithCompanion.kt");
+      }
+
       @Test
       @TestMetadata("jsExternalInheritorsOnly.kt")
       public void testJsExternalInheritorsOnly() {
@@ -7524,6 +7536,12 @@ public class IrBoxJsES6TestGenerated extends AbstractIrBoxJsES6Test {
       runTest("js/js.translator/testData/box/jsExport/exportedDefaultStub.kt");
     }
 
+    @Test
+    @TestMetadata("interfaceWithCompanion.kt")
+    public void testInterfaceWithCompanion() {
+      runTest("js/js.translator/testData/box/jsExport/interfaceWithCompanion.kt");
+    }
+
     @Test
     @TestMetadata("jsExportInClass.kt")
     public void testJsExportInClass() {
@@ -7660,6 +7678,12 @@ public class IrBoxJsES6TestGenerated extends AbstractIrBoxJsES6Test {
       runTest("js/js.translator/testData/box/jsModule/interfaces.kt");
     }
 
+    @Test
+    @TestMetadata("interfacesWithCompanion.kt")
+    public void testInterfacesWithCompanion() {
+      runTest("js/js.translator/testData/box/jsModule/interfacesWithCompanion.kt");
+    }
+
     @Test
     @TestMetadata("kt39378.kt")
     public void testKt39378() {
@@ -7836,6 +7860,12 @@ public class IrBoxJsES6TestGenerated extends AbstractIrBoxJsES6Test {
       runTest("js/js.translator/testData/box/jsQualifier/interfaces.kt");
     }
 
+    @Test
+    @TestMetadata("interfacesWithCompanion.kt")
+    public void testInterfacesWithCompanion() {
+      runTest("js/js.translator/testData/box/jsQualifier/interfacesWithCompanion.kt");
+    }
+
     @Test
     @TestMetadata("simple.kt")
     public void testSimple() {
diff --git a/js/js.tests/tests-gen/org/jetbrains/kotlin/js/test/ir/IrBoxJsTestGenerated.java b/js/js.tests/tests-gen/org/jetbrains/kotlin/js/test/ir/IrBoxJsTestGenerated.java
index 86ba9ba6e91..c60a540b3f4 100644
--- a/js/js.tests/tests-gen/org/jetbrains/kotlin/js/test/ir/IrBoxJsTestGenerated.java
+++ b/js/js.tests/tests-gen/org/jetbrains/kotlin/js/test/ir/IrBoxJsTestGenerated.java
@@ -2499,6 +2499,12 @@ public class IrBoxJsTestGenerated extends AbstractIrBoxJsTest {
         runTest("js/js.translator/testData/box/esModules/jsExport/exportedDefaultStub.kt");
       }
 
+      @Test
+      @TestMetadata("interfaceWithCompanion.kt")
+      public void testInterfaceWithCompanion() {
+        runTest("js/js.translator/testData/box/esModules/jsExport/interfaceWithCompanion.kt");
+      }
+
       @Test
       @TestMetadata("jsExportInClass.kt")
       public void testJsExportInClass() {
@@ -2593,6 +2599,12 @@ public class IrBoxJsTestGenerated extends AbstractIrBoxJsTest {
         runTest("js/js.translator/testData/box/esModules/jsModule/interfaces.kt");
       }
 
+      @Test
+      @TestMetadata("interfacesWithCompanion.kt")
+      public void testInterfacesWithCompanion() {
+        runTest("js/js.translator/testData/box/esModules/jsModule/interfacesWithCompanion.kt");
+      }
+
       @Test
       @TestMetadata("jsExternalInheritorsOnly.kt")
       public void testJsExternalInheritorsOnly() {
@@ -7418,6 +7430,12 @@ public class IrBoxJsTestGenerated extends AbstractIrBoxJsTest {
       runTest("js/js.translator/testData/box/jsExport/exportedDefaultStub.kt");
     }
 
+    @Test
+    @TestMetadata("interfaceWithCompanion.kt")
+    public void testInterfaceWithCompanion() {
+      runTest("js/js.translator/testData/box/jsExport/interfaceWithCompanion.kt");
+    }
+
     @Test
     @TestMetadata("jsExportInClass.kt")
     public void testJsExportInClass() {
@@ -7554,6 +7572,12 @@ public class IrBoxJsTestGenerated extends AbstractIrBoxJsTest {
       runTest("js/js.translator/testData/box/jsModule/interfaces.kt");
     }
 
+    @Test
+    @TestMetadata("interfacesWithCompanion.kt")
+    public void testInterfacesWithCompanion() {
+      runTest("js/js.translator/testData/box/jsModule/interfacesWithCompanion.kt");
+    }
+
     @Test
     @TestMetadata("kt39378.kt")
     public void testKt39378() {
@@ -7730,6 +7754,12 @@ public class IrBoxJsTestGenerated extends AbstractIrBoxJsTest {
       runTest("js/js.translator/testData/box/jsQualifier/interfaces.kt");
     }
 
+    @Test
+    @TestMetadata("interfacesWithCompanion.kt")
+    public void testInterfacesWithCompanion() {
+      runTest("js/js.translator/testData/box/jsQualifier/interfacesWithCompanion.kt");
+    }
+
     @Test
     @TestMetadata("simple.kt")
     public void testSimple() {
diff --git a/js/js.translator/testData/box/esModules/jsExport/interfaceWithCompanion.kt b/js/js.translator/testData/box/esModules/jsExport/interfaceWithCompanion.kt
new file mode 100644
index 00000000000..fa7e53d386b
--- /dev/null
+++ b/js/js.translator/testData/box/esModules/jsExport/interfaceWithCompanion.kt
@@ -0,0 +1,24 @@
+// SKIP_MINIFICATION
+// ES_MODULES
+
+// FILE: api.kt
+package api
+
+@JsExport
+interface A {
+    companion object {
+        fun ok() = "OK"
+    }
+}
+
+// FILE: main.kt
+external interface JsResult {
+    val res: String
+}
+
+@JsModule("./interfaceWithCompanion.mjs")
+external fun jsBox(): JsResult
+
+fun box(): String {
+    return jsBox().res
+}
diff --git a/js/js.translator/testData/box/esModules/jsExport/interfaceWithCompanion.mjs b/js/js.translator/testData/box/esModules/jsExport/interfaceWithCompanion.mjs
new file mode 100644
index 00000000000..bfc988564ff
--- /dev/null
+++ b/js/js.translator/testData/box/esModules/jsExport/interfaceWithCompanion.mjs
@@ -0,0 +1,7 @@
+import * as api from "./interfaceWithCompanion_v5.mjs";
+
+export default function() {
+    return {
+        "res": api.A.getInstance().ok()
+    };
+};
\ No newline at end of file
diff --git a/js/js.translator/testData/box/esModules/jsModule/interfacesWithCompanion.kt b/js/js.translator/testData/box/esModules/jsModule/interfacesWithCompanion.kt
new file mode 100644
index 00000000000..aafb7ff1146
--- /dev/null
+++ b/js/js.translator/testData/box/esModules/jsModule/interfacesWithCompanion.kt
@@ -0,0 +1,20 @@
+// TARGET_BACKEND: JS_IR
+// TARGET_BACKEND: JS_IR_ES6
+// EXPECTED_REACHABLE_NODES: 1238
+// ES_MODULES
+// FILE: bar.kt
+@file:JsModule("./interfacesWithCompanion.mjs")
+package bar
+
+external interface Bar {
+    companion object {
+        fun ok(): String
+    }
+}
+
+// FILE: test.kt
+import bar.Bar
+
+fun box(): String {
+    return Bar.ok()
+}
\ No newline at end of file
diff --git a/js/js.translator/testData/box/esModules/jsModule/interfacesWithCompanion.mjs b/js/js.translator/testData/box/esModules/jsModule/interfacesWithCompanion.mjs
new file mode 100644
index 00000000000..f5344ffe850
--- /dev/null
+++ b/js/js.translator/testData/box/esModules/jsModule/interfacesWithCompanion.mjs
@@ -0,0 +1,3 @@
+export const Bar = {
+    ok() { return "OK" }
+};
diff --git a/js/js.translator/testData/box/jsExport/interfaceWithCompanion.js b/js/js.translator/testData/box/jsExport/interfaceWithCompanion.js
new file mode 100644
index 00000000000..a32b1e03337
--- /dev/null
+++ b/js/js.translator/testData/box/jsExport/interfaceWithCompanion.js
@@ -0,0 +1,12 @@
+$kotlin_test_internal$.beginModule();
+
+module.exports = function() {
+    var { A } = require("main").api
+
+    return {
+        "res": A.ok()
+    };
+};
+
+$kotlin_test_internal$.endModule("lib");
+
diff --git a/js/js.translator/testData/box/jsExport/interfaceWithCompanion.kt b/js/js.translator/testData/box/jsExport/interfaceWithCompanion.kt
new file mode 100644
index 00000000000..ab5354144a6
--- /dev/null
+++ b/js/js.translator/testData/box/jsExport/interfaceWithCompanion.kt
@@ -0,0 +1,24 @@
+// MODULE_KIND: COMMON_JS
+// SKIP_MINIFICATION
+
+// FILE: api.kt
+package api
+
+@JsExport
+interface A {
+    companion object {
+        fun ok() = "OK"
+    }
+}
+
+// FILE: main.kt
+external interface JsResult {
+    val res: String
+}
+
+@JsModule("lib")
+external fun jsBox(): JsResult
+
+fun box(): String {
+    return jsBox().res
+}
diff --git a/js/js.translator/testData/box/jsModule/interfacesWithCompanion.js b/js/js.translator/testData/box/jsModule/interfacesWithCompanion.js
new file mode 100644
index 00000000000..5d76b6229e9
--- /dev/null
+++ b/js/js.translator/testData/box/jsModule/interfacesWithCompanion.js
@@ -0,0 +1,7 @@
+define("bar", [], function() {
+    return {
+        Bar: {
+            ok() { return "OK" }
+        }
+    };
+});
diff --git a/js/js.translator/testData/box/jsModule/interfacesWithCompanion.kt b/js/js.translator/testData/box/jsModule/interfacesWithCompanion.kt
new file mode 100644
index 00000000000..5497e57179f
--- /dev/null
+++ b/js/js.translator/testData/box/jsModule/interfacesWithCompanion.kt
@@ -0,0 +1,23 @@
+// TARGET_BACKEND: JS_IR
+// TARGET_BACKEND: JS_IR_ES6
+// EXPECTED_REACHABLE_NODES: 1238
+// MODULE_KIND: AMD
+// FILE: bar.kt
+@file:JsModule("bar")
+package bar
+
+external interface Bar {
+    companion object {
+        fun ok(): String
+    }
+}
+
+// FILE: test.kt
+import bar.Bar
+
+inline fun Bar.Companion.test() = "CHECK"
+
+fun box(): String {
+    Bar.test()
+    return Bar.ok()
+}
\ No newline at end of file
diff --git a/js/js.translator/testData/box/jsQualifier/interfacesWithCompanion.kt b/js/js.translator/testData/box/jsQualifier/interfacesWithCompanion.kt
new file mode 100644
index 00000000000..abb60b591ee
--- /dev/null
+++ b/js/js.translator/testData/box/jsQualifier/interfacesWithCompanion.kt
@@ -0,0 +1,31 @@
+// TARGET_BACKEND: JS_IR
+// TARGET_BACKEND: JS_IR_ES6
+// EXPECTED_REACHABLE_NODES: 1238
+// FILE: bar.kt
+@file:JsQualifier("bar")
+package bar
+
+external interface Bar {
+    companion object {
+        fun ok(): String
+    }
+}
+
+// FILE: test.kt
+import bar.Bar
+
+fun box(): String {
+    return Bar.ok()
+}
+
+// FILE: test.js
+var bar = function() {
+    var Bar = {
+        ok() {
+            return "OK"
+        }
+    };
+    return {
+        Bar: Bar
+    }
+}();
diff --git a/js/js.translator/testData/typescript-export/interfaces-in-exported-file/interfaces.d.ts b/js/js.translator/testData/typescript-export/interfaces-in-exported-file/interfaces.d.ts
index 2da9c13373a..7c1324ebc4e 100644
--- a/js/js.translator/testData/typescript-export/interfaces-in-exported-file/interfaces.d.ts
+++ b/js/js.translator/testData/typescript-export/interfaces-in-exported-file/interfaces.d.ts
@@ -30,6 +30,15 @@ declare namespace JS_TESTS {
             readonly __doNotUseOrImplementIt: foo.TestInterfaceImpl["__doNotUseOrImplementIt"] & foo.AnotherExportedInterface["__doNotUseOrImplementIt"];
         }
         function processInterface(test: foo.TestInterface): string;
+        interface WithTheCompanion {
+            readonly interfaceField: string;
+            readonly __doNotUseOrImplementIt: {
+                readonly "foo.WithTheCompanion": unique symbol;
+            };
+        }
+        const WithTheCompanion: {
+            companionFunction(): string;
+        };
         function processOptionalInterface(a: foo.OptionalFieldsInterface): string;
         interface InterfaceWithCompanion {
             readonly __doNotUseOrImplementIt: {
@@ -37,4 +46,4 @@ declare namespace JS_TESTS {
             };
         }
     }
-}
\ No newline at end of file
+}
diff --git a/js/js.translator/testData/typescript-export/interfaces-in-exported-file/interfaces.kt b/js/js.translator/testData/typescript-export/interfaces-in-exported-file/interfaces.kt
index b7289819a34..5e848f11755 100644
--- a/js/js.translator/testData/typescript-export/interfaces-in-exported-file/interfaces.kt
+++ b/js/js.translator/testData/typescript-export/interfaces-in-exported-file/interfaces.kt
@@ -43,6 +43,15 @@ external interface OptionalFieldsInterface {
 }
 
 
+interface WithTheCompanion {
+    val interfaceField: String
+
+    companion object {
+        fun companionFunction(): String = "FUNCTION"
+    }
+}
+
+
 fun processOptionalInterface(a: OptionalFieldsInterface): String {
     return "${a.required}${a.notRequired ?: "unknown"}"
 }
@@ -52,6 +61,7 @@ fun processOptionalInterface(a: OptionalFieldsInterface): String {
 interface InterfaceWithCompanion {
     // Emulate added by plugin companion like kotlinx.serialization does
     @Suppress("WRONG_EXPORTED_DECLARATION")
+    @JsExport.Ignore
     companion object {
         fun foo() = "String"
     }
diff --git a/js/js.translator/testData/typescript-export/interfaces-in-exported-file/interfaces__main.ts b/js/js.translator/testData/typescript-export/interfaces-in-exported-file/interfaces__main.ts
index de5842b2b27..be559a4b43b 100644
--- a/js/js.translator/testData/typescript-export/interfaces-in-exported-file/interfaces__main.ts
+++ b/js/js.translator/testData/typescript-export/interfaces-in-exported-file/interfaces__main.ts
@@ -2,6 +2,7 @@ import TestInterfaceImpl = JS_TESTS.foo.TestInterfaceImpl;
 import ChildTestInterfaceImpl = JS_TESTS.foo.ChildTestInterfaceImpl;
 import processInterface = JS_TESTS.foo.processInterface;
 import processOptionalInterface = JS_TESTS.foo.processOptionalInterface;
+import WithTheCompanion = JS_TESTS.foo.WithTheCompanion;
 
 function assert(condition: boolean) {
     if (!condition) {
@@ -20,5 +21,7 @@ function box(): string {
     assert(processOptionalInterface({ required: 4, notRequired: null }) == "4unknown")
     assert(processOptionalInterface({ required: 4, notRequired: 5 }) == "45")
 
+    assert(WithTheCompanion.companionFunction() == "FUNCTION")
+
     return "OK";
 }
\ No newline at end of file
diff --git a/js/js.translator/testData/typescript-export/interfaces/interfaces.d.ts b/js/js.translator/testData/typescript-export/interfaces/interfaces.d.ts
index 2da9c13373a..7c1324ebc4e 100644
--- a/js/js.translator/testData/typescript-export/interfaces/interfaces.d.ts
+++ b/js/js.translator/testData/typescript-export/interfaces/interfaces.d.ts
@@ -30,6 +30,15 @@ declare namespace JS_TESTS {
             readonly __doNotUseOrImplementIt: foo.TestInterfaceImpl["__doNotUseOrImplementIt"] & foo.AnotherExportedInterface["__doNotUseOrImplementIt"];
         }
         function processInterface(test: foo.TestInterface): string;
+        interface WithTheCompanion {
+            readonly interfaceField: string;
+            readonly __doNotUseOrImplementIt: {
+                readonly "foo.WithTheCompanion": unique symbol;
+            };
+        }
+        const WithTheCompanion: {
+            companionFunction(): string;
+        };
         function processOptionalInterface(a: foo.OptionalFieldsInterface): string;
         interface InterfaceWithCompanion {
             readonly __doNotUseOrImplementIt: {
@@ -37,4 +46,4 @@ declare namespace JS_TESTS {
             };
         }
     }
-}
\ No newline at end of file
+}
diff --git a/js/js.translator/testData/typescript-export/interfaces/interfaces.kt b/js/js.translator/testData/typescript-export/interfaces/interfaces.kt
index e63b1288391..ae5024d4fd4 100644
--- a/js/js.translator/testData/typescript-export/interfaces/interfaces.kt
+++ b/js/js.translator/testData/typescript-export/interfaces/interfaces.kt
@@ -38,6 +38,15 @@ external interface OptionalFieldsInterface {
     val notRequired: Int?
 }
 
+@JsExport
+interface WithTheCompanion {
+    val interfaceField: String
+
+    companion object {
+        fun companionFunction(): String = "FUNCTION"
+    }
+}
+
 @JsExport
 fun processOptionalInterface(a: OptionalFieldsInterface): String {
     return "${a.required}${a.notRequired ?: "unknown"}"
@@ -48,6 +57,7 @@ fun processOptionalInterface(a: OptionalFieldsInterface): String {
 interface InterfaceWithCompanion {
     // Emulate added by plugin companion like kotlinx.serialization does
     @Suppress("WRONG_EXPORTED_DECLARATION")
+    @JsExport.Ignore
     companion object {
         fun foo() = "String"
     }
diff --git a/js/js.translator/testData/typescript-export/interfaces/interfaces__main.ts b/js/js.translator/testData/typescript-export/interfaces/interfaces__main.ts
index de5842b2b27..be559a4b43b 100644
--- a/js/js.translator/testData/typescript-export/interfaces/interfaces__main.ts
+++ b/js/js.translator/testData/typescript-export/interfaces/interfaces__main.ts
@@ -2,6 +2,7 @@ import TestInterfaceImpl = JS_TESTS.foo.TestInterfaceImpl;
 import ChildTestInterfaceImpl = JS_TESTS.foo.ChildTestInterfaceImpl;
 import processInterface = JS_TESTS.foo.processInterface;
 import processOptionalInterface = JS_TESTS.foo.processOptionalInterface;
+import WithTheCompanion = JS_TESTS.foo.WithTheCompanion;
 
 function assert(condition: boolean) {
     if (!condition) {
@@ -20,5 +21,7 @@ function box(): string {
     assert(processOptionalInterface({ required: 4, notRequired: null }) == "4unknown")
     assert(processOptionalInterface({ required: 4, notRequired: 5 }) == "45")
 
+    assert(WithTheCompanion.companionFunction() == "FUNCTION")
+
     return "OK";
 }
\ No newline at end of file
diff --git a/wasm/wasm.frontend/src/org/jetbrains/kotlin/wasm/resolve/WasmPlatformConfigurator.kt b/wasm/wasm.frontend/src/org/jetbrains/kotlin/wasm/resolve/WasmPlatformConfigurator.kt
index 0750011a9a5..57ab4119539 100644
--- a/wasm/wasm.frontend/src/org/jetbrains/kotlin/wasm/resolve/WasmPlatformConfigurator.kt
+++ b/wasm/wasm.frontend/src/org/jetbrains/kotlin/wasm/resolve/WasmPlatformConfigurator.kt
@@ -23,7 +23,7 @@ import org.jetbrains.kotlin.wasm.resolve.diagnostics.*
 object WasmJsPlatformConfigurator : PlatformConfiguratorBase(
     additionalDeclarationCheckers = listOf(
         JsNameChecker, JsModuleChecker, JsExternalFileChecker,
-        JsExternalChecker, WasmExternalInheritanceChecker,
+        WasmExternalInheritanceChecker,
         JsRuntimeAnnotationChecker,
         JsExportAnnotationChecker,
         WasmExternalDeclarationChecker,
@@ -50,7 +50,8 @@ object WasmJsPlatformConfigurator : PlatformConfiguratorBase(
         container.useInstance(ExtensionFunctionToExternalIsInlinable)
         container.useInstance(JsQualifierChecker)
         container.useInstance(WasmDiagnosticSuppressor)
-        container.useInstance(JsExportDeclarationChecker(includeUnsignedNumbers = true))
+        container.useInstance(JsExternalChecker(allowCompanionInInterface = false))
+        container.useInstance(JsExportDeclarationChecker(allowCompanionInInterface = false, includeUnsignedNumbers = true))
     }
 
     override fun configureModuleDependentCheckers(container: StorageComponentContainer) {