feat(KT-48814): represent all of the external declarations nullable properties as an optional fields inside d.ts.

compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/export/ExportModel.kt

@@ -59,6 +59,7 @@ class ExportedProperty(
     val isField: Boolean = false,
     val irGetter: IrFunction? = null,
     val irSetter: IrFunction? = null,
+    val isOptional: Boolean = false
 ) : ExportedDeclaration()
 
 // TODO: Cover all cases with frontend and disable error declarations

compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/export/ExportModelGenerator.kt

@@ -139,6 +139,9 @@ class ExportModelGenerator(val context: JsIrBackendContext, val generateNamespac
         specializeType: ExportedType? = null
     ): ExportedDeclaration {
         val parentClass = property.parent as? IrClass
+        val isOptional = property.isEffectivelyExternal() &&
+                property.parent is IrClass &&
+                property.getter?.returnType?.isNullable() == true
 
         return ExportedProperty(
             name = property.getExportedIdentifier(),
@@ -149,7 +152,8 @@ class ExportModelGenerator(val context: JsIrBackendContext, val generateNamespac
             isProtected = property.visibility == DescriptorVisibilities.PROTECTED,
             isField = parentClass?.isInterface == true,
             irGetter = property.getter,
-            irSetter = property.setter
+            irSetter = property.setter,
+            isOptional = isOptional
         )
     }
 

compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/export/ExportModelToTsDeclarations.kt

@@ -133,7 +133,8 @@ class ExportModelToTsDeclarations {
                 ""
             } else {
                 val readonly = if (isMember && !mutable) "readonly " else ""
-                "$prefix$visibility$possibleStatic$keyword$readonly$memberName: $typeToTypeScript;"
+                val optional = if (isOptional) "?" else ""
+                "$prefix$visibility$possibleStatic$keyword$readonly$memberName$optional: $typeToTypeScript;"
             }
         }
     }

js/js.frontend/src/org/jetbrains/kotlin/js/resolve/JsPlatformConfigurator.kt

@@ -25,7 +25,7 @@ object JsPlatformConfigurator : PlatformConfiguratorBase(
         JsRuntimeAnnotationChecker,
         JsDynamicDeclarationChecker,
         JsExportAnnotationChecker,
-        JsExportDeclarationChecker
+        JsExportDeclarationChecker,
     ),
     additionalCallCheckers = listOf(
         JsModuleCallChecker,

js/js.translator/testData/typescript-export/inheritance/inheritance.d.ts

@@ -2,8 +2,8 @@ declare namespace JS_TESTS {
     type Nullable<T> = T | null | undefined
     namespace foo {
         interface I<T, S, U> {
-            x: T;
-            readonly y: S;
+            x?: T;
+            readonly y?: S;
             z(u: U): void;
         }
         interface I2 {

js/js.translator/testData/typescript-export/interfaces/interfaces.d.ts

@@ -1,5 +1,11 @@
 declare namespace JS_TESTS {
     type Nullable<T> = T | null | undefined
+    namespace foo {
+        interface OptionalFieldsInterface {
+            readonly required: number;
+            readonly notRequired?: Nullable<number>;
+        }
+    }
     namespace foo {
         interface TestInterface {
             readonly value: string;
@@ -24,5 +30,6 @@ declare namespace JS_TESTS {
             readonly __doNotUseOrImplementIt: foo.TestInterfaceImpl["__doNotUseOrImplementIt"] & foo.AnotherExportedInterface["__doNotUseOrImplementIt"];
         }
         function processInterface(test: foo.TestInterface): string;
+        function processOptionalInterface(a: foo.OptionalFieldsInterface): string;
     }
 }

js/js.translator/testData/typescript-export/interfaces/interfaces.kt

@@ -31,3 +31,14 @@ class ChildTestInterfaceImpl(): TestInterfaceImpl("Test"), AnotherExportedInterf
 fun processInterface(test: TestInterface): String {
     return "Owner ${test.getOwnerName()} has value '${test.value}'"
 }
+
+@JsExport
+external interface OptionalFieldsInterface {
+    val required: Int
+    val notRequired: Int?
+}
+
+@JsExport
+fun processOptionalInterface(a: OptionalFieldsInterface): String {
+    return "${a.required}${a.notRequired ?: "unknown"}"
+}

js/js.translator/testData/typescript-export/interfaces/interfaces__main.js

@@ -2,6 +2,7 @@
 var TestInterfaceImpl = JS_TESTS.foo.TestInterfaceImpl;
 var ChildTestInterfaceImpl = JS_TESTS.foo.ChildTestInterfaceImpl;
 var processInterface = JS_TESTS.foo.processInterface;
+var processOptionalInterface = JS_TESTS.foo.processOptionalInterface;
 function assert(condition) {
     if (!condition) {
         throw "Assertion failed";
@@ -12,5 +13,8 @@ function box() {
     assert(processInterface(new ChildTestInterfaceImpl()) === "Owner TestInterfaceImpl has value 'Test'");
     // @ts-expect-error "Just test that this code will throw compilation error for a user"
     assert(processInterface({ value: "bar", getOwnerName: function () { return "RandomObject"; } }) === "Owner RandomObject has value 'bar'");
+    assert(processOptionalInterface({ required: 4 }) == "4unknown");
+    assert(processOptionalInterface({ required: 4, notRequired: null }) == "4unknown");
+    assert(processOptionalInterface({ required: 4, notRequired: 5 }) == "45");
     return "OK";
 }

js/js.translator/testData/typescript-export/interfaces/interfaces__main.ts

@@ -1,6 +1,7 @@
 import TestInterfaceImpl = JS_TESTS.foo.TestInterfaceImpl;
 import ChildTestInterfaceImpl = JS_TESTS.foo.ChildTestInterfaceImpl;
 import processInterface = JS_TESTS.foo.processInterface;
+import processOptionalInterface = JS_TESTS.foo.processOptionalInterface;
 
 function assert(condition: boolean) {
     if (!condition) {
@@ -15,5 +16,9 @@ function box(): string {
     // @ts-expect-error "Just test that this code will throw compilation error for a user"
     assert(processInterface({ value: "bar", getOwnerName: () => "RandomObject" }) === "Owner RandomObject has value 'bar'")
 
+    assert(processOptionalInterface({ required: 4 }) == "4unknown")
+    assert(processOptionalInterface({ required: 4, notRequired: null }) == "4unknown")
+    assert(processOptionalInterface({ required: 4, notRequired: 5 }) == "45")
+
     return "OK";
 }

js/js.translator/testData/typescript-export/js-name-in-exported-file/js-name.d.ts

@@ -2,7 +2,7 @@ declare namespace JS_TESTS {
     type Nullable<T> = T | null | undefined
     namespace foo {
         interface Object {
-            readonly constructor: any;
+            readonly constructor?: any;
         }
     }
     namespace foo {

js/js.translator/testData/typescript-export/js-name/js-name.d.ts

@@ -2,7 +2,7 @@ declare namespace JS_TESTS {
     type Nullable<T> = T | null | undefined
     namespace foo {
         interface Object {
-            readonly constructor: any;
+            readonly constructor?: any;
         }
     }
     namespace foo {