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;"
             }
         }
     }