[JS IR BE] Add compiler instrinsic to express undefined value in BE

compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/JsIntrinsics.kt

@@ -308,6 +308,8 @@ class JsIntrinsics(private val irBuiltIns: IrBuiltIns, val context: JsIrBackendC
     val readSharedBox = defineReadSharedBox()
     val writeSharedBox = defineWriteSharedBox()
 
+    val jsUndefined = defineJsUndefinedIntrinsic()
+
     // Helpers:
 
     private fun getInternalFunction(name: String) =
@@ -401,6 +403,14 @@ class JsIntrinsics(private val irBuiltIns: IrBuiltIns, val context: JsIrBackendC
         }
     }
 
+    private fun defineJsUndefinedIntrinsic(): IrSimpleFunction {
+        return irFactory.addFunction(externalPackageFragment) {
+            name = Name.identifier("\$undefined")
+        }.apply {
+            returnType = context.irBuiltIns.nothingNType
+        }
+    }
+
     private fun defineEs6DefaultTypeIntrinsic(): IrSimpleFunction {
         return irFactory.addFunction(externalPackageFragment) {
             name = Name.identifier("DefaultType")

compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/transformers/irToJs/JsIntrinsicTransformers.kt

@@ -235,6 +235,9 @@ class JsIntrinsicTransformers(backendContext: JsIrBackendContext) {
                 val value = args[1]
                 jsAssignment(JsNameRef(Namer.SHARED_BOX_V, box), value)
             }
+            add(intrinsics.jsUndefined) { call: IrCall, context: JsGenerationContext ->
+                JsPrefixOperation(JsUnaryOperator.VOID, JsIntLiteral(1))
+            }
         }
     }