Implement object class in JS BE

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

@@ -73,10 +73,19 @@ class IrElementToJsExpressionTransformer : BaseIrElementToJsNodeTransformer<JsEx
     override fun visitGetValue(expression: IrGetValue, context: JsGenerationContext): JsExpression =
         context.getNameForSymbol(expression.symbol).makeRef()
 
-    override fun visitGetObjectValue(expression: IrGetObjectValue, data: JsGenerationContext): JsExpression {
-        TODO()
+    override fun visitGetObjectValue(expression: IrGetObjectValue, context: JsGenerationContext) = when (expression.symbol.kind) {
+        ClassKind.OBJECT -> {
+            if (expression.type.isUnit()) JsNullLiteral()
+            else {
+                val className = context.getNameForSymbol(expression.symbol)
+                val getInstanceName = className.ident + "_getInstance"
+                JsInvocation(JsNameRef(getInstanceName))
+            }
+        }
+        else -> TODO()
     }
 
+
     override fun visitSetField(expression: IrSetField, context: JsGenerationContext): JsExpression {
         val fieldName = context.getNameForSymbol(expression.symbol)
         val dest = JsNameRef(fieldName, expression.receiver?.accept(this, context))

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

@@ -53,9 +53,33 @@ class JsClassGenerator(private val irClass: IrClass, val context: JsGenerationCo
         }
 
         classBlock.statements += generateClassMetadata()
+        irClass.onlyIf({ kind == ClassKind.OBJECT }) { classBlock.statements += maybeGenerateObjectInstance() }
         return classBlock
     }
 
+    private fun maybeGenerateObjectInstance(): List<JsStatement> {
+        val instanceVarName = "${className.ident}_instance"
+        val getInstanceFunName = "${className.ident}_getInstance"
+        val jsVarNode = context.currentScope.declareName(instanceVarName)
+        val varStmt = JsVars(JsVars.JsVar(jsVarNode, JsNullLiteral()))
+        val function = generateGetInstanceFunction(jsVarNode, getInstanceFunName)
+        return listOf(varStmt, function.makeStmt())
+    }
+
+    private fun generateGetInstanceFunction(instanceVar: JsName, instanceFunName: String): JsFunction {
+        val functionBody = JsBlock()
+        val func = JsFunction(JsFunctionScope(context.currentScope, "getInstance for ${irClass.name} object"), functionBody, "getInstance")
+        func.name = context.currentScope.declareName(instanceFunName)
+
+        functionBody.statements += JsIf(
+            JsBinaryOperation(JsBinaryOperator.REF_EQ, instanceVar.makeRef(), JsNullLiteral()),
+            jsAssignment(instanceVar.makeRef(), JsNew(classNameRef)).makeStmt()
+        )
+        functionBody.statements += JsReturn(instanceVar.makeRef())
+
+        return func
+    }
+
     private fun maybeGeneratePrimaryConstructor() {
         if (!irClass.declarations.any { it is IrConstructor }) {
             val func = JsFunction(JsFunctionScope(context.currentScope, "Ctor for ${irClass.name}"), JsBlock(), "Ctor for ${irClass.name}")