allocate Module.factoryMeta only when needed

lib/FlagDependencyUsagePlugin.js

@@ -90,7 +90,11 @@ class FlagDependencyUsagePlugin {
 							// for a module without side effects we stop tracking usage here when no export is used
 							// This module won't be evaluated in this case
 							// TODO webpack 6 remove this check
-							if (module.factoryMeta.sideEffectFree) return;
+							if (
+								module.factoryMeta !== undefined &&
+								module.factoryMeta.sideEffectFree
+							)
+								return;
 							if (exportsInfo.setUsedForSideEffectsOnly()) {
 								queue.enqueue(module);
 							}

lib/Module.js

@@ -105,8 +105,8 @@ class Module extends DependenciesBlock {
 		// Info from Factory
 		/** @type {TODO} */
 		this.resolveOptions = EMPTY_RESOLVE_OPTIONS;
-		/** @type {object} */
-		this.factoryMeta = {};
+		/** @type {object | undefined} */
+		this.factoryMeta = undefined;
 
 		// Info from Build
 		/** @type {WebpackError[] | undefined} */

lib/optimize/SideEffectsFlagPlugin.js

@@ -68,6 +68,9 @@ class SideEffectsFlagPlugin {
 						cache
 					);
 					if (!hasSideEffects) {
+						if (module.factoryMeta === undefined) {
+							module.factoryMeta = {};
+						}
 						module.factoryMeta.sideEffectFree = true;
 					}
 				}
@@ -76,9 +79,14 @@ class SideEffectsFlagPlugin {
 			});
 			nmf.hooks.module.tap("SideEffectsFlagPlugin", (module, data) => {
 				if (data.settings.sideEffects === false) {
+					if (module.factoryMeta === undefined) {
+						module.factoryMeta = {};
+					}
 					module.factoryMeta.sideEffectFree = true;
 				} else if (data.settings.sideEffects === true) {
-					module.factoryMeta.sideEffectFree = false;
+					if (module.factoryMeta !== undefined) {
+						module.factoryMeta.sideEffectFree = false;
+					}
 				}
 				return module;
 			});
@@ -98,7 +106,10 @@ class SideEffectsFlagPlugin {
 					for (const module of modules) {
 						for (const dep of module.dependencies) {
 							if (dep instanceof HarmonyExportImportedSpecifierDependency) {
-								if (module.factoryMeta.sideEffectFree) {
+								if (
+									module.factoryMeta !== undefined &&
+									module.factoryMeta.sideEffectFree
+								) {
 									const mode = dep.getMode(moduleGraph, true);
 									if (mode.type === "normal-reexport") {
 										let map = reexportMaps.get(module);
@@ -120,7 +131,11 @@ class SideEffectsFlagPlugin {
 								const connection = moduleGraph.getConnection(dep);
 								if (connection) {
 									const refModule = connection.resolvedModule;
-									if (refModule && refModule.factoryMeta.sideEffectFree) {
+									if (
+										refModule &&
+										refModule.factoryMeta !== undefined &&
+										refModule.factoryMeta.sideEffectFree
+									) {
 										connection.active = false;
 									}
 								}