allocate presentationalDependencies array only when needed

lib/DependenciesBlock.js

@@ -55,6 +55,15 @@ class DependenciesBlock {
 		}
 	}
 
+	/**
+	 * Removes all dependencies and blocks
+	 * @returns {void}
+	 */
+	clearDependenciesAndBlocks() {
+		this.dependencies.length = 0;
+		this.blocks.length = 0;
+	}
+
 	/**
 	 * @param {Hash} hash the hash used to track dependencies
 	 * @param {ChunkGraph} chunkGraph the chunk graph

lib/Module.js

@@ -117,8 +117,8 @@ class Module extends DependenciesBlock {
 		this.buildMeta = undefined;
 		/** @type {object} */
 		this.buildInfo = undefined;
-		/** @type {Dependency[]} */
-		this.presentationalDependencies = [];
+		/** @type {Dependency[] | undefined} */
+		this.presentationalDependencies = undefined;
 	}
 
 	// TODO remove in webpack 6
@@ -335,9 +335,23 @@ class Module extends DependenciesBlock {
 	 * @returns {void}
 	 */
 	addPresentationalDependency(presentationalDependency) {
+		if (this.presentationalDependencies === undefined) {
+			this.presentationalDependencies = [];
+		}
 		this.presentationalDependencies.push(presentationalDependency);
 	}
 
+	/**
+	 * Removes all dependencies and blocks
+	 * @returns {void}
+	 */
+	clearDependenciesAndBlocks() {
+		if (this.presentationalDependencies !== undefined) {
+			this.presentationalDependencies.length = 0;
+		}
+		super.clearDependenciesAndBlocks();
+	}
+
 	/**
 	 * @param {WebpackError} warning the warning
 	 * @returns {void}
@@ -551,8 +565,10 @@ class Module extends DependenciesBlock {
 			hash.update(exportInfo.used + "");
 			hash.update(exportInfo.usedName + "");
 		}
-		for (const dep of this.presentationalDependencies) {
-			dep.updateHash(hash, chunkGraph);
+		if (this.presentationalDependencies !== undefined) {
+			for (const dep of this.presentationalDependencies) {
+				dep.updateHash(hash, chunkGraph);
+			}
 		}
 		super.updateHash(hash, chunkGraph);
 	}

lib/NormalModule.js

@@ -557,9 +557,7 @@ class NormalModule extends Module {
 		this._ast = null;
 		this.error = null;
 		this.clearWarningsAndErrors();
-		this.dependencies.length = 0;
-		this.presentationalDependencies.length = 0;
-		this.blocks.length = 0;
+		this.clearDependenciesAndBlocks();
 		this.buildMeta = {};
 		this.buildInfo = {
 			cacheable: false,

lib/javascript/JavascriptGenerator.js

@@ -88,14 +88,16 @@ class JavascriptGenerator extends Generator {
 			);
 		}
 
-		for (const dependency of module.presentationalDependencies) {
-			this.sourceDependency(
-				module,
-				dependency,
-				initFragments,
-				source,
-				generateContext
-			);
+		if (module.presentationalDependencies !== undefined) {
+			for (const dependency of module.presentationalDependencies) {
+				this.sourceDependency(
+					module,
+					dependency,
+					initFragments,
+					source,
+					generateContext
+				);
+			}
 		}
 
 		for (const childBlock of module.blocks) {

lib/optimize/ConcatenatedModule.js

@@ -573,7 +573,6 @@ class ConcatenatedModule extends Module {
 
 		// Graph
 		this.dependencies = [];
-		this.presentationalDependencies = [];
 
 		this._orderedConcatenationList = ConcatenatedModule._createConcatenationList(
 			rootModule,
@@ -592,12 +591,14 @@ class ConcatenatedModule extends Module {
 				)) {
 					this.dependencies.push(d);
 				}
-				for (const d of m.presentationalDependencies.filter(
-					dep =>
-						!(dep instanceof HarmonyImportDependency) ||
-						!modules.has(compilation.moduleGraph.getModule(dep))
-				)) {
-					this.presentationalDependencies.push(d);
+				if (m.presentationalDependencies !== undefined) {
+					for (const d of m.presentationalDependencies.filter(
+						dep =>
+							!(dep instanceof HarmonyImportDependency) ||
+							!modules.has(compilation.moduleGraph.getModule(dep))
+					)) {
+						this.addPresentationalDependency(d);
+					}
 				}
 				// populate file dependencies
 				if (m.buildInfo.fileDependencies) {

lib/optimize/ModuleConcatenationPlugin.js

@@ -113,6 +113,7 @@ class ModuleConcatenationPlugin {
 							if (
 								!module.buildMeta ||
 								module.buildMeta.exportsType !== "namespace" ||
+								module.presentationalDependencies === undefined ||
 								!module.presentationalDependencies.some(
 									d => d instanceof HarmonyCompatibilityDependency
 								)