diff --git a/lib/generate.js b/lib/generate.js
index 862bee5..0e263ef 100644
--- a/lib/generate.js
+++ b/lib/generate.js
@@ -340,7 +340,7 @@ const resolveEntrySource = (entry, sourceMapCache, sourceCache) => {
     const url = entry.url;
 
     // source from `source-id.json`
-    const sourceData = sourceCache[url];
+    const sourceData = sourceCache.get(url);
     if (sourceData) {
         entry.source = sourceData.source;
         return;
@@ -424,7 +424,7 @@ const readCoverageData = async (dir, filename, entryFilter, sourceCache) => {
 };
 
 const readSourceList = async (dir, sourceList) => {
-    const sourceCache = {};
+    const sourceCache = new Map();
 
     for (const filename of sourceList) {
         const content = await Util.readFile(path.resolve(dir, filename));
@@ -436,7 +436,7 @@ const readSourceList = async (dir, sourceList) => {
             continue;
         }
         if (json.url) {
-            sourceCache[json.url] = json;
+            sourceCache.set(json.url, json);
         }
     }
 
@@ -476,16 +476,16 @@ const readFromDir = async (mcr, dir) => {
 
     const entryFilter = mcr.getEntryFilter();
 
-    const results = [];
     for (const filename of coverageList) {
         const coverageData = await readCoverageData(dir, filename, entryFilter, sourceCache);
         if (coverageData) {
-            const res = await mcr.add(coverageData);
-            results.push(res);
+            await mcr.add(coverageData);
         }
     }
 
-    return results;
+    // GC
+    sourceCache.clear();
+
 };
 
 module.exports = {
diff --git a/lib/index.js b/lib/index.js
index 5d945e7..7fe4c95 100644
--- a/lib/index.js
+++ b/lib/index.js
@@ -124,9 +124,8 @@ class CoverageReport {
     // add coverage from dir
     async addFromDir(dir) {
         const time_start = Date.now();
-        const results = await readFromDir(this, dir);
+        await readFromDir(this, dir);
         Util.logTime(`added from dir: ${dir}`, time_start);
-        return results;
     }
 
     // generate report
diff --git a/lib/utils/util.js b/lib/utils/util.js
index 1d1de34..2fc35ec 100644
--- a/lib/utils/util.js
+++ b/lib/utils/util.js
@@ -3,6 +3,9 @@ const { writeFile, readFile } = require('fs/promises');
 const path = require('path');
 const os = require('os');
 const crypto = require('crypto');
+const v8 = require('node:v8');
+const vm = require('node:vm');
+
 const EC = require('eight-colors');
 const CG = require('console-grid');
 const acornWalk = require('acorn-walk');
@@ -788,6 +791,15 @@ const Util = {
         return major1 - major2;
     },
 
+    forceGC: () => {
+        if (!Util.expose_gc) {
+            v8.setFlagsFromString('--expose_gc');
+            Util.expose_gc = true;
+        }
+        const gc = vm.runInNewContext('gc');
+        gc();
+    },
+
     // ==========================================================================================
 
     loggingLevels: {
diff --git a/lib/v8/v8.js b/lib/v8/v8.js
index 62839db..c46fdaf 100644
--- a/lib/v8/v8.js
+++ b/lib/v8/v8.js
@@ -164,7 +164,7 @@ const mergeCssRanges = (itemList) => {
         // ranges: [ {start, end} ]
         const ranges = dedupeFlatRanges(concatRanges);
 
-        resolve(ranges || []);
+        resolve(ranges);
     });
 };
 
@@ -174,14 +174,19 @@ const mergeJsFunctions = async (itemList) => {
 };
 
 const mergeV8DataList = async (dataList) => {
-    let allList = [];
-    dataList.forEach((d) => {
-        allList = allList.concat(d.data);
+    const allList = dataList.map((d) => d.data).flat();
+
+    // separate coverage and empty items
+    const coverageList = [];
+    const allEmptyList = [];
+    allList.forEach((it) => {
+        if (it.empty) {
+            allEmptyList.push(it);
+        } else {
+            coverageList.push(it);
+        }
     });
 
-    // remove empty items
-    const coverageList = allList.filter((it) => !it.empty);
-
     // connect all functions and ranges
     const itemMap = {};
     const sourcePathMap = {};
@@ -215,7 +220,7 @@ const mergeV8DataList = async (dataList) => {
 
     // first time filter for empty, (not for sources)
     // empty and not in item map
-    const emptyList = allList.filter((it) => it.empty).filter((it) => {
+    const emptyList = allEmptyList.filter((it) => {
         if (itemMap[it.id]) {
             return false;
         }
@@ -225,9 +230,10 @@ const mergeV8DataList = async (dataList) => {
         return true;
     });
 
-    // empty list + merged list
-    const mergedList = emptyList.concat(Object.values(itemMap));
+    // merged list + empty list
+    const mergedList = Object.values(itemMap).concat(emptyList);
 
+    // Util.forceGC();
 
     return mergedList;
 };
@@ -254,6 +260,9 @@ const mergeV8Coverage = async (dataList, sourceCache, options) => {
 
     }
 
+    // GC
+    sourceCache.clear();
+
     return mergedList;
 };