module: keep ESM load cache entries in fast mode

benchmark/esm/load-cache.js

@@ -0,0 +1,26 @@
+'use strict';
+
+const common = require('../common.js');
+
+const bench = common.createBenchmark(main, {
+  urls: [1, 100],
+  n: [1e7],
+}, {
+  flags: ['--expose-internals'],
+});
+
+function main({ urls, n }) {
+  const { LoadCache } = require('internal/modules/esm/module_map');
+  const cache = new LoadCache();
+  const job = () => {};
+  const keys = Array.from({ length: urls }, (_, i) => `file:///module-${i}.mjs`);
+  for (let i = 0; i < keys.length; i++) cache.set(keys[i], undefined, job);
+
+  bench.start();
+  for (let i = 0; i < n; i++) {
+    if (cache.get(keys[i % keys.length]) !== job) {
+      throw new Error('Load cache miss');
+    }
+  }
+  bench.end(n);
+}

lib/internal/modules/esm/module_map.js

@@ -6,6 +6,7 @@ const {
   ArrayPrototypeSort,
   JSONStringify,
   ObjectKeys,
+  ObjectSetPrototypeOf,
   SafeMap,
 } = primordials;
 const { kImplicitTypeAttribute } = require('internal/modules/esm/assert');
@@ -108,7 +109,7 @@ class LoadCache extends SafeMap {
     debug(`Storing ${url} (${
       type === kImplicitTypeAttribute ? 'implicit type' : type
     }) in ModuleLoadMap`);
-    const cachedJobsForUrl = super.get(url) ?? { __proto__: null };
+    const cachedJobsForUrl = super.get(url) ?? ObjectSetPrototypeOf({}, null);
     cachedJobsForUrl[type] = job;
     return super.set(url, cachedJobsForUrl);
   }

test/es-module/test-esm-loader-load-cache-prototype.js

@@ -0,0 +1,22 @@
+'use strict';
+// Flags: --expose-internals
+
+require('../common');
+
+const assert = require('assert');
+const { LoadCache } = require('internal/modules/esm/module_map');
+
+const cache = new LoadCache();
+const url = 'file:///module.mjs';
+const job = () => {};
+
+assert.strictEqual(cache.get(url, '__proto__'), undefined);
+assert.strictEqual(cache.has(url, 'constructor'), false);
+cache.set(url, '__proto__', job);
+assert.strictEqual(cache.get(url, '__proto__'), job);
+assert.strictEqual(cache.has(url, '__proto__'), true);
+assert.strictEqual(cache.has(url, 'constructor'), false);
+cache.set(url, 'constructor', job);
+assert.strictEqual(cache.get(url, 'constructor'), job);
+cache.delete(url, '__proto__');
+assert.strictEqual(cache.get(url, '__proto__'), undefined);