lib: refactor lazy loading of undici for fetch method

Object.defineProperty is updated to lazily load the undici dependency
for the fetch method. This change allows for simpler and more reliable
mocking of the fetch method for testing purposes, resolving issues
encountered with premature method invocation during testing.

Fixes: https://github.com/nodejs/node/issues/52015
PR-URL: https://github.com/nodejs/node/pull/52275
Reviewed-By: Moshe Atlow <moshe@atlow.co.il>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Chemi Atlow <chemi@atlow.co.il>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>

lib/internal/bootstrap/web/exposed-window-or-worker.js

@@ -57,32 +57,21 @@ defineReplaceableLazyAttribute(globalThis, 'perf_hooks', ['performance']);
 const { installObjectURLMethods } = require('internal/url');
 installObjectURLMethods();
 
-{
-  // https://fetch.spec.whatwg.org/#fetch-method
-  function set(value) {
-    ObjectDefineProperty(globalThis, 'fetch', {
-      __proto__: null,
-      writable: true,
-      value,
-    });
-  }
-  ObjectDefineProperty(globalThis, 'fetch', {
-    __proto__: null,
-    configurable: true,
-    enumerable: true,
-    set,
-    get() {
-      function fetch(input, init = undefined) {
-        // Loading undici alone lead to promises which breaks lots of tests so we
-        // have to load it really lazily for now.
-        const { fetch: impl } = require('internal/deps/undici/undici');
-        return impl(input, init);
-      }
-      set(fetch);
-      return fetch;
-    },
-  });
-}
+let fetchImpl;
+// https://fetch.spec.whatwg.org/#fetch-method
+ObjectDefineProperty(globalThis, 'fetch', {
+  __proto__: null,
+  configurable: true,
+  enumerable: true,
+  writable: true,
+  value: function value(input, init = undefined) {
+    if (!fetchImpl) { // Implement lazy loading of undici module for fetch function
+      const undiciModule = require('internal/deps/undici/undici');
+      fetchImpl = undiciModule.fetch;
+    }
+    return fetchImpl(input, init);
+  },
+});
 
 // https://xhr.spec.whatwg.org/#interface-formdata
 // https://fetch.spec.whatwg.org/#headers-class

test/parallel/test-fetch-mock.js

@@ -0,0 +1,20 @@
+'use strict';
+require('../common');
+const { mock, test } = require('node:test');
+const assert = require('node:assert');
+
+test('should correctly stub globalThis.fetch', async () => {
+  const customFetch = async (url) => {
+    return {
+      text: async () => 'foo',
+    };
+  };
+
+  mock.method(globalThis, 'fetch', customFetch);
+
+  const response = await globalThis.fetch('some-url');
+  const text = await response.text();
+
+  assert.strictEqual(text, 'foo');
+  mock.restoreAll();
+});