esm: emit experimental warnings in common place

lib/internal/modules/esm/loader.js

@@ -31,6 +31,7 @@ const {
   ERR_UNKNOWN_MODULE_FORMAT
 } = require('internal/errors').codes;
 const { pathToFileURL, isURLInstance, URL } = require('internal/url');
+const { emitExperimentalWarning } = require('internal/util');
 const {
   isAnyArrayBuffer,
   isArrayBufferView,
@@ -53,6 +54,13 @@ const {
   fetchModule,
 } = require('internal/modules/esm/fetch_module');
 
+
+/**
+ * Prevent the specifier resolution warning from being printed twice
+ */
+let emittedSpecifierResolutionWarning = false;
+
+
 /**
  * An ESMLoader instance is used as the main entry point for loading ES modules.
  * Currently, this is a singleton -- there is only one used for loading
@@ -107,6 +115,22 @@ class ESMLoader {
    */
   translators = translators;
 
+  constructor() {
+    if (getOptionValue('--experimental-loader')) {
+      emitExperimentalWarning('Custom ESM Loaders');
+    }
+    if (getOptionValue('--experimental-network-imports')) {
+      emitExperimentalWarning('Network Imports');
+    }
+    if (getOptionValue('--experimental-specifier-resolution') === 'node' && !emittedSpecifierResolutionWarning) {
+      process.emitWarning(
+        'The Node.js specifier resolution flag is experimental. It could change or be removed at any time.',
+        'ExperimentalWarning'
+      );
+      emittedSpecifierResolutionWarning = true;
+    }
+  }
+
   static pluckHooks({
     globalPreload,
     resolve,

lib/internal/modules/esm/resolve.js

@@ -364,7 +364,6 @@ function resolveDirectoryEntry(search) {
 }
 
 const encodedSepRegEx = /%2F|%5C/i;
-let experimentalSpecifierResolutionWarned = false;
 /**
  * @param {URL} resolved
  * @param {string | URL | undefined} base
@@ -379,13 +378,6 @@ function finalizeResolution(resolved, base, preserveSymlinks) {
 
   let path = fileURLToPath(resolved);
   if (getOptionValue('--experimental-specifier-resolution') === 'node') {
-    if (!experimentalSpecifierResolutionWarned) {
-      process.emitWarning(
-        'The Node.js specifier resolution flag is experimental. It could change or be removed at any time.',
-        'ExperimentalWarning');
-      experimentalSpecifierResolutionWarned = true;
-    }
-
     let file = resolveExtensionsWithTryExactName(resolved);
 
     // Directory

lib/internal/process/esm_loader.js

@@ -55,9 +55,6 @@ async function initializeLoader() {
 
   if (!customLoaders.length) return;
 
-  const { emitExperimentalWarning } = require('internal/util');
-  emitExperimentalWarning('--experimental-loader');
-
   let cwd;
   try {
     cwd = process.cwd() + '/';

test/es-module/test-esm-experimental-warnings.mjs

@@ -0,0 +1,32 @@
+import { mustCall } from '../common/index.mjs';
+import { fileURL } from '../common/fixtures.mjs';
+import { match, strictEqual } from 'assert';
+import { spawn } from 'child_process';
+import { execPath } from 'process';
+
+// Verify experimental warnings are printed
+for (
+  const [experiment, arg] of [
+    [/Custom ESM Loaders/, `--experimental-loader=${fileURL('es-module-loaders', 'hooks-custom.mjs')}`],
+    [/Network Imports/, '--experimental-network-imports'],
+    [/specifier resolution/, '--experimental-specifier-resolution=node'],
+  ]
+) {
+  const input = `import ${JSON.stringify(fileURL('es-module-loaders', 'module-named-exports.mjs'))}`;
+  const child = spawn(execPath, [
+    arg,
+    '--input-type=module',
+    '--eval',
+    input,
+  ]);
+
+  let stderr = '';
+  child.stderr.setEncoding('utf8');
+  child.stderr.on('data', (data) => { stderr += data; });
+  child.on('close', mustCall((code, signal) => {
+    strictEqual(code, 0);
+    strictEqual(signal, null);
+    match(stderr, /ExperimentalWarning/);
+    match(stderr, experiment);
+  }));
+}