esm: add a runtime warning when using import assertions

lib/internal/modules/esm/assert.js

@@ -3,6 +3,7 @@
 const {
   ArrayPrototypeFilter,
   ArrayPrototypeIncludes,
+  ObjectKeys,
   ObjectValues,
   ObjectPrototypeHasOwnProperty,
 } = primordials;
@@ -17,6 +18,8 @@ const {
 // The HTML spec has an implied default type of `'javascript'`.
 const kImplicitAssertType = 'javascript';
 
+let alreadyWarned = false;
+
 /**
  * Define a map of module formats to import assertion types (the value of
  * `type` in `assert { type: 'json' }`).
@@ -55,6 +58,16 @@ function validateAssertions(url, format,
                             importAssertions = { __proto__: null }) {
   const validType = formatTypeMap[format];
 
+  if (!alreadyWarned && ObjectKeys(importAssertions).length !== 0) {
+    alreadyWarned = true;
+    process.emitWarning(
+      'Import assertions are not a stable feature of the JavaScript language, ' +
+      'avoid relying on their current behavior and syntax as those might change ' +
+      'in a future version of Node.js.',
+      'ExperimentalWarning',
+    );
+  }
+
   switch (validType) {
     case undefined:
       // Ignore assertions for module formats we don't recognize, to allow new

test/es-module/test-esm-import-assertion-errors.js

@@ -5,6 +5,13 @@ const { rejects } = require('assert');
 const jsModuleDataUrl = 'data:text/javascript,export{}';
 const jsonModuleDataUrl = 'data:application/json,""';
 
+common.expectWarning(
+  'ExperimentalWarning',
+  'Import assertions are not a stable feature of the JavaScript language, ' +
+  'avoid relying on their current behavior and syntax as those might change ' +
+  'in a future version of Node.js.'
+);
+
 async function test() {
   await rejects(
     import('data:text/css,', { assert: { type: 'css' } }),

test/es-module/test-esm-import-assertion-errors.mjs

@@ -1,9 +1,17 @@
-import '../common/index.mjs';
+import { expectWarning } from '../common/index.mjs';
 import { rejects } from 'assert';
 
 const jsModuleDataUrl = 'data:text/javascript,export{}';
 const jsonModuleDataUrl = 'data:application/json,""';
 
+expectWarning(
+  'ExperimentalWarning',
+  'Import assertions are not a stable feature of the JavaScript language, ' +
+  'avoid relying on their current behavior and syntax as those might change ' +
+  'in a future version of Node.js.'
+);
+
+
 await rejects(
   // This rejects because of the unsupported MIME type, not because of the
   // unsupported assertion.

test/es-module/test-esm-import-assertion-validation.js

@@ -1,11 +1,19 @@
 // Flags: --expose-internals
 'use strict';
-require('../common');
+const common = require('../common');
 
 const assert = require('assert');
 
 const { validateAssertions } = require('internal/modules/esm/assert');
 
+common.expectWarning(
+  'ExperimentalWarning',
+  'Import assertions are not a stable feature of the JavaScript language, ' +
+  'avoid relying on their current behavior and syntax as those might change ' +
+  'in a future version of Node.js.'
+);
+
+
 const url = 'test://';
 
 assert.ok(validateAssertions(url, 'builtin', {}));

test/es-module/test-esm-import-assertion-warning.mjs

@@ -0,0 +1,10 @@
+import { expectWarning } from '../common/index.mjs';
+
+expectWarning(
+  'ExperimentalWarning',
+  'Import assertions are not a stable feature of the JavaScript language, ' +
+  'avoid relying on their current behavior and syntax as those might change ' +
+  'in a future version of Node.js.'
+);
+
+await import('data:text/javascript,', { assert: { someUnsupportedKey: 'value' } });

test/es-module/test-esm-json.mjs

@@ -6,7 +6,6 @@ import { describe, it } from 'node:test';
 
 import secret from '../fixtures/experimental.json' assert { type: 'json' };
 
-
 describe('ESM: importing JSON', () => {
   it('should load JSON', () => {
     assert.strictEqual(secret.ofLife, 42);
@@ -17,8 +16,8 @@ describe('ESM: importing JSON', () => {
       fixtures.path('/es-modules/json-modules.mjs'),
     ]);
 
-    assert.match(stderr, /ExperimentalWarning/);
-    assert.match(stderr, /JSON modules/);
+    assert.match(stderr, /ExperimentalWarning: Importing JSON modules/);
+    assert.match(stderr, /ExperimentalWarning: Import assertions/);
     assert.strictEqual(code, 0);
     assert.strictEqual(signal, null);
   });