process: add process.features.typescript

PR-URL: https://github.com/nodejs/node/pull/54295
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Yagiz Nizipli <yagiz@nizipli.com>
Reviewed-By: Marco Ippolito <marcoippolito54@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>

doc/api/process.md

@@ -1976,6 +1976,19 @@ added: v0.5.3
 
 A boolean value that is `true` if the current Node.js build includes support for SNI in TLS.
 
+## `process.features.typescript`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+> Stability: 1.0 - Early development
+
+* {boolean|string}
+
+A value that is `"strip"` if Node.js is run with `--experimental-strip-types`,
+`"transform"` if Node.js is run with `--experimental-transform-types`, and `false` otherwise.
+
 ## `process.features.uv`
 
 <!-- YAML

lib/internal/bootstrap/node.js

@@ -310,6 +310,29 @@ ObjectDefineProperty(process, 'features', {
 }
 
 const { emitWarning, emitWarningSync } = require('internal/process/warning');
+const { getOptionValue } = require('internal/options');
+
+let kTypeStrippingMode = null;
+// This must be a getter, as getOptionValue does not work
+// before bootstrapping.
+ObjectDefineProperty(process.features, 'typescript', {
+  __proto__: null,
+  get() {
+    if (kTypeStrippingMode === null) {
+      if (getOptionValue('--experimental-transform-types')) {
+        kTypeStrippingMode = 'transform';
+      } else if (getOptionValue('--experimental-strip-types')) {
+        kTypeStrippingMode = 'strip';
+      } else {
+        kTypeStrippingMode = false;
+      }
+    }
+    return kTypeStrippingMode;
+  },
+  configurable: true,
+  enumerable: true,
+});
+
 process.emitWarning = emitWarning;
 internalBinding('process_methods').setEmitWarningSync(emitWarningSync);
 

test/es-module/test-typescript.mjs

@@ -353,6 +353,34 @@ test('execute a TypeScript test mocking module', { skip: isWindows && process.ar
   strictEqual(result.code, 0);
 });
 
+test('expect process.features.typescript to be \'strip\' when --experimental-strip-types', async () => {
+  const result = await spawnPromisified(process.execPath, [
+    '--no-warnings',
+    '--experimental-strip-types',
+    '-p', 'process.features.typescript',
+  ]);
+
+  strictEqual(result.stderr, '');
+  strictEqual(result.stdout, 'strip\n');
+  strictEqual(result.code, 0);
+});
+
+test('expect process.features.typescript to be \'transform\' when --experimental-transform-types', async () => {
+  const result = await spawnPromisified(process.execPath, [
+    '--no-warnings',
+    '--experimental-transform-types',
+    '-p', 'process.features.typescript',
+  ]);
+
+  strictEqual(result.stderr, '');
+  strictEqual(result.stdout, 'transform\n');
+  strictEqual(result.code, 0);
+});
+
+test('expect process.features.typescript to be false without type-stripping', async () => {
+  strictEqual(process.features.typescript, false);
+});
+
 test('execute a TypeScript file with union types', async () => {
   const result = await spawnPromisified(process.execPath, [
     '--experimental-strip-types',

test/parallel/test-process-features.js

@@ -3,20 +3,22 @@
 require('../common');
 const assert = require('assert');
 
-const keys = new Set(Object.keys(process.features));
+const actualKeys = new Set(Object.keys(process.features));
+const expectedKeys = new Map([
+  ['inspector', ['boolean']],
+  ['debug', ['boolean']],
+  ['uv', ['boolean']],
+  ['ipv6', ['boolean']],
+  ['tls_alpn', ['boolean']],
+  ['tls_sni', ['boolean']],
+  ['tls_ocsp', ['boolean']],
+  ['tls', ['boolean']],
+  ['cached_builtins', ['boolean']],
+  ['typescript', ['boolean', 'string']],
+]);
 
-assert.deepStrictEqual(keys, new Set([
-  'inspector',
-  'debug',
-  'uv',
-  'ipv6',
-  'tls_alpn',
-  'tls_sni',
-  'tls_ocsp',
-  'tls',
-  'cached_builtins',
-]));
+assert.deepStrictEqual(actualKeys, new Set(expectedKeys.keys()));
 
-for (const key of keys) {
-  assert.strictEqual(typeof process.features[key], 'boolean');
+for (const [key, expected] of expectedKeys) {
+  assert.ok(expected.includes(typeof process.features[key]), `typeof process.features.${key} is not one of [${expected.join(', ')}]`);
 }