From 76321b35e1b3f80f02480de87ff3bd95358f4f58 Mon Sep 17 00:00:00 2001
From: avivkeller <me@aviv.sh>
Date: Sat, 18 Jul 2026 15:40:42 -0700
Subject: [PATCH 1/4] repl: add experimental TypeScript support

Signed-off-by: Aviv Keller <me@aviv.sh>
---
 doc/api/cli.md                         | 12 ++++++++++++
 doc/node.1                             |  5 +++++
 lib/internal/repl/eval.js              | 26 +++++++++++++++++++++++---
 src/node_options.cc                    |  4 ++++
 src/node_options.h                     |  1 +
 test/parallel/test-repl-typescript.mjs | 25 +++++++++++++++++++++++++
 6 files changed, 70 insertions(+), 3 deletions(-)
 create mode 100644 test/parallel/test-repl-typescript.mjs

diff --git a/doc/api/cli.md b/doc/api/cli.md
index b3a42183558dc4..d50727b9b1baf2 100644
--- a/doc/api/cli.md
+++ b/doc/api/cli.md
@@ -1381,6 +1381,18 @@ added:
 
 Enable experimental support for the QUIC protocol.
 
+### `--experimental-repl-typescript`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+> Stability: 1.0 - Early development
+
+Enable experimental support for TypeScript stripping in the REPL. Input that is
+valid JavaScript is evaluated as JavaScript before falling back to TypeScript
+type stripping.
+
 ### `--experimental-sea-config`
 
 <!-- YAML
diff --git a/doc/node.1 b/doc/node.1
index cc51fde7661ac3..754eef63c8aa52 100644
--- a/doc/node.1
+++ b/doc/node.1
@@ -778,6 +778,11 @@ this flag allows Node.js to locate and print their locations.
 .It Fl -experimental-quic
 Enable experimental support for the QUIC protocol.
 .
+.It Fl -experimental-repl-typescript
+Enable experimental support for TypeScript stripping in the REPL. Input that is
+valid JavaScript is evaluated as JavaScript before falling back to TypeScript
+type stripping.
+.
 .It Fl -experimental-sea-config
 Use this flag to generate a blob that can be injected into the Node.js
 binary to produce a single executable application. See the documentation
diff --git a/lib/internal/repl/eval.js b/lib/internal/repl/eval.js
index f988dcc1323055..5eb9a3d1dbf99d 100644
--- a/lib/internal/repl/eval.js
+++ b/lib/internal/repl/eval.js
@@ -27,12 +27,15 @@ const {
   transformModuleSyntax,
 } = require('internal/repl/transform');
 const { getReplInspector, prepareContext } = require('internal/repl/inspector');
+const { getOptionValue } = require('internal/options');
 
 let debug = require('internal/util/debuglog').debuglog('repl', (fn) => {
   debug = fn;
 });
 
 const blankRegExp = /^\s*$/;
+const experimentalREPLTypeScript =
+  getOptionValue('--experimental-repl-typescript');
 
 // Matches the first stack frame that belongs to the REPL
 const internalFrameRegExp =
@@ -119,11 +122,28 @@ function createReplEval(repl) {
     installDynamicImport(context);
     prepareContext(context);
 
+    let code = rawCode;
+    const valid = isValidSyntax(code);
+
+    if (!valid && experimentalREPLTypeScript) {
+      try {
+        const { stripTypeScriptModuleTypes } =
+          require('internal/modules/typescript');
+        code = stripTypeScriptModuleTypes(code, file);
+      } catch (err) {
+        if (err.code !== 'ERR_INVALID_TYPESCRIPT_SYNTAX' &&
+            err.code !== 'ERR_UNSUPPORTED_TYPESCRIPT_SYNTAX') {
+          throw err;
+        }
+        // Let the evaluator report the original JavaScript syntax error. It
+        // also determines whether incomplete input is recoverable.
+      }
+    }
+
     // Wrap bare object literals so `{ a: 1 }` is evaluated as an expression
     // rather than a block statement.
-    let code = rawCode;
-    if (isObjectLiteral(code) && isValidSyntax(code)) {
-      code = `(${StringPrototypeTrim(rawCode)})\n`;
+    if (valid && isObjectLiteral(code)) {
+      code = `(${StringPrototypeTrim(code)})\n`;
     }
 
     // Rewrite ES module syntax into runnable script form.
diff --git a/src/node_options.cc b/src/node_options.cc
index b9d3dd56092e35..d3e382ad79fa31 100644
--- a/src/node_options.cc
+++ b/src/node_options.cc
@@ -730,6 +730,10 @@ EnvironmentOptionsParser::EnvironmentOptionsParser() {
             kAllowedInEnvvar,
             false,
             OptionNamespaces::kPermissionNamespace);
+  AddOption("--experimental-repl-typescript",
+            "experimental TypeScript support in the REPL",
+            &EnvironmentOptions::experimental_repl_typescript,
+            kAllowedInEnvvar);
   AddOption("--experimental-vm-modules",
             "experimental ES Module support in vm module",
             &EnvironmentOptions::experimental_vm_modules,
diff --git a/src/node_options.h b/src/node_options.h
index 3a1add2bb7f8e2..0b57c8166e1c76 100644
--- a/src/node_options.h
+++ b/src/node_options.h
@@ -154,6 +154,7 @@ class EnvironmentOptions : public Options {
   bool allow_wasi = false;
   bool allow_ffi = false;
   bool allow_worker_threads = false;
+  bool experimental_repl_typescript = EXPERIMENTALS_DEFAULT_VALUE;
   bool experimental_vm_modules = EXPERIMENTALS_DEFAULT_VALUE;
   bool async_context_frame = true;
   bool expose_internals = false;
diff --git a/test/parallel/test-repl-typescript.mjs b/test/parallel/test-repl-typescript.mjs
new file mode 100644
index 00000000000000..22cf522b1c5875
--- /dev/null
+++ b/test/parallel/test-repl-typescript.mjs
@@ -0,0 +1,25 @@
+// Flags: --experimental-repl-typescript
+
+import * as common from '../common/index.mjs';
+import assert from 'node:assert';
+import { startNewREPLServer } from '../common/repl.js';
+
+common.skipIfInspectorDisabled();
+
+const { run, output } = startNewREPLServer({
+  terminal: false,
+  prompt: '> ',
+});
+
+await run('let x: number = 3\n');
+assert.match(output.accumulator, /undefined\n> /);
+output.accumulator = '';
+
+await run('x\n');
+assert.match(output.accumulator, /3\n> /);
+output.accumulator = '';
+
+// Syntax which is valid in both languages must retain its JavaScript
+// semantics instead of being type-stripped.
+await run("const foo = () => 'hello world'; foo<Object>(0);\n");
+assert.match(output.accumulator, /false\n> /);

From a5ff7e550a14f0591dbd5cfeaac0998ac302408b Mon Sep 17 00:00:00 2001
From: avivkeller <me@aviv.sh>
Date: Sat, 18 Jul 2026 18:10:35 -0700
Subject: [PATCH 2/4] fixup!

---
 doc/api/cli.md | 1 +
 1 file changed, 1 insertion(+)

diff --git a/doc/api/cli.md b/doc/api/cli.md
index d50727b9b1baf2..809bcf9145eb5d 100644
--- a/doc/api/cli.md
+++ b/doc/api/cli.md
@@ -3834,6 +3834,7 @@ one is included in the list below.
 * `--experimental-print-required-tla`
 * `--experimental-quic`
 * `--experimental-require-module`
+* `--experimental-repl-typescript`
 * `--experimental-shadow-realm`
 * `--experimental-specifier-resolution`
 * `--experimental-stream-iter`

From 86bc7634bf399cc568ad13be41e80285192b8b6f Mon Sep 17 00:00:00 2001
From: avivkeller <me@aviv.sh>
Date: Sat, 18 Jul 2026 20:19:09 -0700
Subject: [PATCH 3/4] fixup! q > p

---
 doc/api/cli.md                         | 2 +-
 test/parallel/test-repl-typescript.mjs | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/doc/api/cli.md b/doc/api/cli.md
index 809bcf9145eb5d..5808eea1d3b9cc 100644
--- a/doc/api/cli.md
+++ b/doc/api/cli.md
@@ -3833,8 +3833,8 @@ one is included in the list below.
 * `--experimental-package-map`
 * `--experimental-print-required-tla`
 * `--experimental-quic`
-* `--experimental-require-module`
 * `--experimental-repl-typescript`
+* `--experimental-require-module`
 * `--experimental-shadow-realm`
 * `--experimental-specifier-resolution`
 * `--experimental-stream-iter`
diff --git a/test/parallel/test-repl-typescript.mjs b/test/parallel/test-repl-typescript.mjs
index 22cf522b1c5875..9cfc10700a3383 100644
--- a/test/parallel/test-repl-typescript.mjs
+++ b/test/parallel/test-repl-typescript.mjs
@@ -22,4 +22,4 @@ output.accumulator = '';
 // Syntax which is valid in both languages must retain its JavaScript
 // semantics instead of being type-stripped.
 await run("const foo = () => 'hello world'; foo<Object>(0);\n");
-assert.match(output.accumulator, /false\n> /);
+assert.match(output.accumulator, /true\n> /);

From 36ba729c3d5f4d8e11cbf1f46fcb9c414ae63e0a Mon Sep 17 00:00:00 2001
From: avivkeller <me@aviv.sh>
Date: Sun, 19 Jul 2026 11:43:50 -0700
Subject: [PATCH 4/4] fixup!

---
 doc/node.1 | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/doc/node.1 b/doc/node.1
index 754eef63c8aa52..ac597e7c6310c7 100644
--- a/doc/node.1
+++ b/doc/node.1
@@ -1978,6 +1978,8 @@ one is included in the list below.
 .It
 \fB--experimental-quic\fR
 .It
+\fB--experimental-repl-typescript\fR
+.It
 \fB--experimental-require-module\fR
 .It
 \fB--experimental-shadow-realm\fR