repl: hide top-level await feature behind a flag

PR-URL: https://github.com/nodejs/node/pull/19604
Refs: https://github.com/nodejs/node/pull/17807
Refs: https://github.com/nodejs/node/pull/15566#issuecomment-353428430
Reviewed-By: Gus Caplan <me@gus.host>

doc/api/repl.md

@@ -179,6 +179,25 @@ Error: foo
 'foo'
 ```
 
+#### `await` keyword
+
+With the `--experimental-repl-await` command line option specified,
+experimental support for the `await` keyword is enabled.
+
+<!-- eslint-skip -->
+```js
+> await Promise.resolve(123)
+123
+> await Promise.reject(new Error('REPL await'))
+Error: REPL await
+    at repl:1:45
+> const timeout = util.promisify(setTimeout);
+undefined
+> const old = Date.now(); await timeout(1000); console.log(Date.now() - old);
+1002
+undefined
+```
+
 ### Custom Evaluation Functions
 
 When a new `repl.REPLServer` is created, a custom evaluation function may be

lib/repl.js

@@ -47,7 +47,6 @@ const {
   makeRequireFunction,
   addBuiltinLibsToObject
 } = require('internal/modules/cjs/helpers');
-const { processTopLevelAwait } = require('internal/repl/await');
 const internalUtil = require('internal/util');
 const { isTypedArray } = require('internal/util/types');
 const util = require('util');
@@ -69,6 +68,10 @@ const {
   ERR_SCRIPT_EXECUTION_INTERRUPTED
 } = require('internal/errors').codes;
 const { sendInspectorCommand } = require('internal/util/inspector');
+const { experimentalREPLAwait } = process.binding('config');
+
+// Lazy-loaded.
+let processTopLevelAwait;
 
 const parentModule = module;
 const replMap = new WeakMap();
@@ -226,7 +229,11 @@ function REPLServer(prompt,
       wrappedCmd = true;
     }
 
-    if (code.includes('await')) {
+    if (experimentalREPLAwait && code.includes('await')) {
+      if (processTopLevelAwait === undefined) {
+        ({ processTopLevelAwait } = require('internal/repl/await'));
+      }
+
       const potentialWrappedCode = processTopLevelAwait(code);
       if (potentialWrappedCode !== null) {
         code = potentialWrappedCode;

src/node.cc

@@ -243,6 +243,11 @@ bool config_experimental_modules = false;
 // that is used by lib/vm.js
 bool config_experimental_vm_modules = false;
 
+// Set in node.cc by ParseArgs when --experimental-repl-await is used.
+// Used in node_config.cc to set a constant on process.binding('config')
+// that is used by lib/repl.js.
+bool config_experimental_repl_await = false;
+
 // Set in node.cc by ParseArgs when --loader is used.
 // Used in node_config.cc to set a constant on process.binding('config')
 // that is used by lib/internal/bootstrap/node.js
@@ -3463,6 +3468,10 @@ static void PrintHelp() {
 #if defined(NODE_HAVE_I18N_SUPPORT)
          "  --experimental-modules     experimental ES Module support\n"
          "                             and caching modules\n"
+#endif  // defined(NODE_HAVE_I18N_SUPPORT)
+         "  --experimental-repl-await  experimental await keyword support\n"
+         "                             in REPL\n"
+#if defined(NODE_HAVE_I18N_SUPPORT)
          "  --experimental-vm-modules  experimental ES Module support\n"
          "                             in vm module\n"
 #endif  // defined(NODE_HAVE_I18N_SUPPORT)
@@ -3622,6 +3631,7 @@ static void CheckIfAllowedInEnv(const char* exe, bool is_env,
     // Node options, sorted in `node --help` order for ease of comparison.
     "--enable-fips",
     "--experimental-modules",
+    "--experimental-repl-await",
     "--experimental-vm-modules",
     "--expose-http2",   // keep as a non-op through v9.x
     "--force-fips",
@@ -3819,6 +3829,8 @@ static void ParseArgs(int* argc,
       new_v8_argc += 1;
     } else if (strcmp(arg, "--experimental-vm-modules") == 0) {
       config_experimental_vm_modules = true;
+    } else if (strcmp(arg, "--experimental-repl-await") == 0) {
+      config_experimental_repl_await = true;
     }  else if (strcmp(arg, "--loader") == 0) {
       const char* module = argv[index + 1];
       if (!config_experimental_modules) {

src/node_config.cc

@@ -89,6 +89,9 @@ static void Initialize(Local<Object> target,
   if (config_experimental_vm_modules)
     READONLY_BOOLEAN_PROPERTY("experimentalVMModules");
 
+  if (config_experimental_repl_await)
+    READONLY_BOOLEAN_PROPERTY("experimentalREPLAwait");
+
   if (config_pending_deprecation)
     READONLY_BOOLEAN_PROPERTY("pendingDeprecation");
 

src/node_internals.h

@@ -181,6 +181,11 @@ extern bool config_experimental_modules;
 // that is used by lib/vm.js
 extern bool config_experimental_vm_modules;
 
+// Set in node.cc by ParseArgs when --experimental-repl-await is used.
+// Used in node_config.cc to set a constant on process.binding('config')
+// that is used by lib/repl.js.
+extern bool config_experimental_repl_await;
+
 // Set in node.cc by ParseArgs when --loader is used.
 // Used in node_config.cc to set a constant on process.binding('config')
 // that is used by lib/internal/bootstrap/node.js

test/parallel/test-repl-top-level-await.js

@@ -7,7 +7,7 @@ const repl = require('repl');
 
 common.crashOnUnhandledRejection();
 
-// Flags: --expose-internals
+// Flags: --expose-internals --experimental-repl-await
 
 const PROMPT = 'await repl > ';