repl: check for NODE_REPL_EXTERNAL_MODULE

PR-URL: https://github.com/nodejs/node/pull/29778
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>

doc/api/cli.md

@@ -1121,6 +1121,14 @@ Path to the file used to store the persistent REPL history. The default path is
 `~/.node_repl_history`, which is overridden by this variable. Setting the value
 to an empty string (`''` or `' '`) disables persistent REPL history.
 
+### `NODE_REPL_EXTERNAL_MODULE=file`
+<!-- YAML
+added: REPLACEME
+-->
+
+Path to a Node.js module which will be loaded in place of the built-in REPL.
+Overriding this value to an empty string (`''`) will use the built-in REPL.
+
 ### `NODE_TLS_REJECT_UNAUTHORIZED=value`
 
 If `value` equals `'0'`, certificate validation is disabled for TLS connections.

lib/internal/main/repl.js

@@ -19,6 +19,11 @@ prepareMainThreadExecution();
 
 markBootstrapComplete();
 
+if (process.env.NODE_REPL_EXTERNAL_MODULE) {
+  require('internal/modules/cjs/loader')
+    .Module
+    ._load(process.env.NODE_REPL_EXTERNAL_MODULE, undefined, true);
+} else {
   // --input-type flag not supported in REPL
   if (getOptionValue('--input-type')) {
     // If we can't write to stderr, we'd like to make this a noop,
@@ -54,3 +59,4 @@ if (getOptionValue('[has_eval_string]')) {
                getOptionValue('--inspect-brk'),
                getOptionValue('--print'));
   }
+}

test/fixtures/external-repl-module.js

@@ -0,0 +1,3 @@
+'use strict';
+
+console.log('42');

test/pseudo-tty/test-repl-external-module.js

@@ -0,0 +1,14 @@
+'use strict';
+
+require('../common');
+const fixtures = require('../common/fixtures');
+const { execSync } = require('child_process');
+
+execSync(process.execPath, {
+  encoding: 'utf8',
+  stdio: 'inherit',
+  env: {
+    ...process.env,
+    NODE_REPL_EXTERNAL_MODULE: fixtures.path('external-repl-module.js'),
+  },
+});

test/pseudo-tty/test-repl-external-module.out

@@ -0,0 +1 @@
+42