repl: properly handle repl.repl

The repl property is set so that it's possible to inspect the
instances own properties during runtime. This was never tested and
it was also only exported in case the instance was started with
`.start()` instead of using the constructor directly. In case that
more than a single instance was created, all instances got access
to the first instance.

From now on the repl property is only exported in case the repl is
starte as standalone program.

lib/repl.js

@@ -234,10 +234,14 @@ function REPLServer(prompt,
     throw new ERR_INVALID_REPL_EVAL_CONFIG();
   }
 
-  // Add this listener only once and use a WeakSet that contains the REPLs
-  // domains. Otherwise we'd have to add a single listener to each REPL instance
-  // and that could trigger the `MaxListenersExceededWarning`.
-  if (!options[kStandaloneREPL] && !addedNewListener) {
+  if (options[kStandaloneREPL]) {
+    // It is possible to introspect the running REPL accessing this variable
+    // from inside the REPL. This is useful for anyone working on the REPL.
+    exports.repl = this;
+  } else if (!addedNewListener) {
+    // Add this listener only once and use a WeakSet that contains the REPLs
+    // domains. Otherwise we'd have to add a single listener to each REPL
+    // instance and that could trigger the `MaxListenersExceededWarning`.
     process.prependListener('newListener', (event, listener) => {
       if (event === 'uncaughtException' &&
           process.domain &&
@@ -895,14 +899,8 @@ exports.start = function(prompt,
                          useGlobal,
                          ignoreUndefined,
                          replMode) {
-  const repl = new REPLServer(prompt,
-                              source,
-                              eval_,
-                              useGlobal,
-                              ignoreUndefined,
-                              replMode);
-  if (!exports.repl) exports.repl = repl;
-  return repl;
+  return new REPLServer(
+    prompt, source, eval_, useGlobal, ignoreUndefined, replMode);
 };
 
 REPLServer.prototype.setupHistory = function setupHistory(historyFile, cb) {

test/parallel/test-repl-options.js

@@ -24,6 +24,9 @@ const common = require('../common');
 const ArrayStream = require('../common/arraystream');
 const assert = require('assert');
 const repl = require('repl');
+const cp = require('child_process');
+
+assert.strictEqual(repl.repl, undefined);
 
 common.expectWarning({
   DeprecationWarning: {
@@ -120,3 +123,25 @@ assert.strictEqual(r4.ignoreUndefined, false);
 assert.strictEqual(r4.replMode, repl.REPL_MODE_SLOPPY);
 assert.strictEqual(r4.historySize, 30);
 r4.close();
+
+// Check the standalone REPL
+{
+  const child = cp.spawn(process.execPath, ['--interactive']);
+  let output = '';
+
+  child.stdout.setEncoding('utf8');
+  child.stdout.on('data', (data) => {
+    output += data;
+  });
+
+  child.on('exit', common.mustCall(() => {
+    const results = output.replace(/^> /mg, '').split('\n').slice(2);
+    assert.deepStrictEqual(results, ['undefined', '']);
+  }));
+
+  child.stdin.write(
+    'assert.ok(util.inspect(repl.repl, {depth: -1}).includes("REPLServer"));\n'
+  );
+  child.stdin.write('.exit');
+  child.stdin.end();
+}