repl: add replDefaults to customize the writer

So far it was not possible to modify the inspection defaults used by
the REPL from the running instance itself. This introduces a new
property on `util.inspect` which is only used inside the REPL and which
allows to modify the used inspection defaults at any point of time.

PR-URL: #26375
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/repl.md

@@ -256,13 +256,32 @@ function isRecoverableError(error) {
 
 By default, [`repl.REPLServer`][] instances format output using the
 [`util.inspect()`][] method before writing the output to the provided `Writable`
-stream (`process.stdout` by default). The `useColors` boolean option can be
-specified at construction to instruct the default writer to use ANSI style
-codes to colorize the output from the `util.inspect()` method.
+stream (`process.stdout` by default). The `showProxy` inspection option is set
+to true by default and the `colors` option is set to true depending on the
+REPL's `useColors` option.
 
-It is possible to fully customize the output of a [`repl.REPLServer`][] instance
-by passing a new function in using the `writer` option on construction. The
-following example, for instance, simply converts any input text to upper case:
+The `useColors` boolean option can be specified at construction to instruct the
+default writer to use ANSI style codes to colorize the output from the
+`util.inspect()` method.
+
+If the REPL is run as standalone program, it is also possible to change the
+REPL's [inspection defaults][`util.inspect()`] from inside the REPL by using the
+`inspect.replDefaults` property which mirrors the `defaultOptions` from
+[`util.inspect()`][].
+
+```console
+> util.inspect.replDefaults.compact = false;
+false
+> [1]
+[
+  1
+]
+>
+```
+
+To fully customize the output of a [`repl.REPLServer`][] instance pass in a new
+function for the `writer` option on construction. The following example, for
+instance, simply converts any input text to upper case:
 
 ```js
 const repl = require('repl');

lib/internal/repl.js

@@ -1,6 +1,7 @@
 'use strict';
 
 const REPL = require('repl');
+const { kStandaloneREPL } = require('internal/repl/utils');
 
 module.exports = Object.create(REPL);
 module.exports.createInternalRepl = createRepl;
@@ -11,6 +12,7 @@ function createRepl(env, opts, cb) {
     opts = null;
   }
   opts = {
+    [kStandaloneREPL]: true,
     ignoreUndefined: false,
     terminal: process.stdout.isTTY,
     useGlobal: true,

lib/internal/repl/recoverable.js → lib/internal/repl/utils.js

@@ -70,5 +70,6 @@ function isRecoverableError(e, code) {
 }
 
 module.exports = {
-  isRecoverableError
+  isRecoverableError,
+  kStandaloneREPL: Symbol('kStandaloneREPL')
 };

lib/repl.js

@@ -73,7 +73,10 @@ const { sendInspectorCommand } = require('internal/util/inspector');
 const experimentalREPLAwait = require('internal/options').getOptionValue(
   '--experimental-repl-await'
 );
-const { isRecoverableError } = require('internal/repl/recoverable');
+const {
+  isRecoverableError,
+  kStandaloneREPL
+} = require('internal/repl/utils');
 const {
   getOwnNonIndexProperties,
   propertyFilter: {
@@ -499,10 +502,25 @@ function REPLServer(prompt,
   }
   self.useColors = !!options.useColors;
 
-  if (self.useColors && self.writer === writer) {
-    // Turn on ANSI coloring.
-    self.writer = (obj) => util.inspect(obj, self.writer.options);
-    self.writer.options = { ...writer.options, colors: true };
+  if (self.writer === writer) {
+    // Conditionally turn on ANSI coloring.
+    writer.options.colors = self.useColors;
+
+    if (options[kStandaloneREPL]) {
+      Object.defineProperty(util.inspect, 'replDefaults', {
+        get() {
+          return writer.options;
+        },
+        set(options) {
+          if (options === null || typeof options !== 'object') {
+            throw new ERR_INVALID_ARG_TYPE('options', 'Object', options);
+          }
+          return Object.assign(writer.options, options);
+        },
+        enumerable: true,
+        configurable: true
+      });
+    }
   }
 
   function filterInternalStackFrames(structuredStack) {

node.gyp

@@ -174,7 +174,7 @@
       'lib/internal/repl.js',
       'lib/internal/repl/await.js',
       'lib/internal/repl/history.js',
-      'lib/internal/repl/recoverable.js',
+      'lib/internal/repl/utils.js',
       'lib/internal/socket_list.js',
       'lib/internal/test/binding.js',
       'lib/internal/test/heap.js',

test/parallel/test-repl-inspect-defaults.js

@@ -0,0 +1,29 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const cp = require('child_process');
+const child = cp.spawn(process.execPath, ['-i']);
+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');
+  assert.deepStrictEqual(
+    results,
+    [
+      '[ 42, 23 ]',
+      '1',
+      '[ 42, ... 1 more item ]',
+      ''
+    ]
+  );
+}));
+
+child.stdin.write('[ 42, 23 ]\n');
+child.stdin.write('util.inspect.replDefaults.maxArrayLength = 1\n');
+child.stdin.write('[ 42, 23 ]\n');
+child.stdin.end();