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>

Author: BridgeAR

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
 constrepl=require('repl');

lib/internal/repl.js

@@ -1,6 +1,7 @@
 'use strict';
 
 constREPL=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.jslib/internal/repl/recoverable.js renamed to 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');
 constexperimentalREPLAwait=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(){
+returnwriter.options;
+},
+set(options){
+if(options===null||typeofoptions!=='object'){
+thrownewERR_INVALID_ARG_TYPE('options','Object',options);
+}
+returnObject.assign(writer.options,options);
+},
+enumerable: true,
+configurable: true
+});
+}
 }
 
 functionfilterInternalStackFrames(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';
+constcommon=require('../common');
+constassert=require('assert');
+constcp=require('child_process');
+constchild=cp.spawn(process.execPath,['-i']);
+letoutput='';
+
+child.stdout.setEncoding('utf8');
+child.stdout.on('data',(data)=>{
+output+=data;
+});
+
+child.on('exit',common.mustCall(()=>{
+constresults=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();