repl: add syntax highlighting, auto-indentation, and signature hints

[hemanth]

Description

Add REPL enhancements enabled by default:

1. Inline Syntax Highlighting

Uses the Acorn tokenizer (already in deps/) to colorize JavaScript input in real-time:

• Keywords (const, function, if, etc.) → Magenta
• Strings → Green
• Numbers → Yellow
• Booleans/null/undefined → Yellow
• Regular expressions → Red
• Comments → Gray

Highlighting is applied at display time only — repl.line remains plain text, so cursor positioning and evaluation are unaffected.

2. Auto-Indentation

Automatically inserts 2-space indentation on continuation lines based on brace/bracket/paren depth when entering multiline input.

3. Function Signature Hints

When the user types functionName(, the function's parameter list is displayed as a dimmed hint below the input line. Uses the V8 Inspector protocol with throwOnSideEffect: true for safety.

Usage

These features are enabled by default when useColors and terminal are true. Individual features can be controlled via repl.start() options (e.g., syntaxHighlighting: false).

Motivation

This addresses the feature requests from #48164, which was closed as not planned. These are quality-of-life improvements that bring the Node.js REPL closer to modern shell experiences (Python's IPython, Ruby's Pry, etc.).

Refs: #48164

[nodejs-github-bot]

Review requested:

• @nodejs/config

[codecov]

Codecov Report

❌ Patch coverage is 73.65269% with 132 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.20%. Comparing base (14166a5) to head (eec5763).
⚠️ Report is 21 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/internal/repl/utils.js	70.79%	131 Missing and 1 partial ⚠️

Additional details and impacted files

@@ Coverage Diff @@## main #64443 +/- ##
==========================================
- Coverage 90.23% 90.20% -0.03% 
==========================================
Files 741 741 Lines 241375 242165 +790 Branches 45483 45628 +145 ==========================================
+ Hits 217800 218454 +654 - Misses 15129 15247 +118 - Partials 8446 8464 +18 

Files with missing lines	Coverage Δ
lib/repl.js	94.19% <100.00%> (+0.18%)	⬆️
src/node_options.cc	76.65% <100.00%> (-0.08%)	⬇️
src/node_options.h	98.02% <100.00%> (+<0.01%)	⬆️
lib/internal/repl/utils.js	87.56% <70.79%> (-8.83%)	⬇️

... and 56 files with indirect coverage changes

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[avivkeller]

can these be idiomatic PRs?

[hemanth]

@avivkeller for sure! having some challenges on my local machine, so depending on the GH CI to report me the build time errors.

[edsadr]

this one has been in my list forever; I thik this should dogfood util.styleText and make use of everything about coloring already existing in core

[hemanth]

Agree. util.styleText reminded me of #43371

[avivkeller]

Also note that #64034 makes some behind-the-scenes improvements as well, and may conflict with these idiomatic PRs code-wise

[hemanth]

Rebased on main, removed the --experimental-repl-enhancements flag — features are now on by default.

Highlighting, auto-indent, and signature hints are all display-only (repl.line stays plain text), so they are safe to ship ungated. Each can still be disabled via repl.start() options or TERM=dumb.

Adapted to the new getReplInspector API and the refactored line queue from recent main changes.

cc @avivkeller@RafaelGSS@edsadr

[avivkeller]

As I said previously, I think these should be idiomatic PRs

[hemanth]

Split into idiomatic single-feature PRs per @avivkeller's review:

1. repl: add inline syntax highlighting #64609 — repl: add inline syntax highlighting
2. repl: add auto-indentation for multiline input #64607 — repl: add auto-indentation for multiline input
3. repl: add function signature hints #64608 — repl: add function signature hints

Closing this monolith PR in favor of the above.