repl: support .help expression for introspection

[hemanth]

Description

Extend the .help REPL command to accept an optional expression, providing interactive introspection without adding anything to the REPL context.

Without an argument, .help prints the command listing exactly as before. With an argument, the expression is evaluated in the REPL context and introspection info about the resulting value is printed.

Usage

> .help Array
Function: Array
  Length:     1
  Prototype methods:
    at, concat, copyWithin, entries, every, fill, filter, find, ...

> .help process.versions
Object {
  Properties:
    node: '27.0.0-pre'
    ...
}

> .help fetchData
AsyncFunction: fetchData
  Parameters: url
  Length:     1

> .help 42
number: 42

Implementation Details

• .help expression evaluates the expression via the REPL's own eval (so custom evaluators, context vars, etc. all work) and prints type, function kind (Function/AsyncFunction/GeneratorFunction/Class), signature, own properties categorized into Methods/Accessors/Properties, and the prototype chain
• Uses util.styleText for colorized output (respects NO_COLOR)
• Evaluation errors are reported without breaking the REPL
• No changes to the REPL context: per review feedback, the earlier global help() approach was dropped in favor of extending the existing dot-command

Refs: #51763

[codecov]

Codecov Report

❌ Patch coverage is 95.23810% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.23%. Comparing base (e7864e6) to head (a9e0938).
⚠️ Report is 36 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/internal/repl/utils.js	96.51%	6 Missing ⚠️
lib/repl.js	82.35%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #64479      +/-   ##
==========================================
- Coverage   90.24%   90.23%   -0.01%     
==========================================
  Files         741      739       -2     
  Lines      241665   241931     +266     
  Branches    45543    45608      +65     
==========================================
+ Hits       218092   218313     +221     
- Misses      15105    15127      +22     
- Partials     8468     8491      +23     

Files with missing lines	Coverage Δ
lib/repl.js	93.87% <82.35%> (-0.14%)	⬇️
lib/internal/repl/utils.js	96.41% <96.51%> (+0.02%)	⬆️

... and 51 files with indirect coverage changes

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[avivkeller]

.help makes sense, but I don't think polluting the global object with help() does

[hemanth]

hmm, so it would be like .help [param]? like today .help just prints the help message. _help is odd?

[ljharb]

having any repl command that doesn't start with . is odd, i'd think

[hemanth]

nods, but we do have just the _ which holds the value of the pervious line that was executed.

[hemanth]

Reworked as suggested: the global help() is gone, and the existing .help command now takes an optional expression instead.

• .help with no argument prints the command listing, unchanged.
• .help expression evaluates the expression via the REPL's own eval and prints introspection info (type, function signature, properties/methods/accessors, prototype chain). Since it goes through the regular eval path, context variables and custom evaluators work as expected.

Nothing is added to the REPL context anymore, so there's no pollution in useGlobal mode and no shadowing of user-defined help. Docs updated and tests rewritten to drive .help through the REPL input stream.

//cc @avivkeller @ljharb && @nodejs/repl