repl: add inline syntax highlighting by hemanth · Pull Request #64609 · nodejs/node

hemanth · 2026-07-19T22:31:23Z

Split from #64443 per review feedback to use idiomatic single-feature PRs.

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

Keywords → Magenta
Strings → Green
Numbers → Yellow
Regular expressions → Red
Comments → Gray

Highlighting is display-only — repl.line stays plain text. Can be disabled via syntaxHighlighting: false or TERM=dumb.

Dogfoods util.styleText per @edsadr's suggestion.

Refs: #48164

avivkeller

I have a few concerns here.

Node provides util.inspect.styles which defines colors, so we don't need to define them separately
This involves hooking into and monkey patching REPL internals. Rather than monkey patching them, they should be adapted to work with new code.
This is a lot of code to maintain, and it'll require manual editing every time a new keyword is added to V8. AST parsing provides some of that info for us, and we can probably reduce overhead by relying on it.

(Note that I have an alternate implementation at #64591. That PR and this one being opened around the same time is a coincidence of the REPL inspector landing)

hemanth · 2026-07-19T23:07:37Z

Colors now come from util.inspect.styles via stylizeWithColor — no more hardcoded color names
Dropped the _writeToOutput/_refreshLine monkey-patch, using readline's colorize hook instead
Token→style mapping uses Acorn's token.type.keyword/label — no manual keyword list to maintain

For context: this is a split from #64443 which predates #64591. Will let you all decide which PR to pick 🤝

PTAL @avivkeller

avivkeller · 2026-07-19T23:11:06Z

+const { stylizeWithColor } = require('internal/util/inspect');
+const { Parser } = require('internal/deps/acorn/acorn/dist/acorn');
+
+const tokenizer = FunctionPrototypeBind(Parser.tokenizer, Parser);
+
+/**
+ * Maps an Acorn token to a util.inspect style name.
+ * Returns undefined for tokens that should not be highlighted.
+ * @param {object} token The Acorn token.
+ * @returns {string|undefined} The inspect style name.
+ */
+function tokenStyle(token) {
+  const { label, keyword } = token.type;
+
+  if (keyword !== undefined) {
+    if (label === 'true' || label === 'false') return 'boolean';
+    if (label === 'null') return 'null';
+    if (label === 'import' || label === 'export') return 'module';
+    return 'special';
+  }
+
+  switch (label) {
+    case 'name':
+      switch (token.value) {
+        case 'undefined':
+          return 'undefined';
+        case 'NaN':
+        case 'Infinity':
+          return 'number';
+        default:
+          return undefined;
+      }
+    case 'num':
+      return 'number';
+    case 'bigint':
+      return 'bigint';
+    case 'string':
+    case 'template':
+    case '`':
+      return 'string';
+    case 'regexp':
+      return 'regexp';
+    default:
+      return undefined;
+  }
+}
+
+/**
+ * Applies ANSI syntax highlighting to a JavaScript code string.
+ * Uses the Acorn tokenizer and util.inspect.styles for colors,
+ * so the color scheme stays in sync with util.inspect output.
+ * @param {string} code The JavaScript code string to highlight.
+ * @returns {string} The highlighted code string.
+ */
+function syntaxHighlight(code) {
+  if (code.length === 0) return code;
+
+  let result = '';
+  let offset = 0;
+
+  function write(start, end, inspectStyle) {
+    result +=
+      StringPrototypeSlice(code, offset, start) +
+      stylizeWithColor(StringPrototypeSlice(code, start, end), inspectStyle);
+    offset = end;
+  }
+
+  try {
+    const iterator = tokenizer(code, {
+      __proto__: null,
+      allowHashBang: true,
+      ecmaVersion: 'latest',
+      onComment(_block, _text, start, end) {
+        write(start, end, 'undefined');
+      },
+    });
+
+    for (const token of iterator) {
+      const inspectStyle = tokenStyle(token);
+      if (inspectStyle !== undefined) {
+        write(token.start, token.end, inspectStyle);
+      }
+    }
+  } catch {
+    // Acorn throws for unfinished strings, templates, and comments.
+    // Tokens reported before the error are still useful while typing.
+  }
+
+  return result + StringPrototypeSlice(code, offset);
+}
+


While I respect that you changed this following my review, this code is plagiarized from my own. It's important to see your own spin on things.

Hey @avivkeller — "plagiarized" is a strong word. The original implementation was in #64443 which you reviewed and suggested splitting into separate PRs. While I was working on that split, you opened #64591 independently. When your review on this PR suggested using stylizeWithColor and the colorize hook, I adopted those patterns from your PR to address your feedback — that felt like the right thing to do.

If the code ended up too close to yours, a co-author credit would have been the collaborative path forward. That said, you are one of the maintainers — feel free to close this PR if you'd prefer to go with #64591.

I don't think closing this is the right call. My PR doesn't get any special priority just because I'm a maintainer, as we treat all PRs equally, and that includes mine.

That said, regarding your statements:

"I adopted those patterns from your PR." Adopting patterns means using existing code as a reference to write something new. What actually happened here is that my changes were inserted into yours almost 1:1 (which is also why the lint is failing).

"if the code ended up too close to yours": if you'd referenced my code and it came out similar, that would be completely fine. Referencing each other's work is a normal part of collaboration. Copying it verbatim is a different thing.

Totally understand, it was a cherry-pick there, as I was addressing your review wanted to use the same patterns you have defined.

What do you suggest we do with PR?

Do a new implementation of the same challenge?

Close this on behalf?

Revert this PR to the pervious state and enhance?

Any other alternatives.

Use the Acorn tokenizer to colorize JavaScript input in real-time. Keywords are magenta, strings green, numbers yellow, regexps red, and comments gray. Highlighting is display-only — repl.line stays plain text. Can be disabled via `syntaxHighlighting: false` in repl.start() or TERM=dumb. Refs: nodejs#48164 Signed-off-by: hemanth <hemanth.hm@gmail.com>

codecov · 2026-07-21T08:16:53Z

Codecov Report

❌ Patch coverage is 68.36735% with 31 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.10%. Comparing base (0bcc6ef) to head (f74ff7b).
⚠️ Report is 15 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/internal/repl/utils.js	67.39%	30 Missing ⚠️
lib/repl.js	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #64609      +/-   ##
==========================================
- Coverage   90.14%   90.10%   -0.04%     
==========================================
  Files         741      741              
  Lines      242080   242189     +109     
  Branches    45563    45572       +9     
==========================================
+ Hits       218213   218226      +13     
- Misses      15375    15472      +97     
+ Partials     8492     8491       -1

Files with missing lines	Coverage Δ
lib/repl.js	`92.87% <83.33%> (-0.05%)`	⬇️
lib/internal/repl/utils.js	`94.12% <67.39%> (-2.80%)`	⬇️

... and 39 files with indirect coverage changes

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nodejs-github-bot added needs-ci PRs that need a full CI run. repl Issues and PRs related to the REPL subsystem. labels Jul 19, 2026

hemanth mentioned this pull request Jul 19, 2026

repl: add syntax highlighting, auto-indentation, and signature hints #64443

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avivkeller requested changes Jul 19, 2026

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hemanth force-pushed the repl/syntax-highlighting branch from b39fe8f to 756e16e Compare July 19, 2026 23:07

avivkeller reviewed Jul 19, 2026

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hemanth force-pushed the repl/syntax-highlighting branch 2 times, most recently from 53b4780 to 405267e Compare July 19, 2026 23:41

hemanth force-pushed the repl/syntax-highlighting branch from 405267e to f74ff7b Compare July 21, 2026 06:18

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