repl: add function signature hints by hemanth · Pull Request #64610 · nodejs/node

hemanth · 2026-07-19T22:46:16Z

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

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

Requires preview to be enabled and the inspector to be available.

Refs: #48164

avivkeller · 2026-07-19T22:47:44Z

@hemanth every time a PR is opened and closed it pings maintainers. May I suggest verifying the branch is correct before opening a PR?

avivkeller

Tests?

When typing a function name followed by '(', the REPL now displays a dimmed hint below the input line showing the function's parameters. For example, typing 'console.log(' shows: // console.log(...data) Uses the inspector to safely extract parameter lists via Function.prototype.toString(), with throwOnSideEffect to prevent unintended evaluation. PR-URL: nodejs#64610

hemanth · 2026-07-21T06:27:04Z

Multiple PRs, multiple branches! After splitting the larger PR it has caused few side-effects, cleaned up the branch, please review @avivkeller thank you!

When typing a function name followed by '(', the REPL now displays a dimmed hint below the input line showing the function's parameters. For example, typing 'console.log(' shows: // console.log(...data) Uses the inspector to safely extract parameter lists via Function.prototype.toString(), with throwOnSideEffect to prevent unintended evaluation. PR-URL: nodejs#64610

avivkeller · 2026-07-21T06:43:03Z

  _builtinLibs = value;
 }

+const { styleText } = require('util');


This should be imported with everything else

avivkeller · 2026-07-21T06:44:12Z

+    // Find the function name before the opening parenthesis.
+    // Look backwards from cursor for pattern: identifier(
+    let parenPos = -1;
+    for (let i = cursor - 1; i >= 0; i--) {
+      const ch = StringPrototypeCharCodeAt(line, i);
+      if (ch === 40) { // '('
+        parenPos = i;
+        break;
+      }
+      // If we hit anything other than whitespace or content after '(',
+      // stop looking.
+      if (ch !== 32 && ch !== 9) break; // space, tab
+    }
+
+    if (parenPos < 0) return;
+
+    // Extract the expression before the '(' - supports simple names
+    // (e.g. `foo(`) and dotted access (e.g. `console.log(`).
+    // Parenthesized expressions like `(fn)(` are not handled;
+    // the hint is silently skipped in that case.
+    const nameEnd = parenPos;
+    let nameStart = nameEnd;
+    for (let i = nameEnd - 1; i >= 0; i--) {
+      const ch = StringPrototypeCharCodeAt(line, i);
+      // Allow identifier chars and dots for property access.
+      if ((ch >= 65 && ch <= 90) || (ch >= 97 && ch <= 122) ||
+          (ch >= 48 && ch <= 57) || ch === 95 || ch === 36 || ch === 46) {
+        nameStart = i;
+      } else {
+        break;
+      }
+    }


How do we get the expression for auto completion? Can we reuse that logic here?

avivkeller · 2026-07-21T06:44:38Z

+      throwOnSideEffect: true,
+      timeout: 200,
+      contextId: repl[contextSymbol],
+    }).then((preview) => {


Would it be cleaner to make this function asynchronous?

avivkeller · 2026-07-21T06:46:03Z

+    }).catch((err) => {
+      process.emitWarning(
+        `Signature hint failed: ${err?.message || err}`,
+        'REPLWarning',
+      );
+    });


I think not handling the error at all is smarter, if something goes wrong, we should bubble it up so that we know, right?

avivkeller · 2026-07-21T06:46:52Z

+    let showSignatureHint;
+    let clearSignatureHint;
+    if (preview && this.terminal && process.features.inspector &&
+        process.env.TERM !== 'dumb') {
+      ({ showSignatureHint, clearSignatureHint } = setupSignatureHint(
+        this,
+        kContextId,
+      ));
+    } else {
+      showSignatureHint = () => {};
+      clearSignatureHint = () => {};
+    }


Rather than setting functions to no-ops, we can use ?.()

avivkeller · 2026-07-21T06:48:06Z

+class REPLStream extends Stream {
+  readable = true;
+  writable = true;
+
+  constructor() {
+    super();
+    this.lines = [''];
+    this.writeCount = 0;
+  }
+  async run(data) {
+    for (const entry of data) {
+      this.emit('data', entry);
+      await this.settle();
+    }
+    this.emit('data', '\n');
+  }
+  settle() {
+    return new Promise((resolve) => {
+      let last = this.writeCount;
+      let quiet = 0;
+      const check = () => {
+        if (this.writeCount !== last) {
+          last = this.writeCount;
+          quiet = 0;
+        } else if (++quiet >= 10) {
+          resolve();
+          return;
+        }
+        setImmediate(check);
+      };
+      setImmediate(check);
+    });
+  }
+  write(chunk) {
+    this.writeCount++;
+    const chunkLines = chunk.toString('utf8').split('\n');
+    this.lines[this.lines.length - 1] += chunkLines[0];
+    if (chunkLines.length > 1) {
+      this.lines.push(...chunkLines.slice(1));
+    }
+    this.emit('line', this.lines[this.lines.length - 1]);
+    return true;
+  }
+  async wait() {
+    this.lines = [''];
+    for await (const [line] of events.on(this, 'line')) {
+      if (line.includes(PROMPT)) {
+        return this.lines;
+      }
+    }
+  }
+  pause() {}
+  resume() {}
+}
+
+async function runAndWait(cmds, repl) {
+  const promise = repl.inputStream.wait();
+  for (const cmd of cmds) {
+    await repl.inputStream.run(cmd);
+  }
+  return promise;
+}


Does common/repl have something that can do this for us, or is this boilerplate really needed? It seems like more code to maintain

avivkeller

I know I'm leaving a lot of reviews, and I'm very sorry, keep in mind, these are just reviews, and you're not obligated to accept them

avivkeller · 2026-07-21T06:50:58Z

+/**
+ * Wraps text with an ANSI color via util.styleText.
+ * Uses the fast path (validateStream: false) to skip stream checks.
+ * @param {string} colorName The color name (e.g. 'magenta', 'green').
+ * @param {string} text The text to colorize.
+ * @returns {string} The colorized text.
+ */
+function style(colorName, text) {
+  return styleText(colorName, text, kStyleOpts);
+}
+


I wonder if we can leech off preview and avoid some styling boiler plate?

As in, make this a sub part of preview.

codecov · 2026-07-21T08:25:20Z

Codecov Report

❌ Patch coverage is 89.80892% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.13%. Comparing base (0bcc6ef) to head (c4307f5).
⚠️ Report is 15 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/internal/repl/utils.js	88.48%	15 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #64610      +/-   ##
==========================================
- Coverage   90.14%   90.13%   -0.01%     
==========================================
  Files         741      741              
  Lines      242080   242248     +168     
  Branches    45563    45593      +30     
==========================================
+ Hits       218213   218343     +130     
- Misses      15375    15404      +29     
- Partials     8492     8501       +9

Files with missing lines	Coverage Δ
lib/repl.js	`93.02% <100.00%> (+0.09%)`	⬆️
lib/internal/repl/utils.js	`95.77% <88.48%> (-1.16%)`	⬇️

... and 40 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 closed this Jul 19, 2026

hemanth reopened this Jul 19, 2026

hemanth force-pushed the repl/signature-hints-new branch from b064b71 to 6beba0c Compare July 19, 2026 23:31

jasnell reviewed Jul 20, 2026

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hemanth force-pushed the repl/signature-hints-new branch from fc38014 to ae8b308 Compare July 21, 2026 06:23

hemanth force-pushed the repl/signature-hints-new branch from ae8b308 to 7777029 Compare July 21, 2026 06:25

hemanth force-pushed the repl/signature-hints-new branch from 7777029 to be1b29e Compare July 21, 2026 06:29

hemanth force-pushed the repl/signature-hints-new branch from be1b29e to c4307f5 Compare July 21, 2026 06:31

avivkeller reviewed Jul 21, 2026

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