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repl: add function signature hints - #64610

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repl: add function signature hints#64610
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hemanth:repl/signature-hints-new

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@hemanth

@hemanthhemanth commented Jul 19, 2026

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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

@nodejs-github-botnodejs-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
@hemanthhemanth closed this Jul 19, 2026
@hemanthhemanth reopened this Jul 19, 2026
@avivkeller

avivkeller commented Jul 19, 2026

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

@hemanth
hemanthforce-pushed the repl/signature-hints-new branch from b064b71 to 6beba0cCompareJuly 19, 2026 23:31
jasnell
jasnell reviewed Jul 20, 2026
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Comment threaddoc/api/repl.md Outdated
avivkeller
avivkeller requested changes Jul 21, 2026
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Comment threaddoc/api/repl.md Outdated
Comment threaddoc/api/repl.md Outdated
Comment threaddoc/api/repl.md Outdated
Comment threadlib/internal/repl/utils.js Outdated
Comment threadlib/repl.js Outdated
avivkeller
avivkeller reviewed Jul 21, 2026
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Comment threadlib/internal/repl/utils.js
avivkeller
avivkeller reviewed Jul 21, 2026
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Tests?

Comment threadlib/internal/repl/utils.js Outdated
Comment threadlib/internal/repl/utils.js Outdated
@hemanth
hemanthforce-pushed the repl/signature-hints-new branch from fc38014 to ae8b308CompareJuly 21, 2026 06:23
@hemanth
hemanthforce-pushed the repl/signature-hints-new branch from ae8b308 to 7777029CompareJuly 21, 2026 06:25
@hemanth

hemanth commented Jul 21, 2026

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

@hemanth
hemanthforce-pushed the repl/signature-hints-new branch from 7777029 to be1b29eCompareJuly 21, 2026 06:29
@hemanth
hemanthforce-pushed the repl/signature-hints-new branch from be1b29e to c4307f5CompareJuly 21, 2026 06:31
avivkeller
avivkeller reviewed Jul 21, 2026
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Comment threadlib/internal/repl/utils.js Outdated
Comment threadlib/internal/repl/utils.js
Comment on lines +926 to +957
// 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;
}
}

@avivkelleravivkellerJul 21, 2026

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

Comment threadlib/internal/repl/utils.js
throwOnSideEffect: true,
timeout: 200,
contextId: repl[contextSymbol],
}).then((preview) => {

@avivkelleravivkellerJul 21, 2026

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Would it be cleaner to make this function asynchronous?

Comment threadlib/internal/repl/utils.js Outdated
Comment threadlib/repl.js
Comment on lines +680 to +691
let showSignatureHint;
let clearSignatureHint;
if (preview && this.terminal && process.features.inspector &&
process.env.TERM !== 'dumb') {
({ showSignatureHint, clearSignatureHint } = setupSignatureHint(
this,
kContextId,
));
} else {
showSignatureHint = () => {};
clearSignatureHint = () => {};
}

@avivkelleravivkellerJul 21, 2026

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

Comment threadtest/parallel/test-repl-signature-hints.mjs Outdated
avivkeller
avivkeller reviewed Jul 21, 2026
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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

Comment threadlib/internal/repl/utils.js Outdated
@codecov

codecovBot commented Jul 21, 2026

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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 linesPatch %Lines
lib/internal/repl/utils.js88.48%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 linesCoverage Δ
lib/repl.js93.02% <100.00%> (+0.09%)⬆️
lib/internal/repl/utils.js95.77% <88.48%> (-1.16%)⬇️

... and 40 files with indirect coverage changes

🚀 New features to boost your workflow:
  • ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
  • 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

@hemanth
repl: add function signature hints
86623b4
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
hemanthforce-pushed the repl/signature-hints-new branch from c4307f5 to 86623b4CompareJuly 21, 2026 16:56
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