fix: use format strings in debug calls by fasttime · Pull Request #21247 · eslint/eslint

fasttime · 2026-08-23T08:57:57Z

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What is the purpose of this pull request? (put an "X" next to an item)

[ ] Documentation update
[X] Bug fix (template)
[ ] New rule (template)
[ ] Changes an existing rule (template)
[ ] Add autofix to a rule
[ ] Add a CLI option
[ ] Add something to the core
[ ] Other, please explain:

What did you do? Please include the actual source code causing the issue.

Linted files whose names contain a percent sign, with debug output enabled:

mkdir repro && cd repro
echo 'module.exports = [];' > eslint.config.js
echo 'var x = 1;' > 'a%%b.js'
echo 'var y = 2;' > 'c%sd.js'
DEBUG='eslint:*' npx eslint .

What did you expect to happen?

The debug output should show the file names as they are on disk.

What actually happened? Please include the actual, raw output from ESLint.

The percent sequences are consumed as format placeholders:

eslint:config-loader Calculating config for file .../a%b.js +0ms
eslint:config-loader Calculating config for file .../c+2msd.js

a%%b.js is printed as a%b.js, and in c%sd.js the %s swallows the +2ms timestamp that debug appends when printing to a TTY terminal, splicing it into the middle of the path and dropping it from the end of the line.

This happens because these the message is built with a template literal:

debug(`Calculating config for file ${filePath}`);

The interpolated string then becomes the format string, so any % sequence it happens to contain is interpreted by debug rather than printed literally. debug collapses %% to % itself (common.js), and the remaining specifiers are resolved by Node.js' util.format against the appended arguments.

The same problem occurs for other debug messages produced from a template literal that interpolates variable strings (like filenames or URLs).

What changes did you make? (Give an overview)

Use format strings

Converted debug() calls that used template literals to format strings with placeholders, which is how other parts of the codebase already do it (e.g. debug("%s\n%s", message, ex.stack) in lib/languages/js/index.js):

-debug(`Calculating config for file ${filePath}`);
+debug("Calculating config for file %s", filePath);

Values passed as arguments (not in the format string) are substituted verbatim and are not scanned for placeholders, so the output remains correct regardless of the file name.

Using format strings has another less evident effect: the message is only assembled if debugging is enabled, whereas a template literal is interpolated unconditionally before debug is called. Deferring the message calculation avoids building a string that is possibly discarded, which seems desirable even if the benefit on memory usage or performance should be negligible.

Internal rule

Added an internal rule internal-rules/no-debug-template-literals that reports a template literal used as the format argument of a debug call.

This should prevent the issue from creeping back in, considering that debug messages aren't typically verified in unit tests.

The rule and tests are partly generated with Claude Opus 5 and GPT-5.6.

Is there anything you'd like reviewers to focus on?

Summary by CodeRabbit

Documentation
- Updated the debug output example to reflect the current quoted file-pattern format.
Developer Experience
- Standardized diagnostic output formatting for clearer, more consistent debug messages.
- Added automated checks to prevent unsupported template-literal formatting in debug messages.
- Added comprehensive validation for formatting conversions, escaping, newlines, nested templates, and other edge cases.

netlify · 2026-08-23T08:58:03Z

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fasttime · 2026-08-23T11:15:25Z

 						"There are suppressions left that do not occur anymore. To resolve this, re-run the command with `--prune-suppressions` to remove unused suppressions. To ignore unused suppressions, use `--pass-on-unpruned-suppressions`.",
 					);
-					debug(JSON.stringify(unusedSuppressions, null, 2));
+					debug("%O", unusedSuppressions);


"%O" will format the argument on multiple lines when debugging is enabled (see https://github.com/debug-js/debug/blob/4.4.3/README.md#formatters). For example:

eslint:cli { eslint:cli 'file.js': { eslint:cli 'no-undef': { count: 1 }, eslint:cli 'no-unused-vars': { count: 1 }, eslint:cli 'no-var': { count: 1 } eslint:cli } eslint:cli } +14ms

fasttime · 2026-08-23T11:41:55Z

 		}

-		debug(`Using file patterns: ${normalizedPatterns}`);
+		debug("Using file patterns: %s", normalizedPatterns);


normalizedPatterns is an array which was being previously coerced into a string, producing an output like:

eslint:eslint Using file patterns: lib/,tests/ +0ms

With the %s formatter, the output now becomes:

eslint:eslint Using file patterns: [ 'lib/', 'tests/' ] +0ms

fasttime · 2026-08-23T12:43:19Z

-		debug(`Using config loader ${this.#configLoader.constructor.name}`);
-


Now that there is only one ConfigLoader class, I think this message is no longer needed.

coderabbitai · 2026-09-01T06:12:10Z

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

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📒 Files selected for processing (1)

tools/internal-rules/no-debug-template-literals.js

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📝 Walkthrough

Walkthrough

The change adds an internal rule for parameterized debug() formatting, enables and tests the rule, and converts existing debug calls across ESLint. The debug documentation example now shows quoted file patterns.

Changes

Parameterized debug logging

Layer / File(s)	Summary
Rule matching and suggestions `tools/internal-rules/no-debug-template-literals.js`	Adds detection for configured debug methods and suggestions that convert eligible template literals to format strings.
Rule enforcement and coverage `eslint.config.js`, `tests/tools/internal-rules/no-debug-template-literals.js`	Enables the rule as an error and tests diagnostics, formatting, escaping, comments, nested templates, and configurable methods.
CLI, configuration, and cache logging `lib/cli-engine/lint-result-cache.js`, `lib/cli.js`, `lib/config/config-loader.js`, `lib/eslint/eslint-helpers.js`, `lib/eslint/eslint.js`, `docs/src/use/configure/debug.md`	Converts debug messages to placeholder-based arguments and updates the file-pattern output example.
Code-path and lint-fix logging `lib/linter/code-path-analysis/*`, `lib/linter/linter.js`	Converts code-path and `verifyAndFix` diagnostics to parameterized logging and permits the `debug.dump` format.

Estimated code review effort: 3 (Moderate) | ~25 minutes

Merge Risk: ⚪ Minimal · up to 48a3a

This change preserves parameterized debug formatting enforcement and corrects the internal fixer callback shape. No current merge-blocking risk remains.

Suggested reviewers: aladdin-add

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the primary change: replacing template-literal interpolation with format strings in debug calls.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 10 functions across 11 files.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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Actionable comments posted: 1

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Inline comments:
In `@tools/internal-rules/no-debug-template-literals.js`:
- Line 96: Update the quasi detection in the no-debug template literal rule to
match percent sequences followed by either a letter or another percent,
preventing suggestions from doubling existing %% escapes; add a regression test
covering the 100% output case and preserve existing detection behavior.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Team

Run ID: d23b7921-4e5a-466e-8ee6-c41464d73559

📥 Commits

Reviewing files that changed from the base of the PR and between c6cc6c5 and d11e1ad.

📒 Files selected for processing (12)

docs/src/use/configure/debug.md
eslint.config.js
lib/cli-engine/lint-result-cache.js
lib/cli.js
lib/config/config-loader.js
lib/eslint/eslint-helpers.js
lib/eslint/eslint.js
lib/linter/code-path-analysis/code-path-analyzer.js
lib/linter/code-path-analysis/debug-helpers.js
lib/linter/linter.js
tests/tools/internal-rules/no-debug-template-literals.js
tools/internal-rules/no-debug-template-literals.js

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DMartens

Changes make sense and LGTM, thanks.
I only have an optional suggestion for the suggestion.

DMartens · 2026-09-01T21:45:20Z

+				}
+
+				// No suggestion is provided when there are multiple arguments.
+				const suggest =


This condition (node.arguments.length === 1) could be moved into the fixer, to conditionally return null.

Updated in c4f754c, thanks!

mdjermanovic · 2026-09-03T07:39:36Z

+					suggest: [
+						{
+							messageId: "replaceWithFormatString",
+							fix: createFix(formatArgument, node),


createFix has only one parameter?

Yes, there should be only one parameter, my mistake. Thanks.

mdjermanovic

LGTM, thanks!

This PR contains the following updates: | Package | Change | [Age](https://docs.renovatebot.com/merge-confidence/) | [Confidence](https://docs.renovatebot.com/merge-confidence/) | |---|---|---|---| | [eslint](https://eslint.org) ([source](https://github.com/eslint/eslint)) | [`10.9.1` → `10.10.0`](https://renovatebot.com/diffs/npm/eslint/10.9.1/10.10.0) | ![age](https://developer.mend.io/api/mc/badges/age/npm/eslint/10.10.0?slim=true) | ![confidence](https://developer.mend.io/api/mc/badges/confidence/npm/eslint/10.9.1/10.10.0?slim=true) | --- ### Release Notes <details> <summary>eslint/eslint (eslint)</summary> ### [`v10.10.0`](https://github.com/eslint/eslint/releases/tag/v10.10.0) [Compare Source](eslint/eslint@v10.9.1...v10.10.0) #### Features - [`264b434`](eslint/eslint@264b434) feat: add `d` and `v` flags to `no-unexpected-multiline` ([#21305](eslint/eslint#21305)) (Gihyeon Jeong / 정기현) - [`c6cc6c5`](eslint/eslint@c6cc6c5) feat: check `Object.prototype` property names in `new-cap` ([#21269](eslint/eslint#21269)) (crimsonjay0) - [`5661fa6`](eslint/eslint@5661fa6) feat: no-extra-bind false negatives with class fields and static blocks ([#21260](eslint/eslint#21260)) (synthex-byte) #### Bug Fixes - [`bb47dc6`](eslint/eslint@bb47dc6) fix: update dependency file-entry-cache to v11 ([#20801](eslint/eslint#20801)) (Milos Djermanovic) - [`427ac0a`](eslint/eslint@427ac0a) fix: use format strings in debug calls ([#21247](eslint/eslint#21247)) (Francesco Trotta) - [`9d81532`](eslint/eslint@9d81532) fix: support `__proto__` in `/* exported */` comments ([#21261](eslint/eslint#21261)) (sethamus) - [`87e0a08`](eslint/eslint@87e0a08) fix: prefer-object-has-own autofix breaks when Object is shadowed ([#21282](eslint/eslint#21282)) (김채영) - [`8e2cb14`](eslint/eslint@8e2cb14) fix: `new-cap` false positive for `UTC` calls with `properties: false` ([#21275](eslint/eslint#21275)) (Pixel) - [`9f4a364`](eslint/eslint@9f4a364) fix: Ignore static imports in no-unreachable ([#21276](eslint/eslint#21276)) (Taha Kotil) #### Documentation - [`2417cad`](eslint/eslint@2417cad) docs: Update README (GitHub Actions Bot) - [`9cecb8a`](eslint/eslint@9cecb8a) docs: document `\c` control letter escapes in no-control-regex ([#21286](eslint/eslint#21286)) (한국) - [`8724829`](eslint/eslint@8724829) docs: update compat table links ([#21263](eslint/eslint#21263)) (fnx) - [`5634542`](eslint/eslint@5634542) docs: Clarify eqeqeq suggestion behavior ([#21256](eslint/eslint#21256)) (Müslüm Yılmaz) #### Chores - [`b3d876b`](eslint/eslint@b3d876b) chore: disable npm audit in ecosystem tests ([#21306](eslint/eslint#21306)) (Francesco Trotta) - [`1696682`](eslint/eslint@1696682) ci: restore EMFILE test on Node.js 26 ([#21297](eslint/eslint#21297)) (Marry (Subin Yang)) - [`2c7f5d6`](eslint/eslint@2c7f5d6) chore: update github/codeql-action action to v4.37.9 ([#21296](eslint/eslint#21296)) (renovate\[bot]) - [`3c753f1`](eslint/eslint@3c753f1) chore: update eslint ([#21289](eslint/eslint#21289)) (renovate\[bot]) - [`1c73469`](eslint/eslint@1c73469) chore: update ecosystem plugins ([#21280](eslint/eslint#21280)) (ESLint Bot) - [`08a02be`](eslint/eslint@08a02be) test: add error locations to `no-extra-boolean-cast` ([#21266](eslint/eslint#21266)) (lumir) - [`77bb1db`](eslint/eslint@77bb1db) chore: update github/codeql-action action to v4.37.8 ([#21270](eslint/eslint#21270)) (renovate\[bot]) - [`007e81a`](eslint/eslint@007e81a) ci: skip EMFILE test on Node.js 26 ([#21265](eslint/eslint#21265)) (lumir) - [`0430280`](eslint/eslint@0430280) chore: improve ecosystem tests compatibility on Windows ([#21178](eslint/eslint#21178)) (crimsonjay0) </details> --- ### Configuration 📅 **Schedule**: (UTC) - Branch creation - At any time (no schedule defined) - Automerge - At any time (no schedule defined) 🚦 **Automerge**: Enabled. ♻ **Rebasing**: Whenever PR is behind base branch, or you tick the rebase/retry checkbox. 🔕 **Ignore**: Close this PR and you won't be reminded about this update again. --- - [ ] If you want to rebase/retry this PR, check this box --- This PR has been generated by [Mend Renovate](https://github.com/renovatebot/renovate).  Reviewed-on: https://codeberg.org/huskas-2189/Bookmark/pulls/261

github-project-automation Bot added this to Triage Aug 23, 2026

github-project-automation Bot moved this to Needs Triage in Triage Aug 23, 2026

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fasttime commented Aug 23, 2026

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fasttime added 5 commits September 1, 2026 07:49

fix: use format strings in debug calls

7355235

tweaks

dabb9fd

remove debug output for ConfigLoader class name

c30648e

no suggestion if the template literal contains a format specifier

1b550e4

clarify comments

d11e1ad

fasttime force-pushed the debug-format branch from 6f75266 to d11e1ad Compare September 1, 2026 06:12

fasttime marked this pull request as ready for review September 1, 2026 06:26

fasttime requested a review from a team as a code owner September 1, 2026 06:26

coderabbitai Bot reviewed Sep 1, 2026

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Comment thread tools/internal-rules/no-debug-template-literals.js

DMartens previously approved these changes Sep 1, 2026

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DMartens mentioned this pull request Sep 1, 2026

fix: update dependency file-entry-cache to v11 #20801

Merged

3 tasks

move the check for multiple arguments to the fixer function

c4f754c

fasttime dismissed DMartens’s stale review via c4f754c September 2, 2026 19:32

mdjermanovic reviewed Sep 3, 2026

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remove sperfluous parameter

48a3ad4

DMartens approved these changes Sep 3, 2026

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snitin315 approved these changes Sep 4, 2026

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mdjermanovic added the accepted There is consensus among the team that this change meets the criteria for inclusion label Sep 4, 2026

mdjermanovic moved this from Needs Triage to Implementing in Triage Sep 4, 2026

mdjermanovic approved these changes Sep 4, 2026

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mdjermanovic merged commit 427ac0a into main Sep 4, 2026
45 checks passed

mdjermanovic deleted the debug-format branch September 4, 2026 12:59

github-project-automation Bot moved this from Implementing to Complete in Triage Sep 4, 2026

		debug(`Using config loader ${this.#configLoader.constructor.name}`);

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What is the purpose of this pull request? (put an "X" next to an item)

What changes did you make? (Give an overview)

Use format strings

Internal rule

Is there anything you'd like reviewers to focus on?

Summary by CodeRabbit

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fasttime commented Aug 23, 2026 •

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