Custom Linting for JupyterLab

Status: In Progress · Proposal

August 2026¶

Summary of Work Completed¶

New rules were added to the ESLint plugin and enabled downstream; existing rules were improved:

• new jupyter/no-dynamic-translation rule to improve translation coverage jupyterlab/eslint-plugin#91

• new rules aimed at preventing memory leaks:

  • ensuring signal cleanup require-signal-cleanup, require-signal-this-arg and prefer-signal-this-arg jupyterlab/eslint-plugin#78

  • ensuring cleanup of disposables require-disposable-ownership and require-disposable-transfer jupyterlab/eslint-plugin#77, jupyterlab/eslint-plugin#89, jupyterlab/eslint-plugin#93

• jupyter/galata-prefer-filebrowser-helper suggesting to use the filebrowser utilities for Playwright UI tests over custom code jupyterlab/eslint-plugin#73, jupyterlab/eslint-plugin#97

In addition to the galata-prefer-filebrowser-helper rule we are in various stages of developing additional rules that encourage use of methods with known reliability when developing new UI tests for Jupyter frontends:

• galata-prefer-notebook-cell-helper jupyterlab/eslint-plugin#105

• galata-prefer-sidebar-activity-helper jupyterlab/eslint-plugin#101

• galata-prefer-menu-helper jupyterlab/eslint-plugin#85

We are working on improving two rules:

• adding autofix to jupyter/no-schema-enum and detection of missing type information in the correct formulation jupyterlab/eslint-plugin#106

• expanding the jupyter/no-untranslated-string rule to catch more gaps in translation jupyterlab/eslint-plugin#102

The ESLint plugin was updated to the latest version in the JupyterLab extension-template (jupyterlab/extension-template#156, https://github.com/jupyterlab/extension-template/releases/tag/v4.6.4).

Finally, we assisted community members adding a new jupyter/incorrect-translator-usage rule (jupyterlab/eslint-plugin#76). This, and the jupyter/prefer-lazy-imports rule (jupyterlab/eslint-plugin#108) that is being developed outside of the CFP funding envelope, demonstrate the multiplier effect of this proposal: the community sees the value of a static analysis toolset nudging the entire ecosystem of frontend extensions towards better code quality with user-facing results (better translation, fewer errors, lower memory usage, faster load times).

We track most of the issues and pull requests associated with this proposal in this public GitHub project.

Deliverables/Milestones¶

• Goal: develop and maintain custom ESLint rules tailored to the JupyterLab/Notebook codebase.

  • Done

• Impact Metric: Adoption of the additional rules by the community

  • Done

• Impact: Reduction in time to CI failure on violation of best practices

  • Done

• Impact: reduction in late-stage CI failures

  • Done

Challenges or Risks¶

None

Budget Update¶

As of 25th August 2026 we used 15,120.50 USD out of 17,496 USD budget.

Part of the budget was used to support the visual testing improvements, with the end-goal of preventing future regressions in the test suite reliability by codifying all the best practices developed throughout the work on the proposal focused on the visual regression tests.

Next Steps¶

We intend to complete the work on the pull requests that are currently open. We do not plan any major addition going forward given that we are close to exhausting the budget.

June 2026¶

Summary of Work Completed¶

New rules were added to the ESLint plugin and enabled downstream; existing rules were improved:

• new jupyter/no-pageconfig-base-url jupyterlab/eslint-plugin#68

• new jupyter/no-schema-enum rule jupyterlab/eslint-plugin#55, jupyterlab/eslint-plugin#60, jupyterlab/eslint-plugin#58

• new jupyter/require-soft-assertions-before-snapshots rule jupyterlab/eslint-plugin#53

• improved jupyter/plugin-activation-args rule jupyterlab/eslint-plugin#64, jupyterlab/eslint-plugin#54, jupyterlab/eslint-plugin#56

• improved jupyter/no-untranslated-string jupyterlab/eslint-plugin#49

We performed necessary maintenance and improved the documentation

• fixing release workflow jupyterlab/eslint-plugin#46

• add CONTRIBUTING.md jupyterlab/eslint-plugin#67

The existing linter rules in JupyterLab were tightened and new ones were added:

• spell checking for documentation jupyterlab/jupyterlab#18398

• regular expression linting jupyterlab/jupyterlab#18908

• rule to forbid serial mode in regression testing jupyterlab/jupyterlab#18838

• GitHub Actions security linter jupyterlab/jupyterlab#18898

• adopted shell script and documentation linter jupyterlab/jupyterlab#18858

• forbid explicit any jupyterlab/jupyterlab#18847

• and progressively remove it from the codebase jupyterlab/jupyterlab#18869

• enforce that certain CSS rules use variables jupyterlab/jupyterlab#18614

The blog post describing the ESLint plugin was published: https://blog.jupyter.org/catching-jupyter-specific-bugs-before-ci-does-announcing-jupyter-eslint-plugin-fc65ae414630

The ESLint plugin was included in the latest version of JupyterLab extension-template (jupyterlab/extension-template#141, https://github.com/jupyterlab/extension-template/releases/tag/v4.6.0)

We started adopting the eslint plugin in official Jupyter extensions:

• jupyterlab/jupyter-collaboration#602

• jupyterlab/jupyterlab-git#1491

We track most of the issues and pull requests associated with this proposal in this public GitHub project.

Deliverables/Milestones¶

• Goal: develop and maintain custom ESLint rules tailored to the JupyterLab/Notebook codebase.

  • Done

• Impact Metric: Adoption of the additional rules by the community

  • following projects adopted this rule organically:

    • jupyter-chat (jupyterlab/jupyter-chat#410)

    • ipyparallel (ipython/ipyparallel#1009)

  • community extensions started pulling the package via extension-template updates:

    • https://github.com/d-torrance/jupyterlab-macaulay2

    • https://github.com/QuantStack/jupyter-ai-tutor

    • https://github.com/ejuet/zenodo-jupyterlab-extension-prototype

• Impact: Reduction in time to CI failure on violation of best practices

  • Done - the lint job completes and reports back within 6 minutes, the frequency of review comments discussing the best practice significantly decreases as most frequent issues are now caught by pre-commit, or fixed by linter integration of coding agents authoring the patches

• Impact: reduction in late-stage CI failures

  • Done

Challenges or Risks¶

None

Budget Update¶

As of 23rd June 2026 we used 8,591 USD out of 17,496 USD budget.

20% of the budget was used to support the visual testing improvements, with the end-goal of preventing future regressions in the test suite reliability by codifying all the best practices developed throughout the work on the proposal focused on the visual regression tests.

Next Steps¶

• Tightening of select rules in extension-template and Notebook repositories based on lessons from the adoption of stricter rules in JupyterLab, accounting for distinct needs of these two repositories

• Adding linting rules for galata visual testing to prevent flaky tests from being committed in the first place

• Further improve the ESLint plugin, based on any community feedback received

Beyond these, we intend to spend the remainder of the budget on ensuring no flaky tests are present, enabling easy validation of the efficacy of the linting rules preventing flakiness of UI tests.

April 2026¶

Summary of Work Completed¶

• The ESLint plugin was transition to the JupyterLab organization (https://github.com/jupyterlab/eslint-plugin) and published on npm as jupyter/eslint-plugin

• The plugin was adopted by JupyterLab (jupyterlab/jupyterlab#18615), Notebook (jupyter/notebook#7856) and JupyterLite (jupyterlite/jupyterlite#1917)

• The documentation was published (https://eslint-plugin.readthedocs.io/en/latest/)

• Existing rules were improved, and new rules were added, including:

  • new jupyter/no-translation-concatenation rule jupyterlab/eslint-plugin#44

  • new jupyter/no-untranslated-string rule jupyterlab/eslint-plugin#41

  • new jupyter/token-format rule jupyterlab/eslint-plugin#40

  • improved jupyter/plugin-description rule jupyterlab/eslint-plugin#37

  • improvements to @jupyter/plugin-activation-args rule jupyterlab/eslint-plugin#31, jupyterlab/eslint-plugin#36

• Downstream tests are now on CI to ensure no regressions are introduced and allowing early evaluation of new rules/rule changes

We track some of the issues and pull requests associated with this proposal in this public GitHub project.

Deliverables/Milestones¶

• Goal: develop and maintain custom ESLint rules tailored to the JupyterLab/Notebook codebase.

  • Done

• Impact Metric: Adoption of the additional rules by the community

  • jupyter-chat (jupyterlab/jupyter-chat#410) adopted this rule organically

• Impact: improvement in the code quality

  • Adoption of the plugin highlighted multiple previously unknown issues with translation, documentation, and dependency injection that were since fixed

Challenges or Risks¶

None

Budget Update¶

As of 26th April 2026 we used 3,102 USD out of 17,496 USD budget.

Next Steps¶

• Further expansion of new bespoke rules based on the discussions tracked in JupyterLab repository issues

• Tightening of existing rules in JupyterLab, extension-template and Notebook repositories

February 2026¶

Summary of Work Completed¶

• ESLint plugin with custom rules was created https://github.com/Darshan808/eslint-plugin-jupyter

• ESLint upgrade to v9 is in progress jupyterlab/jupyterlab#18434

We track some of the issues and pull requests associated with this proposal in this public GitHub project.

Deliverables/Milestones¶

• Goal: develop and maintain custom ESLint rules tailored to the JupyterLab/Notebook codebase.

  • The plugin now exists, is pending transfer to JupyterLab-governed repository

• Impact Metric: Adoption of the additional rules by the community

  • Multiple community members expressed interest in adopting the ESLint plugin during the Feb 25th, 2026 Jupyter Community Call

Challenges or Risks¶

• Security vulnerabilities in ESLint v8 forced an early migration to v9. As a consequence the early work aimed at making the plugin compatible with both versions was shelved in favour of prioritising migration of JupyterLab codebase to the newer version.

Budget Update¶

As of 20th Feb 2026 we used 330 USD out of 17,496 USD budget.

Next Steps¶

• Transition the ESLint plugin to the JupyterLab organization

• Expansion of new bespoke rules based on the discussions tracked in JupyterLab repository issues

• Tightening of existing rules in JupyterLab, extension-template and Notebook repositories