feat: implement unnecessaryUseStricts rule for TypeScript plugin

[Copilot]

Implements unnecessaryUseStricts rule to detect and remove redundant "use strict" directives in ES modules, where strict mode is automatically enabled.

// ❌ Unnecessary in modules
"use strict";

export function calculate(value: number) {
    return value * 2;
}

// ✅ Correct
export function calculate(value: number) {
    return value * 2;
}

Changes

• Rule implementation (packages/ts/src/rules/unnecessaryUseStricts.ts)

  • Detects "use strict" directives at the start of ES modules
  • Provides autofix that removes the directive including trailing whitespace
  • Only triggers for files with import/export statements (ts.isExternalModule)

• Tests (packages/ts/src/rules/unnecessaryUseStricts.test.ts)

  • Validates detection in various module patterns
  • Confirms non-modules are allowed to keep "use strict"
  • Tests consecutive line handling and whitespace removal

• Documentation (packages/site/src/content/docs/rules/ts/unnecessaryUseStricts.mdx)

  • Explains ES module strict mode behavior
  • Provides MDN references

• Integration

  • Added to packages/ts/src/plugin.ts (alphabetical order)
  • Marked as implemented in packages/comparisons/src/data.json
  • Updated cspell.json with new terms

Equivalent to Biome's noRedundantUseStrict rule.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• biomejs.dev
  • Triggering command: /home/REDACTED/work/_temp/ghcca-node/node/bin/node /home/REDACTED/work/_temp/ghcca-node/node/bin/node --enable-source-maps /home/REDACTED/work/_temp/copilot-developer-action-main/dist/index.js (dns block)
• telemetry.astro.build
  • Triggering command: /opt/hostedtoolcache/node/24.13.0/x64/bin/node node /home/REDACTED/work/flint/flint/packages/site/node_modules/.bin/../astro/astro.js sync git conf�� get --local node credential.userngit aKGoldberg@usersrev-parse (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
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Original prompt

---

This section details on the original issue you should resolve

<issue_title>🚀 Feature: Implement unnecessaryUseStricts rule (TypeScript)</issue_title>
<issue_description>### Feature Request Checklist

• I have pulled the latest main branch of the repository.
• I have searched for related issues and found none that matched my issue.

Overview

Per flint.fyi/rules, Flint is intended to have a unnecessaryUseStricts rule in the TypeScript plugin (ts). It'll behave roughly equivalently to the existing implementations in other linters to start. This issue tracks adding that rule.

Adding this rule will entail creating the following new source files:

• packages/ts/src/rules/unnecessaryUseStricts.ts: implementation of the rule itself
• packages/ts/src/rules/unnecessaryUseStricts.test.ts: tests for the rule
• packages/site/src/content/docs/rules/ts/unnecessaryUseStricts.mdx: documentation of the rule

Additionally, the following files will need to be edited:

• packages/comparisons/src/data.json: Comparisons data will need to mention the rule is now "status": "implemented"
• packages/ts/src/plugin.ts: Included rules should have the new one inserted in alphabetical order

Additional Info

Existing rules in other linters:

• Biome: noRedundantUseStrict

❤️‍🔥</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes 🚀 Feature: Implement unnecessaryUseStricts rule (TypeScript) #2246

---

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[changeset-bot]

⚠️ No Changeset found

Latest commit: 23b4e6a

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Project	Deployment	Actions	Updated (UTC)
flint	Ready	Preview, Comment	Jan 31, 2026 11:36pm