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Agent skill

No Unchecked Indexed Access

by thedaviddias in thedaviddias/Front-End-Checklist

A skill your agent uses when auditing tsconfig.json for missing safety flags, reviewing code that uses array index access in data processing pipelines, or investigating runtime undefined errors that…

MITAuto-check passed

Install No Unchecked Indexed Access

skills CLI
$ npx skills add thedaviddias/Front-End-Checklist --skill no-unchecked-indexed-access -a claude-code

Project install by default; add -g for ~/.claude/skills/.

GitHub CLI
$ gh skill install thedaviddias/Front-End-Checklist no-unchecked-indexed-access --agent claude-code

Project scope by default; add --scope user for a personal install. Needs GitHub CLI 2.90.0 or later (public preview).

Manual copy
$ git clone --depth 1 https://github.com/thedaviddias/Front-End-Checklist.git skills-src && mkdir -p .claude/skills && cp -r skills-src/skills/no-unchecked-indexed-access .claude/skills/no-unchecked-indexed-access && rm -rf skills-src

Use ~/.claude/skills/ instead of .claude/skills for a personal install. The folder must contain SKILL.md.

Claude Code skills documentation · loads skills from .claude/skills/

Facts

Skill name
no-unchecked-indexed-access
GitHub stars
74k
Token cost
~584 tokens
SKILL.md length
250 words
Files
2 (incl. references)
Skills in repo
390
Repo updated
First seen
Licence
MIT

At a glance

A skill your agent uses when auditing tsconfig.json for missing safety flags, reviewing code that uses array index access in data processing pipelines, or investigating runtime undefined errors that…

  • Auditing tsconfig.json for missing safety flags
  • SKILL.md covers Quick Reference, Check, Fix and Explain, plus 1 more section
  • Instructions only: no scripts, shell commands, URLs or credentials in SKILL.md
  • Reviewing code that uses array index access in data processing pipelines

What it does

No Unchecked Indexed Access is an agent skill from thedaviddias/Front-End-Checklist. Use when auditing tsconfig.json for missing safety flags, reviewing code that uses array index access in data processing pipelines, or investigating runtime undefined errors that TypeScript did not catch.

Its SKILL.md is about 580 tokens, which your agent loads only when the skill is triggered. The skill folder holds 2 other files, including reference files (for example `references/rule.md`).

It works with TypeScript. The repository describes itself as: 🗂 The essential checklist for modern web development, for humans and AI agents. The licence is MIT.

When your agent uses it

  • Auditing tsconfig.json for missing safety flags
  • Reviewing code that uses array index access in data processing pipelines
  • Investigating runtime undefined errors that TypeScript did not catch

Example prompts

  • “/no-unchecked-indexed-access”

What it can do on your machine

Read from SKILL.md and the folder at commit e8d14d0. It shows what the files ask for, not the result of running them.

  • Tool permissions

    Pre-approves nothing: there is no allowed-tools line, so your agent's usual permission prompts apply.

    From allowed-tools in the SKILL.md frontmatter.

  • Runs code

    No scripts in the folder and no shell commands in SKILL.md.

    From the folder's file list and the shell code blocks in SKILL.md.

  • Network

    Links to these hosts (documentation or services it may open):

    • frontendchecklist.io

    From URLs in SKILL.md, links to its own repository left out.

  • Credentials

    Names no API keys, tokens, secrets or passwords.

    From names ending in _API_KEY, _TOKEN, _SECRET, _KEY or _PASSWORD in SKILL.md.

Context cost

No Unchecked Indexed Access loads about 584 tokens when it runs, and up to ~2.1k if it reads all its reference files. Until then it costs about 58 tokens; SKILL.md has 250 words of instructions outside code blocks.

Always · name and description, kept in context so the agent knows when to use it
~58
When it runs · the whole SKILL.md, loaded when a task matches
~584
With references · SKILL.md plus every file in references/, read only if the agent opens them
~2.1k

Estimates: characters ÷ 4, the usual rule of thumb; real counts depend on the model's tokenizer. Scripts and assets cost tokens only if the agent reads them.

Safety

Auto-check passed

The automated check found no risky patterns in SKILL.md.

Automated static check — not a guarantee. Review scripts before installing. It scans the text of SKILL.md for risky patterns (piping downloads into a shell, reading credential files, hidden Unicode, destructive commands); files beside SKILL.md are not scanned.

SKILL.md

The full file from thedaviddias/Front-End-Checklist at commit e8d14d0, republished under its MIT licence (© thedaviddias). 250 words, ~584 tokens.

Download SKILL.mdSave it as .claude/skills/no-unchecked-indexed-access/SKILL.md (or your agent's skills folder). This skill also uses 1 other file; get the full folder from GitHub.
name
no-unchecked-indexed-access
description
Use when auditing tsconfig.json for missing safety flags, reviewing code that uses array index access in data processing pipelines, or investigating runtime undefined errors that TypeScript did not catch.
metadata.category
javascript
metadata.priority
medium
metadata.difficulty
intermediate
metadata.estimatedTime
15
metadata.source
frontendchecklist.io
metadata.url
https://frontendchecklist.io/rules/javascript/no-unchecked-indexed-access

Enable noUncheckedIndexedAccess to catch out-of-bounds array bugs

TypeScript's default behaviour types array[0] as T, pretending the access is always safe. In reality, the array might be empty or the index might be out of range, leading to silent undefined values that cause downstream crashes. Enabling noUncheckedIndexedAccess surfaces these potential failures as type errors at compile time, requiring developers to prove the access is safe before the code runs.

Quick Reference

  • Without the flag, array[0] is typed as T even if the array is empty
  • noUncheckedIndexedAccess changes array[n] to return T | undefined
  • Forces an explicit check before using any array element accessed by index
  • Optional chaining and nullish coalescing handle most false positives cleanly

Check

Check whether noUncheckedIndexedAccess is enabled in the project's tsconfig.json. If not, scan for array index accesses (arr[0], arr[i]) that could fail if the array is shorter than expected.

Fix

Enable noUncheckedIndexedAccess in tsconfig.json, then update any array index accesses that TypeScript now flags to include an explicit undefined check or use optional chaining before the accessed value is used.

Explain

Explain what noUncheckedIndexedAccess does, why the default TypeScript behaviour is unsound for array index access, and how optional chaining and Array.at() interact with the flag.

Code Review

Review all array index accesses in this file (arr[0], arr[i], object[key]). Flag any location where the accessed value is used without a preceding undefined check, even if noUncheckedIndexedAccess is not yet enabled in the project.

For full implementation details, code examples, and framework-specific guidance, see references/rule.md.

Rule page: https://frontendchecklist.io/rules/javascript/no-unchecked-indexed-access

© thedaviddias, MIT. Rendered from Markdown: HTML in the file is shown as text, images as links, and headings moved down two levels. Raw file

Files

SKILL.md and 1 other file (references) in skills/no-unchecked-indexed-access of thedaviddias/Front-End-Checklist.

  • SKILL.md
  • references/rule.md

Open the folder on GitHubat commit e8d14d0

Compare with similar skills

No Unchecked Indexed Access next to the 5 skills that share the most tags, products or categories with it. Stars are the repository's; “used in” counts other GitHub owners with a copy.

No Unchecked Indexed Access compared with similar skills
SkillStarsUsed inTokensAuto-checkLicenceRepo updated
No Unchecked Indexed Access this skillthedaviddias/Front-End-Checklist74k—~584Automated safety check: PassMIT
MCP Server Builderanthropics/skills180k63 repos~2.3kAutomated safety check: PassApache-2.0
Web Artifacts Builderanthropics/skills180k40 repos~769Automated safety check: PassApache-2.0
Typescript Advanced Typesrolling-scopes/rsschool-app10k25 repos~4.2kAutomated safety check: PassMPL-2.0
Code Review ChecklistshareAI-lab/learn-claude-code78k4 repos~1.1kAutomated safety check: PassMIT
MCP Server BuildershareAI-lab/learn-claude-code78k4 repos~1.2kAutomated safety check: PassMIT

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Works with

Questions about No Unchecked Indexed Access

What does No Unchecked Indexed Access do?

A skill your agent uses when auditing tsconfig.json for missing safety flags, reviewing code that uses array index access in data processing pipelines, or investigating runtime undefined errors that…. No Unchecked Indexed Access is an agent skill from thedaviddias/Front-End-Checklist.json for missing safety flags, reviewing code that uses array index access in data processing pipelines, or investigating runtime undefined errors that TypeScript did not catch.

When should I use No Unchecked Indexed Access?

No Unchecked Indexed Access fits situations like: auditing tsconfig.json for missing safety flags; reviewing code that uses array index access in data processing pipelines; investigating runtime undefined errors that TypeScript did not catch.

How do I install No Unchecked Indexed Access in Claude Code?

Run `npx skills add thedaviddias/Front-End-Checklist --skill no-unchecked-indexed-access -a claude-code`. Or copy the skill folder (skills/no-unchecked-indexed-access in thedaviddias/Front-End-Checklist) into .claude/skills/no-unchecked-indexed-access in your project. Claude Code loads it when a task matches its description.

How do I install No Unchecked Indexed Access in Codex?

Run `npx skills add thedaviddias/Front-End-Checklist --skill no-unchecked-indexed-access -a codex`. Or copy the skill folder (skills/no-unchecked-indexed-access in thedaviddias/Front-End-Checklist) into .agents/skills/no-unchecked-indexed-access in your project. Codex loads it when a task matches its description.

Can I use No Unchecked Indexed Access in Cursor, Gemini CLI or GitHub Copilot?

Cursor, Gemini CLI, GitHub Copilot and OpenCode also load SKILL.md folders. With the skills CLI, run `npx skills add thedaviddias/Front-End-Checklist --skill no-unchecked-indexed-access -a cursor` (or -a gemini-cli, github-copilot or opencode for the others). To copy it by hand, put the folder in .cursor/skills/no-unchecked-indexed-access, .gemini/skills/no-unchecked-indexed-access, .github/skills/no-unchecked-indexed-access and .opencode/skills/no-unchecked-indexed-access in your project.

What does No Unchecked Indexed Access need to run?

SKILL.md names no scripts, command-line tools or credentials: No Unchecked Indexed Access is instructions for the agent only.

Does No Unchecked Indexed Access access the network?

SKILL.md names 1 domain. As links in the text: frontendchecklist.io. This is read from the text; nothing was executed.

Is No Unchecked Indexed Access safe to install?

Our automated static check of SKILL.md found no risky patterns, such as piping downloads into a shell, reading credential files or hidden Unicode. It is not a guarantee. Review the folder before installing.

What licence does No Unchecked Indexed Access use?

No Unchecked Indexed Access is published under the MIT licence (the repository's licence). It allows redistribution, so the full SKILL.md is shown on this page.

How many tokens does No Unchecked Indexed Access use?

About 584 tokens (SKILL.md is roughly 2.3k characters). Agents keep only the skill's name and description in context until a task matches; then they load SKILL.md in full. Its references folder adds about 1.6k tokens, read only when the agent opens those files.

What are the alternatives to No Unchecked Indexed Access?

Skills that share tags, products or a category with No Unchecked Indexed Access: MCP Server Builder (anthropics/skills, 180k stars), Web Artifacts Builder (anthropics/skills, 180k stars), Typescript Advanced Types (rolling-scopes/rsschool-app, 10k stars) and Code Review Checklist (shareAI-lab/learn-claude-code, 78k stars). The comparison table on this page puts their stars, adoption, token cost, safety result and licence side by side.

Who maintains No Unchecked Indexed Access?

thedaviddias (a GitHub user) maintains it in thedaviddias/Front-End-Checklist, which has 74,421 GitHub stars. The repository holds 390 skills in this directory. The repository was last updated on October 6, 2026.

Source: thedaviddias/Front-End-Checklist on GitHub. Facts on this page come from the repository at the commit we read; the author's words are quoted as theirs.