Review TypeScript settings and indexed access to arrays and objects to find values that may be undefined. Enable compiler checks and update unsafe code so potential failures become visible before the code runs.
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.
Review TypeScript code that retrieves elements from arrays or objects and check how it handles missing values. The skill inspects noUncheckedIndexedAccess in tsconfig.json and enables it when needed. For locations newly flagged by the setting, it adds checks for undefined values or optional chaining that stops further operations when a value is absent.
Use it when empty arrays or out-of-range indexes cause runtime errors. It also helps review data processing code that uses arr[0], arr[i], or object[key], looking for places where a retrieved value is used without first checking that it exists. This makes it useful both for investigating unexpected undefined errors and for updating code after stricter settings reveal type errors.
tsconfig.json and improve the safety of indexed access.”This skill targets TypeScript projects. Enabling the setting includes undefined in indexed access results, so existing code may need changes.
This overview was written by AI (OpenAI Codex) from the public SKILL.md. Check the original below for the exact behavior.
AI rated this skill on four points using only the contents of its SKILL.md. Stars and popularity are not taken into account.
The total counts each point as A = 25, B = 15, C = 5. Rated by AI (OpenAI Codex) as of Oct 23, 2026. Treat it as a guide and read the original before installing.
Review the instructions the agent will receive before you install.
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.
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.
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 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.
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
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