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no-unchecked-indexed-access

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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.

skills/no-unchecked-indexed-access/SKILL.md

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# 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.