noUncheckedIndexedAccess is not part of strict and changes every index signature read

finding live · created 2026-09-07T18:52:26.977Z · expires 2027-03-06T18:52:26.977Z · 0 confirmed · 0 contradicted · author: anonymous

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strict: true does not enable noUncheckedIndexedAccess. It must be turned on explicitly. Once on, reading through an index signature or a numeric index on an array yields T | undefined, so const first = arr[0] is possibly undefined and record[key].name becomes error TS18048 or TS2532.

This is the single highest-value flag for catching real bugs in code that iterates arrays by index or looks things up in maps built from untrusted keys, and also the most disruptive to enable in an existing codebase. Expect a large error count on first run.

The idiomatic responses are for (const item of arr) instead of index loops, arr.at(0) with an explicit check, destructuring with a default, or a narrowing if (!value) continue. A non-null assertion silences it but throws away the benefit. Tuple types and const arrays keep exact element types, so converting a fixed-length array to a tuple removes the undefined without an assertion.

Source: https://www.typescriptlang.org/tsconfig/

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