require-no-unchecked-indexed-access

Require "noUncheckedIndexedAccess": true in compilerOptions to include undefined in the type of array index access and index-signature reads.

Targeted pattern scope

The compilerOptions.noUncheckedIndexedAccess field in any tsconfig*.json file.

What this rule reports

This rule reports when noUncheckedIndexedAccess is absent or set to false.

Why this rule exists

Without noUncheckedIndexedAccess, accessing an array element by index (e.g., arr[0]) returns the element type T even though the array might be empty. TypeScript assumes the access is in-bounds and produces no warning, so code like:

const first = items[0].name; // No error even if items is empty

compiles without complaint but throws TypeError: Cannot read properties of undefined at runtime.

Enabling noUncheckedIndexedAccess changes the return type of indexed access to T | undefined, forcing developers to check for undefined before using the value — the same pattern required for Map.get() and Record access.

The auto-fixer adds "noUncheckedIndexedAccess": true to compilerOptions.

❌ Incorrect

{
    "compilerOptions": {
        "strict": true
    }
}

noUncheckedIndexedAccess is not part of strict and must be set explicitly.

✅ Correct

{
    "compilerOptions": {
        "strict": true,
        "noUncheckedIndexedAccess": true
    }
}

When not to use it

Disable this rule for codebases that rely heavily on index access where the in-bounds guarantee is established by surrounding logic, and the additional undefined checks would add noise without catching real bugs.

Package documentation

Rule catalog ID: R021

Targeted pattern scope​

What this rule reports​

Why this rule exists​

❌ Incorrect​

✅ Correct​

When not to use it​

Package documentation​

Further reading​