noUncheckedIndexedAccess

With noUncheckedIndexedAccess enabled, unchecked array indexing and access through index signatures include undefined when the key might not exist. This encourages guards before using values. It does not check array bounds at runtime or rewrite the indexing operation.

The array has only indexes 0 and 1. The guard ensures both an existing numeric entry and an out-of-bounds undefined value are handled safely, with or without the extra compiler flag.

Array access with an index typed number is not proof that the index exists.

Save as nouncheckedindexedaccess.ts. The following program passed strict TypeScript compilation, and the displayed output was captured by executing its emitted JavaScript with Node.js 22.16.0.

TypeScript source

const scores = [9, 7];
function readScore(index: number): string {
  const score = scores[index];
  return score === undefined ? "Missing" : `Score: ${score}`;
}
console.log(readScore(1));
console.log(readScore(4));

Verified output

Score: 7
Missing

Compile: npx tsc --strict --target es2022 --module commonjs nouncheckedindexedaccess.ts; run: node nouncheckedindexedaccess.js. For Node.js API examples, install @types/node first. For browser projects, use the browser-specific instructions below.

The example demonstrates the code-level behavior; the related configuration or test commands are shown separately when needed.

The array has only indexes 0 and 1. The guard ensures both an existing numeric entry and an out-of-bounds undefined value are handled safely, with or without the extra compiler flag.

Predict what changes if you alter one input. Then run the code to compare your reasoning with the real output.

Array access with an index typed number is not proof that the index exists.

Identify whether the failure is a type-checking issue, a configuration issue, or a runtime behavior issue before choosing a fix.

Compile once with --noUncheckedIndexedAccess and once without. Remove the guard and compare diagnostics.

Write down your expected result first. For configuration examples, compare compiler output and diagnostics; for test examples, make the assertion fail deliberately to verify that the tool reports it.

For the exact current options and platform behavior, consult the official documentation. These examples were verified using the tool versions described above.

With noUncheckedIndexedAccess enabled, unchecked array indexing and access through index signatures include undefined when the key might not exist.

Run the primary example, then complete the practice task or environment-specific lab.