noUncheckedIndexedAccess

An index signature is a guess

Reading a key that was never written gives `undefined` at runtime. This is the flag that says so in the type.

Since: TypeScript 4.1
strict: Not in strict
In your tsconfig: "noUncheckedIndexedAccess": true
Compiled with: "strict": true

The same snippet both times. Only the option changed.

With it off

"noUncheckedIndexedAccess": false

const rows: Record<string, string> = {}

export const first: string = rows.a

Compiles clean

With it on

"noUncheckedIndexedAccess": true

const rows: Record<string, string> = {}

export const first: string = rows.a

Emits TS2322

Why the compiler bothers

The most surprising thing about `strict` is that it does not include this. `Record<string, string>` claims every one of the infinitely many string keys holds a string, which is never true of any real object, and the same hole is in every array access — `items[5]` on a three-element array is `undefined` and typed as the element. It is not under `strict` because turning it on rewrites every loop body and every lookup in a codebase, and that was judged too much to impose by default. It is still the right setting.

Takeaway

Turn it on early in a project or never. Retrofitting it is the expensive one.

Where to go next

22 options