noPropertyAccessFromIndexSignature

Dotted access means declared

Reserves `obj.key` for properties that were actually declared, and pushes everything from an index signature to `obj["key"]`.

Since: TypeScript 4.2
strict: Not in strict
In your tsconfig: "noPropertyAccessFromIndexSignature": true

The same snippet both times. Only the option changed.

With it off

"noPropertyAccessFromIndexSignature": false

const env: { [key: string]: string } = {}

export const mode = env.NODE_ENV

Compiles clean

With it on

"noPropertyAccessFromIndexSignature": true

const env: { [key: string]: string } = {}

export const mode = env.NODE_ENV

Emits TS4111

Why the compiler bothers

It is a notation rule rather than a soundness one, and it earns its place because of what a reader can then infer: with it on, a dot means somebody wrote that property down, and a bracket means the key is data. Without it, `env.NODE_ENV` and `config.timeout` look identical and only one of them is checked against anything. It pairs naturally with `noUncheckedIndexedAccess`, which supplies the other half — that the value might not be there at all.

Takeaway

Dot for what the type declares, bracket for what the data happens to hold.

Where to go next

22 options