exactOptionalPropertyTypes

Missing is not the same as undefined

`{ retries?: number }` normally also permits `{ retries: undefined }`. This flag makes the question mark mean only "may be absent".

Since: TypeScript 4.4
strict: Not in strict
In your tsconfig: "exactOptionalPropertyTypes": true
Compiled with: "strict": true

The same snippet both times. Only the option changed.

With it off

"exactOptionalPropertyTypes": false

type Options = { retries?: number }

export const options: Options = { retries: undefined }

Compiles clean

With it on

"exactOptionalPropertyTypes": true

type Options = { retries?: number }

export const options: Options = { retries: undefined }

Emits TS2375

Why the compiler bothers

The two are different at runtime and the difference is load-bearing: `"retries" in options` is false for one and true for the other, `Object.keys` lists one and not the other, and a merge that spreads defaults will have the explicit `undefined` overwrite the default while the absent key does not. Without this flag the type system cannot tell them apart, so the one API shape where it matters most — an options object with defaults — is exactly the one it cannot describe.

Takeaway

Write `retries?: number | undefined` when you really do accept both.

Where to go next

22 options