Unions

NonNullable

Removes null and undefined from a type.

What it is

Worth reading the definition for. It used to be a distributive conditional; since TypeScript 4.8 it is T & {}, because intersecting with the empty object type removes exactly the two types that cannot have properties. Shorter, faster, and no longer distributive — which matters only in the corners.

Examples

```
NonNullable<string | null>
```
→string
```
NonNullable<string | undefined | null>
```
→string
```
NonNullable<null>
```
→never

Each resolved type above was printed by TypeScript 6.0.3, not written by hand.

What it does not do

It does not touch a nested property. NonNullable<{ a: string | null }> is unchanged — the nullability is inside, and this works on the outside.
It does not narrow a value at run time. A variable typed string | null still needs a check; this only describes the type after one.

Takeaway

The definition changed under everyone in 4.8 and almost nobody noticed, which is the argument for quoting definitions instead of remembering them.