What is `NonNullable<T>` utility type?

NonNullable<T> removes null and undefined from a type:

type MaybeString = string | null | undefined;
type DefinitelyString = NonNullable<MaybeString>; // string

function processValue(value: string | null | undefined): void {
  if (value !== null && value !== undefined) {
    // Type narrowing
    const processed: NonNullable<typeof value> = value; // string
    console.log(processed.toUpperCase());
  }
}

// Useful with arrays
type ArrayItem<T> = T extends (infer U)[] ? NonNullable<U> : never;
type StringArrayItem = ArrayItem<(string | null)[]>; // string