Proposal: SetNonNullable

[JohannCooper]

Overview

Similar in syntax to the SetOptional and SetRequired types, SetNonNullable creates a type that makes the given keys non-nullable.

Example

type Foo = {
  a: number;
  b: string | undefined;
  c?: boolean | null;
}

type SomeNonNullable = SetNonNullable<Foo, 'b' | 'c'>;
// type SomeNonNullable = {
//   a: number;
//   b: string; // Can no longer be undefined.
//   c?: boolean; // Can no longer be null, but is still optional.
// }

Use Case

Most use cases that apply to the native NonNullable type apply here as well. The obvious difference is that SetNonNullable applies to types with keys. I have found this useful when storing form data within an object. After the form data has been validated to contain all required fields, it is useful to have a type to represent a valid, completed form.

For example,

type EmptyForm = {
  name: string | null;
  age: number | undefined;
};

type CompletedForm = SetNonNullable<EmptyForm, 'name' | 'age'>;

I personally try not to use null. However many forms are fetched from external sources where I cannot control the form structure. I often see a mix of undefined and null used to represented empty fields.