Making nullable properties optional in TypeScript
20th May 2022
Sometimes optional properties turn out like nullable in the build system.
Let’s say you have a type like this:
type City = {
name: string;
population: number | null;
mayor: string | null;
dbId: string | null;
};But you would like a type like this:
type City = {
name: string;
population?: number | null;
mayor?: string | null;
dbId?: string | null;
};What we need is an OptionalNullable<T> type. This can be achieved by using a PickNullable and a PickNotNullable utility type and combining them together.
type PickNullable<T> = {
[P in keyof T as null extends T[P] ? P : never]: T[P];
};
type PickNotNullable<T> = {
[P in keyof T as null extends T[P] ? never : P]: T[P];
};
type OptionalNullable<T> = {
[K in keyof PickNullable<T>]?: T[K];
} & {
[K in keyof PickNotNullable<T>]: T[K];
};
// use it:
const city: OptionalNullable<City> = {
name: "Los Angeles",
};Thanks Tobias S for helping me on Stack Overflow.