What does NonNullable<T> do?

TypeScriptJunior~1 min read

Definition

In the standard library it is implemented like this:

javascript

type NonNullable<T> = T extends null | undefined ? never : T;

That is: if T is null or undefined, the result is never; otherwise T itself is returned.

Usage examples

javascript

type A = NonNullable<string | null | undefined>; // string
type B = NonNullable<number | undefined>;        // number
type C = NonNullable<null | undefined>;          // never
type D = NonNullable<string | null>;             // string

Practical use

Safely accessing values after a check:

javascript

function getLength(value: string | null) {
  if (value) {
    let safe: NonNullable<typeof value> = value; // now definitely a string
    return safe.length;
  }
  return 0;
}

Typing API responses after filtering out null:

javascript

const users = [ "Tom", null, "Bob" ];
const valid = users.filter(Boolean) as NonNullable<typeof users[number]>[];
// valid: string[]

Combined with other utility types:

javascript

type User = { name?: string | null };
type SafeUser = NonNullable<User["name"]>; // string

In short

Utility	Purpose
`NonNullable<T>`	Removes `null` and `undefined`
`Partial<T>`	Makes all fields optional
`Required<T>`	Makes all fields required
`Readonly<T>`	Makes all fields read-only
`Extract<T, U>`	Keeps the intersection of types
`Exclude<T, U>`	Removes the intersection of types

Short Answer

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What does NonNullable<T> do?

#Definition

#Usage examples

#Practical use

#In short

Short Answer

Definition

Usage examples

Practical use

In short