IfNotExtends.md

typelab / conditions / IfNotExtends

type IfNotExtends<T1, T2, Then, Else> = T1 extends T2 ? Else : Then;

Resolves to Then if T1 is not assignable to T2, otherwise resolves to Else.

Type Parameters

Type Parameter	Default type	Description
T1	‐	The first type to compare.
T2	‐	The second type to compare.
Then	‐	The type to return if T1 is not assignable to T2.
Else	never	The type to return if T1 is assignable to T2. Defaults to never.

Returns

Then if T1 is not assignable to T2, Else otherwise.

Example

type Yes = IfNotExtends<string, '', 'yes', 'no'>; // 'yes'
type No = IfNotExtends<'', string, 'yes', 'no'>; // 'no'
type YesOrNo = IfNotExtends<string | number, string, 'yes', 'no'>; // 'yes' | 'no'