feat(types): IsNull, IfNull

Signed-off-by: Lexus Drumgold <[email protected]>

src/types/__tests__/if-null.spec-d.ts

@@ -0,0 +1,19 @@
+/**
+ * @file Type Tests - IfNull
+ * @module tutils/types/tests/unit-d/IfNull
+ */
+
+import type TestSubject from '../if-null'
+
+describe('unit-d:types/IfNull', () => {
+  type False = false
+  type True = true
+
+  it('should equal False if IsNull<T> extends false', () => {
+    expectTypeOf<TestSubject<string[], True, False>>().toEqualTypeOf<False>()
+  })
+
+  it('should equal True if IsNull<T> extends true', () => {
+    expectTypeOf<TestSubject<null, True, False>>().toEqualTypeOf<True>()
+  })
+})

src/types/__tests__/is-null.spec-d.ts

@@ -0,0 +1,16 @@
+/**
+ * @file Type Tests - IsNull
+ * @module tutils/types/tests/unit-d/IsNull
+ */
+
+import type TestSubject from '../is-null'
+
+describe('unit-d:types/IsNull', () => {
+  it('should equal false if T does not extend null', () => {
+    expectTypeOf<TestSubject<boolean>>().toEqualTypeOf<false>()
+  })
+
+  it('should equal true if T extends null', () => {
+    expectTypeOf<TestSubject<null>>().toEqualTypeOf<true>()
+  })
+})

src/types/if-null.ts

@@ -0,0 +1,20 @@
+/**
+ * @file Type Definitions - IfNull
+ * @module tutils/types/IfNull
+ */
+
+import type IsNull from './is-null'
+
+/**
+ * Conditional type that resolves depending on whether or not `T` extends
+ * `null`.
+ *
+ * @see {@linkcode IsNull}
+ *
+ * @template T - Type to evaluate
+ * @template True - Type if `T` extends `null`
+ * @template False - Type if `T` does not extend `null`
+ */
+type IfNull<T, True, False> = IsNull<T> extends true ? True : False
+
+export type { IfNull as default }

src/types/index.ts

@@ -30,6 +30,7 @@ export type { default as IfFunction } from './if-function'
 export type { default as IfJsonPrimitive } from './if-json-primitive'
 export type { default as IfNever } from './if-never'
 export type { default as IfNil } from './if-nil'
+export type { default as IfNull } from './if-null'
 export type { default as IndexSignature } from './index-signature'
 export type { default as IsAny } from './is-any'
 export type { default as IsArray } from './is-array'
@@ -39,6 +40,7 @@ export type { default as IsFunction } from './is-function'
 export type { default as IsJsonPrimitive } from './is-json-primitive'
 export type { default as IsNever } from './is-never'
 export type { default as IsNil } from './is-nil'
+export type { default as IsNull } from './is-null'
 export type { default as IsTuple } from './is-tuple'
 export type { default as Join } from './join'
 export type { default as JsonArray } from './json-array'

src/types/is-null.ts

@@ -0,0 +1,13 @@
+/**
+ * @file Type Definitions - IsNull
+ * @module tutils/types/IsNull
+ */
+
+/**
+ * Returns a boolean indicating if `T` extends `null`.
+ *
+ * @template T - Type to evaluate
+ */
+type IsNull<T> = T extends null ? true : false
+
+export type { IsNull as default }