feat: introduce asNullable

sanitizers/src/asNullable.ts

@@ -0,0 +1,8 @@
+import { Sanitizer, Result } from ".";
+
+export const asNullable = <T>(sanitizer: Sanitizer<T>) : Sanitizer<T | null> =>
+  (value, path) => value === null
+    ? Result.ok(null)
+      : value === undefined 
+      ? Result.error([{ path: 'path', expected: 'not undefined' }])
+        : sanitizer(value, path)

sanitizers/src/index.ts

@@ -8,6 +8,7 @@ export { asFlatMapped } from './asFlatMapped'
 export { asInteger } from './asInteger'
 export { asMapped } from './asMapped'
 export { asMatching } from './asMatching'
+export { asNullable } from './asNullable'
 export { asNumber } from './asNumber'
 export { asObject } from './asObject'
 export { asOptional } from './asOptional'

sanitizers/test/asNullable.test.ts

@@ -0,0 +1,33 @@
+import { expect } from 'chai'
+import { asString, asNullable, Result } from '../src';
+
+
+describe('asNullable', () => {
+    it('sanitizes using the nested sanitizer', async () => {
+        const asNullableString = asNullable(asString)
+        const result = asNullableString('abc', '')
+        expect(result).to.deep.equal(Result.ok('abc'))
+      })
+
+      it('returns nested sanitizer errors', async () => {
+        const asNullableString = asNullable(asString)
+        const result = asNullableString(false, 'path')
+        expect(result).to.deep.equal(
+          Result.error([{ path: 'path', expected: 'string' }])
+        )
+      })
+
+      it('sanitizes undefined', async () => {
+        const asNullableString = asNullable(asString)
+        const result = asNullableString(undefined, 'path')
+        expect(result).to.deep.equal(
+            Result.error([{ path: 'path', expected: 'not undefined' }])
+          )
+      })
+
+      it('sanitizes null', async () => {
+        const asNullableString = asNullable(asString)
+        const result = asNullableString(null, 'path')
+        expect(result).to.deep.equal(Result.ok(null))
+      })
+});