Add is.optional and assert.optional

Fixes #111

readme.md

@@ -587,6 +587,21 @@ is.all(is.string, '🦄', [], 'unicorns');
 //=> false
 ```
 
+##### .optional(value, predicate)
+
+Returns `true` if `value` is `undefined` or satisfies the given `predicate`.
+
+```js
+is.optional(undefined, is.string);
+//=> true
+
+is.optional('🦄', is.string);
+//=> true
+
+is.optional(123, is.string);
+//=> false
+```
+
 ##### .validDate(value)
 
 Returns `true` if the value is a valid date.
@@ -682,6 +697,23 @@ handleMovieRatingApiResponse({rating: 0.87, title: 'The Matrix'});
 handleMovieRatingApiResponse({rating: '🦄'});
 ```
 
+### Optional assertion
+
+Asserts that `value` is `undefined` or satisfies the provided `assertion`.
+
+```ts
+import {assert} from '@sindresorhus/is';
+
+assert.optional(undefined, assert.string);
+// Passes without throwing
+
+assert.optional('🦄', assert.string);
+// Passes without throwing
+
+assert.optional(123, assert.string);
+// Throws: Expected value which is `string`, received value of type `number`
+```
+
 ## Generic type parameters
 
 The type guards and type assertions are aware of [generic type parameters](https://www.typescriptlang.org/docs/handbook/generics.html), such as `Promise<T>` and `Map<Key, Value>`. The default is `unknown` for most cases, since `is` cannot check them at runtime. If the generic type is known at compile-time, either implicitly (inferred) or explicitly (provided), `is` propagates the type so it can be used later.