Function optional

optional<T>(predicate: TypePredicateFn<T>): TypePredicateFn<T | undefined>
Create a type predicate function that passes if input is undefined or passes the given predicate.
Type Parameters
- T
Parameters
- predicate: TypePredicateFn<T>
Returns TypePredicateFn<T | undefined>
Example
```
const isOptionalString = optional(isString);
isOptionalString('hello'); // true
isOptionalString(undefined); // true
isOptionalString(null); // false
```
- Defined in src/predicate/factory/optional.ts:15