object: add defaultTo

fixes #7

src/object.test.ts

@@ -1,7 +1,7 @@
 import { expect } from 'chai'
 
 import { FefeError } from './errors'
-import { object, optional, ObjectOptions } from './object'
+import { object, defaultTo, optional, ObjectOptions } from './object'
 import { string } from './string'
 
 describe('object()', () => {
@@ -58,6 +58,14 @@ describe('object()', () => {
   })
 })
 
+describe('defaultTo()', () => {
+  it('should return an object options object with default value/function', () => {
+    const validator = string()
+    const options: ObjectOptions<string> = defaultTo(validator, 'foo')
+    expect(options).to.eql({ validator, default: 'foo' })
+  })
+})
+
 describe('optional()', () => {
   it('should return an optional object options object', () => {
     const validator = string()

src/object.ts

@@ -68,6 +68,13 @@ export function object<D extends ObjectDefinition> (
   }
 }
 
+export function defaultTo<R> (validator: Validator<R>, _default: R | (() => R)): ObjectOptions<R> {
+  return {
+    validator,
+    default: _default
+  }
+}
+
 export function optional<R> (validator: Validator<R>): ObjectOptions<R> {
   return {
     validator,