[typescript] possibly undefined mutators (on render) are difficult to use (#129)

* [typescript] show mutator example - undefined mutators are difficult

* Fixed and tested with final-form/final-form#68

Author: rosskevin

src/ReactFinalForm.d.test.tsx

@@ -1,5 +1,6 @@
 import * as React from 'react'
 import { Form, Field } from './index'
+import { Mutator } from 'final-form/dist'
 
 const onSubmit = async (values: any) => {
   console.log(values)
@@ -63,3 +64,36 @@ const simpleSubscription = () => (
     )}
   </Form>
 )
+
+const setValue: Mutator = ([name, newValue], state, { changeValue }) => {
+  changeValue(state, name, value => newValue)
+}
+
+const mutated = () => (
+  <Form onSubmit={onSubmit} mutators={{ setValue }}>
+    {({
+      handleSubmit,
+      mutators: { setValue },
+      submitting,
+      pristine,
+      values
+    }) => (
+      <form onSubmit={handleSubmit}>
+        <Field
+          name="firstName"
+          component="input"
+          type="text"
+          placeholder="First Name"
+        />
+        <button
+          type="button"
+          onClick={e => setValue('firstName', 'Kevin')}
+          disabled={submitting || pristine}
+        >
+          Reset
+        </button>
+        <pre>{JSON.stringify(values)}</pre>
+      </form>
+    )}
+  </Form>
+)

src/index.d.ts

@@ -44,7 +44,7 @@ export interface SubsetFormApi {
   change: (name: string, value: any) => void
   focus: (name: string) => void
   initialize: (values: object) => void
-  mutators?: { [key: string]: Function }
+  mutators: { [key: string]: Function }
   reset: () => void
 }