add tests

__tests__/base.js

@@ -1,5 +1,5 @@
 "use strict"
-import produce, {setAutoFreeze, setUseProxies, nothing} from "../src/immer"
+import produce, {setAutoFreeze, setUseProxies, nothing, isDraft} from "../src/immer"
 import deepFreeze from "deep-freeze"
 import cloneDeep from "lodash.clonedeep"
 import * as lodash from "lodash"
@@ -1311,6 +1311,40 @@ function runBaseTest(name, useProxies, freeze, useListener) {
             return freeze ? deepFreeze(data) : data
         }
     })
+
+    describe(`isDraft - ${name}`, () => {
+        beforeAll(() => {
+            setAutoFreeze(freeze)
+            setUseProxies(useProxies)
+        })
+
+        it('returns true for object drafts', () => {
+            produce({}, state => {
+                expect(isDraft(state)).toBeTruthy()
+            })
+        })
+        it('returns true for array drafts', () => {
+            produce([], state => {
+                expect(isDraft(state)).toBeTruthy()
+            })
+        })
+        it('returns true for objects nested in object drafts', () => {
+            produce({ a: { b: {} } }, state => {
+                expect(isDraft(state.a)).toBeTruthy()
+                expect(isDraft(state.a.b)).toBeTruthy()
+            })
+        })
+        it('returns false for new objects added to a draft', () => {
+            produce({}, state => {
+                state.a = {}
+                expect(isDraft(state.a)).toBeFalsy()
+            })
+        })
+        it('returns false for a returned draft', () => {
+            const result = produce({}, state => state)
+            expect(isDraft(result)).toBeFalsy()
+        })
+    })
 }
 
 function enumerableOnly(x) {