Document mock error

README.md

@@ -177,5 +177,50 @@ test('should not be impacted by the previous test', () => {
 });
 ```
 
+If needing to change the mock implementation of `localStorage` methods, please use jest's
+[mockImplementationOnce](https://jestjs.io/docs/en/mock-function-api#mockfnmockimplementationoncefn).
+Using other methods may result in the current method mock being permanently overwritten.
+
+```js
+// does NOT work
+test('should override the setItem mock and reset it to its previous implementation', () => {
+  const KEY = 'foo',
+    VALUE = 'bar';
+
+  localStorage.setItem.mockImplementation(() => {
+    throw 'Something went wrong!'
+  })
+  expect(() => {
+    localStorage.setItem(KEY, VALUE)
+  }).toThrow('Something went wrong!')
+
+  localStorage.setItem.mockClear();
+  expect(() => {
+    localStorage.setItem(KEY, VALUE)
+  }).not.toThrow('Something went wrong!')
+});
+
+// does work
+test('should override the setItem mock and reset it to its previous implementation', () => {
+  const KEY = 'foo',
+    VALUE = 'bar';
+
+  localStorage.setItem.mockImplementationOnce(() => {
+    throw 'Something went wrong!'
+  })
+  expect(() => {
+    localStorage.setItem(KEY, VALUE)
+  }).toThrow('Something went wrong!')
+
+  localStorage.setItem.mockClear();
+  expect(() => {
+    localStorage.setItem(KEY, VALUE)
+  }).not.toThrow('Something went wrong!')
+
+  expect(localStorage.getItem(KEY)).toEqual(VALUE)
+});
+
+```
+
 See the [contributing guide](./CONTRIBUTING.md) for details on how you can
 contribute.

__tests__/index.js

@@ -1,5 +1,5 @@
 describe('storage', () =>
-  [localStorage, sessionStorage].map(storage => {
+  [localStorage, sessionStorage].map((storage) => {
     // https://html.spec.whatwg.org/multipage/webstorage.html#storage
     beforeEach(() => {
       storage.clear();

__tests__/setup.test.js

@@ -1,37 +1,35 @@
-import { LocalStorage } from '../src/localstorage';
-
 describe('setup', () => {
-  const orignalImpGlobsl = {};
+  const orignalImpGlobals = {};
 
-  const setupGloabls = (restore = false) => {
+  const setupGlobals = (restore = false) => {
     [
       '_localStorage',
       'localStorage',
       '_sessionStorage',
       'sessionStorage',
-    ].forEach(globalKey => {
+    ].forEach((globalKey) => {
       if (restore) {
         delete global[globalKey];
-        global[globalKey] = orignalImpGlobsl[globalKey];
+        global[globalKey] = orignalImpGlobals[globalKey];
       } else {
-        orignalImpGlobsl[globalKey] = global[globalKey];
+        orignalImpGlobals[globalKey] = global[globalKey];
         delete global[globalKey];
       }
     });
   };
 
-  const restoreGlobals = () => setupGloabls(true);
+  const restoreGlobals = () => setupGlobals(true);
 
   beforeEach(() => {
-    setupGloabls();
+    setupGlobals();
     jest.resetModuleRegistry();
   });
 
   afterEach(() => {
     restoreGlobals();
   });
 
-  ['_localStorage', '_sessionStorage'].forEach(gKey => {
+  ['_localStorage', '_sessionStorage'].forEach((gKey) => {
     it(`[${gKey}] should define a property on the global object with writable false`, () => {
       require('../src/setup');
       expect(global[gKey.replace('_', '')].constructor.name).toBe(

src/localstorage.js

@@ -2,7 +2,7 @@ export class LocalStorage {
   constructor(jest) {
     Object.defineProperty(this, 'getItem', {
       enumerable: false,
-      value: jest.fn(key => this[key] || null),
+      value: jest.fn((key) => this[key] || null),
     });
     Object.defineProperty(this, 'setItem', {
       enumerable: false,
@@ -13,14 +13,14 @@ export class LocalStorage {
     });
     Object.defineProperty(this, 'removeItem', {
       enumerable: false,
-      value: jest.fn(key => {
+      value: jest.fn((key) => {
         delete this[key];
       }),
     });
     Object.defineProperty(this, 'clear', {
       enumerable: false,
       value: jest.fn(() => {
-        Object.keys(this).map(key => delete this[key]);
+        Object.keys(this).map((key) => delete this[key]);
       }),
     });
     Object.defineProperty(this, 'toString', {
@@ -31,7 +31,7 @@ export class LocalStorage {
     });
     Object.defineProperty(this, 'key', {
       enumerable: false,
-      value: jest.fn(idx => Object.keys(this)[idx] || null),
+      value: jest.fn((idx) => Object.keys(this)[idx] || null),
     });
   } // end constructor