Tara Timmerman

Posted on May 12

Mocking localStorage in Jest (w/o breaking your brain 😂)

#webdev #testing #jest #typescript

This guide is for a TypeScript setup. If you're using plain JavaScript, you'll still find it helpful, just tweak the type-y bits!

⚡ TL;DR (Copy-Paste Ready)

Install dev deps

npm install --save-dev jest ts-jest @types/jest jest-localstorage-mock

Update jest.config.js

// jest.config.js
module.exports = {
  preset: "ts-jest",
  testEnvironment: "jsdom",
  setupFilesAfterEnv: ["jest-localstorage-mock"],
  // …other config…
};

Write tests

describe("localStorage mock", () => {
  beforeEach(() => localStorage.clear());

  it("stores and retrieves a value", () => {
    localStorage.setItem("key", "value");
    expect(localStorage.getItem("key")).toBe("value");
  });
  // …more tests…
});

Ever had a test accidentally wipe out your real browser data? Or tests fail intermittently because of leftover localStorage? I have, and all I can say is:

"No, thank you." - Me

But fear not! Mocking localStorage in Jest is the solution. It keeps your tests clean, safe, and gloriously isolated.

🎯 Why Bother Mocking?

Mocking localStorage with jest-localstorage-mock gives you:

🔁 A fresh, clean slate for every test.
🧼 Zero pollution across test cases.
🧪 Predictable behavior, no sneaky real browser interactions.
⚡ Faster, more reliable test runs.

🧰 What You’ll Need

Assuming you're working in a TypeScript project, install these dev dependencies to get started:

npm install --save-dev jest ts-jest @types/jest jest-localstorage-mock

This will install:

jest: The testing engine.
ts-jest: So Jest can understand TypeScript.
@types/jest: For autocomplete and type checking.
jest-localstorage-mock: The localStorage mock you didn’t know you needed.

🛠️ Setting Up Jest for localStorage Mocking

Create or update your jest.config.js at the root of your project:

// jest.config.js

/** @type {import('ts-jest').JestConfigWithTsJest} */
module.exports = {
  preset: "ts-jest",
  testEnvironment: "jsdom",
  transform: {
    "^.+\\.tsx?$": "ts-jest",
  },
  testMatch: ["**/?(*.)+(spec|test).[jt]s?(x)"],
  setupFilesAfterEnv: ["jest-localstorage-mock"], // Mocks localStorage before each test
};

With this setup, Jest will:

Run TypeScript using ts-jest,
Mock localStorage using jest-localstorage-mock,
Fake a browser with jsdom because localStorage is a browser API so we need a pretend browser to make it work.

🧪 Example Tests Using localStorage

With localStorage mocked, you can now write tests that interact with it just like you would in your app’s actual code.

describe("localStorage mock", () => {
  beforeEach(() => {
    localStorage.clear();
  });

  test("should store and retrieve a simple value", () => {
    localStorage.setItem("theme", "dark");
    expect(localStorage.getItem("theme")).toBe("dark");
  });

  test("should handle JSON data", () => {
    const settings = { notifications: true };
    localStorage.setItem("app-settings", JSON.stringify(settings));

    const saved = JSON.parse(localStorage.getItem("app-settings") || "{}");
    expect(saved.notifications).toBe(true);
  });

  test("should remove an item", () => {
    localStorage.setItem("activeTab", "dashboard");
    localStorage.removeItem("activeTab");

    expect(localStorage.getItem("activeTab")).toBe(null);
  });

  test("should clear all items", () => {
    localStorage.setItem("a", "1");
    localStorage.setItem("b", "2");
    localStorage.clear();

    expect(localStorage.getItem("a")).toBe(null);
    expect(localStorage.getItem("b")).toBe(null);
  });
});

💡 Quick Reality Check: Don’t Store Sensitive Stuff

This part’s serious: don’t store sensitive data like tokens, passwords, or anything personal in localStorage, especially outside tests.

It’s readable via browser dev tools and wide open to XSS attacks. Your tests won’t care, but real users will.

🙌 That’s It. Go Forth and Test!

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