sevensc / mock-local-storage

Mock lodcalStorage for headless unit tests

mock-local-storage

Mock localStorage for headless unit tests

Inspired by StackOverflow answers and wrapped into npm package.

Motivation

Used to mock localStorage to run headless tests of cache implementation in terminal (ie. without browser).

Installation

npm install mock-local-storage --save-dev

Usage

Mocha

Require in Mocha, which will replace localStorage and sessionStorage on the global and window objects:

mocha --require mock-local-storage

If you are using jsdom-global, make sure it is required before mock-local-storage:

mocha --require jsdom-global --require mock-local-storage

Other testing frameworks

In a node environment you can mock the window.localStorage as follows:

global.window = {}
import localStorage from 'mock-local-storage'
window.localStorage = global.localStorage

This is very useful when you want to run headless tests on code mant for the browser that use localStorage

You can even store this in a file that is reused across tests:

mock-localstorage.js

global.window = {}
import localStorage from 'mock-local-storage'
window.localStorage = global.localStorage

using-localstorage.test.js

import './mock-localstorage'

// unit tests follow here

Caveats

There are some caveats with using index operator. Browser's localStorage works with strings and stringifyes objects stored via localStorage[key] notation, but this implementation does not.

Additionally, test code can provide a callback to be invoked on item insertion. Mock implementation will invoke it when localStorage.setItem() is called (but not with localStorage[key] notation).

It can be used to emulate allocation errors, like this:

describe('test with mock localStorage', () => {
    afterEach(() => {
        localStorage.clear();
		// remove callback
        localStorage.itemInsertionCallback = null;
    });
    it('emulate quota exceeded error', () => {
        localStorage.length.should.equal(0);
		// register callback
        localStorage.itemInsertionCallback = (len) => {
            if (len >= 5) {
                let err = new Error('Mock localStorage quota exceeded');
                err.code = 22;
                throw err;
            }
        };
        let handled = false;
        try {
            for (let i = 0; i < 10; ++i) {
                localStorage.setItem(i, i);
            }
        } catch (e) {
            if (e.code == 22) {
                // handle quota exceeded error
                handled = true;
            }
        }
        handled.should.be.true;
        localStorage.length.should.equal(5);
    });
});

Tests

npm install
npm test

License

MIT