Mock localStorage for headless unit tests

Inspired by StackOverflow answers and wrapped into npm package.

This package has moved from the letsrock-today repository into KaiSforza's GitLab repository.

There is a copy of the code in Github, as well, but issues and pull requests will only be watched in Gitlab.

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

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

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

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

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

This is very useful when you want to run headless tests on code meant 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 'mock-local-storage'
window.localStorage = global.localStorage

using-localstorage.test.js

import './mock-localstorage'

// unit tests follow here

Besides mocking of conventional localStorage interface, this implementation provides a way for test code to register 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);
    });
});

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.

localStorage.itemInsertionCallback won't be invoked with localStorage[key] notation.

npm install
npm test

Please feel free to send us occusionall MRs along with unit tests, we'll merge them if they successfully build and pass unit tests. Consider to always provide unit tests, illustrating your problem, along with PR to avoid future regression.

MIT