Mocking window.location in Jest

Recently I had to write some tests for a piece of JavaScript code that used window.location. This left me trying to figure out how to mock the window.location so that I could pass in dummy data and ensure that the data I got back was what was expected. Here was how I managed to solve the issue.

I found a few different articles about mocking but some said they only worked in older versions of jest so I had to find a more up to date solution. There were also some packages that give this functionality, but I proffered to try and work it out without a package.

Make a new Object

In the end it turns out the simplest way was to delete it and make it as a new object, thanks to this post on Stack Overflow.

delete global.window.location;
global.window = Object.create(window);
global.window.location = {};

Using typescript in the test helped me see what keys were required in my location object, so I added all of these in and passed in some dummy values that I could use in my tests.

global.window.location = {
    ancestorOrigins: null,
    hash: null,
    host: 'dummy.com',
    port: '80',
    protocol: 'http:',
    hostname: 'dummy.com',
    href: 'http://dummy.com?page=1&name=testing',
    origin: 'http://dummy.com',
    pathname: null,
    search: null,
    assign: null,
    reload: null,
    replace: null,
};

Now I could set some data in the initial mock and know what the initial state should be and what the end result should be.

Before Each

As this was quite a lot of setup code I decided to move this into a beforeEach() method so it will be run before each test.

This also helps ensure each test isn’t affected by any previous tests which could cause anomalous results.

Now, when I want a different initial state in a test, I can just overwrite the part that needs updating for that test instead of overwriting everything.

describe('removeValueFromParam', () => {
    it('removes a specific value from a given param', () => {
        // Overwrite the default href for this test
        window.location.href = 'http://dummy.com/?page=1&name=testing%2Ctest';

        removeValueFromParam('name', 'testing');

        expect(window.location.href).toEqual('http://dummy.com/?page=1&name=test')
    });
});

To string

The tests kept failing on certain tests. I realised it was failing when the window.location was coming back as undefined instead of a string. Somewhere in the code it was calling window.location.toString() but the mock I had created didn’t have this method.

To match this functionality, I then added a toString() method to my mock. I could hard code a response and make a specific test pass.

// Mock window.location
global.window.location = {
    href: 'http://dummy.com?page=1&name=testing',
    /*
    * Other settings
    */
    toString: () => {
        return 'http://dummy.com?page=1&name=testing';
    },
};

The issue now was that different tests needed different responses from the toString() method, so I decided to look up what the standard method does.

The toString() stringifier method of the Location interface returns a string containing the whole URL. It is a read-only version of Location.href.

• https://developer.mozilla.org/en-US/docs/Web/API/Location/toString

Therefore, I just needed to return the location.href to match the default behaviour.

// Mock window.location
global.window.location = {
    href: 'http://dummy.com?page=1&name=testing',
    /*
    * Other settings
    */
    toString: () => {
        return global.window.location.href;
    },
};

Now, the tests were all passing. If your code makes use of other methods in window.location then you should be able to add these to your mock too.

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