Mocking globals in Jest
<p>Is there any way in Jest to mock global objects, such as <code>navigator</code>, or <code>Image</code>*? I've pretty much given up on this, and left it up to a series of mockable utility methods. For example:</p>

<pre><code>// Utils.js
export isOnline() {
    return navigator.onLine;
}
</code></pre>

<p>Testing this tiny function is simple, but crufty and not deterministic at all. I can get 75% of the way there, but this is about as far as I can go:</p>

<pre><code>// Utils.test.js
it('knows if it is online', () =&gt; {
    const { isOnline } = require('path/to/Utils');

    expect(() =&gt; isOnline()).not.toThrow();
    expect(typeof isOnline()).toBe('boolean');
});
</code></pre>

<p>On the other hand, if I am okay with this indirection, I can now access <code>navigator</code> via these utilities:</p>

<pre><code>// Foo.js
import { isOnline } from './Utils';

export default class Foo {
    doSomethingOnline() {
        if (!isOnline()) throw new Error('Not online');

        /* More implementation */            
    }
}
</code></pre>

<p>...and deterministically test like this...</p>

<pre><code>// Foo.test.js
it('throws when offline', () =&gt; {
    const Utils = require('../services/Utils');
    Utils.isOnline = jest.fn(() =&gt; isOnline);

    const Foo = require('../path/to/Foo').default;
    let foo = new Foo();

    // User is offline -- should fail
    let isOnline = false;
    expect(() =&gt; foo.doSomethingOnline()).toThrow();

    // User is online -- should be okay
    isOnline = true;
    expect(() =&gt; foo.doSomethingOnline()).not.toThrow();
});
</code></pre>

<p>Out of all the testing frameworks I've used, Jest feels like the most complete solution, but any time I write awkward code just to make it testable, I feel like my testing tools are letting me down.</p>

<p>Is this the only solution or do I need to add Rewire?</p>

<p>*Don't smirk. <code>Image</code> is fantastic for pinging a remote network resource.</p>