Mocking

Since Stencil's testing capabilities are built on top of Jest, the mocking features of Jest can be utilized to mock out libraries or certain parts of your code. For further information have a look at the Jest docs.

Mocking a Library

To create a mock for a library that is imported from node_modules, you can simply create a folder __mocks__ on the same directory level as node_modules (usually in your project's root folder), then create a file in there with the same name as the package you want to mock, and that mock will automatically be applied.

For example, if you want to mock md5, you'd create a file __mocks__/md5.ts with the following content:

export default () => 'fakehash';

If you want to mock a scoped package like @capacitor/core, you'll have to create the file as __mocks__/@capacitor/core.ts.

Mocking Your Own Code

To create a mock for some of your own code, you'll have to create the mocks folder on a different layer.

Let's say you have a file src/helpers/utils.ts that exposes a getRandomInt helper, and a service that provides a function which uses this helper.

// src/helpers/utils.ts

export const getRandomInt = (min: number, max: number) =>
  Math.round(Math.random() * (max - min)) + min;

// src/services/foo.ts

import { getRandomInt } from '../helpers/utils';

export const bar = () => getRandomInt(0, 10);

To mock this function, you create a file src/helpers/__mocks__/utils.ts and write your mock in that file.

// src/helpers/__mocks__/utils.ts

export const getRandomInt = () => 42;

Because Jest only auto-mocks node modules, you'll also have to let your test know that you want it to apply that mock, by calling jest.mock().

// src/foo.spec.ts

jest.mock('./helpers/utils');

import { bar } from './services/foo';

describe('Foo', () => {
  it('bar()', () => {
    expect(bar()).toBe(42);
  });
});

It's important that you call jest.mock('...') before your import.

Instead of creating a file in a __mocks__ folder, there is an alternative approach of providing a mock: the jest.mock() function takes a module factory function as an optional second argument. The following test will work the same as the one before, without having to create a src/helpers/__mocks__/utils.ts file.

// src/foo.spec.ts

jest.mock('./helpers/utils', () => ({
    getRandomInt: () => 42,
}));

import { foo } from './services/foo';

describe('Foo', () => {
  it('bar()', () => {
    expect(bar()).toBe(42);
  });
});

Mocking in E2E Tests

If you use newE2EPage in an end-to-end test, your component's code will be executed in a browser context (Stencil will launch a headless Chromium instance using Puppeteer). However your mocks will only be registered in the Node.js context, which means that your component will still call the original implementation. If you need to mock something in the browser context, you can either have a look at using Jest with Puppeteer, or possibly switch to using newSpecPage, which creates a virtual (mocked) DOM in the node context.

// src/components/foo/foo.tsx

import { h, Component, Method } from '@stencil/core';
import { getRandomInt } from '../../helpers/utils';

@Component({ tag: 'foo-component' })
export class Foo {
    @Method()
    async bar() {
        return getRandomInt(0, 10);
    }

    render() {
        return <div />;
    }
}

// src/foo.e2e.ts

jest.mock('./helpers/utils', () => ({
    getRandomInt: () => 42,
}));

import { newSpecPage } from '@stencil/core/testing';
import { Foo } from './components/foo/foo';

describe('Foo', () => {
    it('bar()', async () => {
        const page = await newSpecPage({
            components: [Foo],
            html: '<foo-component></foo-component>',
        });
        const foo = page.body.querySelector('foo-component');

        if (!foo) {
            throw new Error('Could not find Foo component');
        }

        expect(await foo.bar()).toBe(42);
    });
});