Use Jest as test framework

[codeandcats]

Another enhancement request: I was wondering how you'd feel about replacing Mocha with Jest?

Motivation

• Chai's API can be a little inconsistent with whether you should use brackets or not after assertions. Jest's API is straight-forward, no having to guess the correct combination of nested property names, and every assertion is a function, not a property getter, so you always use brackets. e.g. expect(thing).toBeTruthy().

• Support for spying on mock functions comes out of box. No need to use a separate library like Sinon. Naive example:

    describe('AdvancedMath', () => {
      it('should square numbers', () => {
        const basicMath: Partial<BasicMath> = {
          multiply: jest.fn().mockReturnValue(3);
        };
  
        const advancedMath = new AdvancedMath(basicMath);
        const result = advancedMath.square(5);
        expect(result).toBe(25);
        expect(basicMath.multiply).toHaveBeenCalledOnce();
        expect(basicMath.multiply).toHaveBeenCalledWith(5, 5);
      });
    });

• Support for mocking modules out of the box. No need to use a separate library like mockery. Unlike mockery you don't need to ensure your mock is registered before the library is required in the code that you're testing.

• Comes with Instanbul support built-in, allowing you to easily track code-coverage and even fail when minimum thresholds for functions, statements, branches and lines are not met.

• One test library to rule them all! 💍

Indirectly related: Also wanted to ask how you'd feel about putting the tests in the same directory as the files being tested.

This has the following advantages:

• Makes it clear that foo.ts is not tested when it doesn't have an accompanying foo.test.ts file right next to it.

• No need to maintain a mirrored directory structure of your tests.

[laurence-myers]

Sure, feel free to raise a PR. 👍

(I agree re: Chai's BDD API, but they also offer a "TDD" API which uses classic single-function assertions.)

I'd prefer to keep the tests in a separate directory for now, it makes it easier to exclude test files from the TSC config and published artifacts. The code coverage report could serve to tell us if a file is tested.

[codeandcats]

Okay, cool. I might convert it and shoot through a PR when I have some time.

No worries at test location, though I will say...

1. Don't forget that tsconfig has an exclude glob option.

2. Also, the ts-jest plugin for jest runs directly against the .ts files without generating .js files so test output files never pollute your artefacts directory.

Imho, it's super nice when creating a component to have a folder for the component, with code, the view, styles and tests all in same location. Much less jumping around the project to find related files.

[laurence-myers]

This has been released in v.0.0.13