Instance matcher and mocks confusion

Hi, we just tried to move to your library from chai. But we were unable to find a matcher for instances. Is there any such matcher yet? How would I write the following chai expect in earl?

const service = new Service();
chai.expect(service).to.be.instanceOf(Service);

Same applies for the property matcher. Sometimes we want to check if an object has a property.

chai.expect(service).to.have.property("prop");

Of course we can circumvent both with equal like

const service = new Service();
chai.expect(service instanceof Service).toEqual(true);
chai.expect(Object.prototype.hasOwnProperty.call(service, "prop")).toEqual(true);

but the method chai offers seems a little bit easier in my opinion.

Also a Regex matcher would be very nice to have. Currently I have to do something like this:

expect(new RegExp(/Error while starting the server.*/).test(msg)).toEqual(true);

which works but the error message is very unspecific being only Error: false not equal to true.

[Kampfmoehre]

No problem. Your library is awesome so far. Testing with typescript always felt a little bit strange. Mocha is difficult when you have multiple different test suites that need different setups (we split our tests in unit, api and integration) and jest always feels way to slow with typescript. We also checked out alsatian but it wont work with the many reporters we need.

The biggest obstacle I had with Typescript are mocks. They almost never are typed and/or easy to use at the same time and I am far from understanding how to type them properly with sinon or jest. The best thing that I have found so far is typemoq which is inspired by C# and thus I am familiar with it. But in the TS/JS world it seems a little bit too much code for mocking. But it creates correctly typed mocks to use and I don’t have to mock everything but only the parts I need.

As far as I have tested your library, I am unsure if and how we use your mocking functionality. In the example above I would need to provide a logger. Most of our code is written with dependency injection so it should be easy to just provide fake instances in tests, right? But often Typescript is making things hard here.

This is how many of our services are written:

export class Service {
  private readonly logger: LoggerBase;

  constructor(loggerFactory: (name: string|Function) => LoggerBase) {
    this.logger = loggerFactory(this.constructor);
}

Now LoggerBase is defined as an interface that has logger function like info, warn, error and so on which each take an object, a string and template params (for the string). Now since it doesn’t create it’s own logger, it should be easy to just provide a fake one when instantiating from our test, shouldn’t it? Well since all those methods are defined in LoggerBase, typescript wants to have them all, when we create a fake logger:

new Service(() => ({info: ()=> {}}));
// Typescript throws an error like `Type '{ info: () => void; }' is missing the following properties from type 'LoggerBase': trace, debug, warn, error, fatal`

But I only need info for my test. Nearly every mocking framework I tried so far does not handle this very well. I always have to work with casts and loose types or have to silence eslint and typescript. Since most of them are written for JS I guess they don’t care much.

What I am really looking for is something like typemoq provides:

const mock = Mock.of(LoggerBase); // Difficult because LoggerBase is only an interface here so it would be an empty object at runtime
mock.setup("info").withArgs("arg1", AnyString(), ...).return("Whatever"); // ... makes no sense for logger to return something but you'll get the point)

new Service(() => mock.object) // this should have the correct type so ts won't complain and no casts are needed

expect(mock.info).toHaveBeenCalledWith("arg1", "arg2", "...");
// or
mock.verify(); // Throws if not called with the correct arguments

[krzkaczor]

Mocks were redesigned now. Head to https://earljs.dev/docs/guides/mocks for the new docs.