How to mock Task Promise chaining in Jest?

[ReDrUm]

Hi,

I'm familiar with how to mock simple-git using jest.mock which works fine for single method calls. But if an implementation is using chaining via the Task Promises API, then the mocking becomes much more challenging.

For example the docs show you can do await git.init().addRemote('origin', '...remote.git');

Which under the hood is achieved because each API method (such as init and addRemote) internally invokes this._runTask which looks like the below:

protected _runTask<T>(task: SimpleGitTask<T>, then?: SimpleGitTaskCallback<T>) {
      const chain = this._executor.chain();
      const promise = chain.push(task);

      if (then) {
         taskCallback(task, promise, then);
      }

      return Object.create(this, {
         then: { value: promise.then.bind(promise) },
         catch: { value: promise.catch.bind(promise) },
         _executor: { value: chain },
      });
   }

Do you have an example of how one might mock simple-git to support for the same chaining? e.g.

class MockedSimpleGit {
    constructor() { }
    async init() {
        return ...?
    }
    async addRemote() {
        return ...?
    }
}

jest.mock('simple-git', () => ({
    __esModule: true,
    ...jest.requireActual('simple-git'),
    default: jest.fn().mockImplementation(() => new MockedSimpleGit())
}));

Thanks

[steveukx]

The mock would differ depending on how you're calling simple-git, but you could use a combination of chaining and data:

class MockedSimpleGit {
    constructor() { }
    async init() {
        return this
    }
    async addRemote() {
        return { ..mock data.. };
    }
}