Create mock for a symbol in typescript

Question

Say I have a component:

export class Component {
  constructor(@injectable(IDependency) dep: IDependency) {}

  // other methods/properties do not matter
}

and the dependency it requires is defined as:

export const IDependency = Symbol('IDependency');
export interface IDependency { /* does not matter */ }

Now I want to unit-test the component, but because number of dependencies can be significant (not like 20, but 5 is already significant to me), I do not want to manually create all required mocks. Instead I want an IoC container to automatically mock anything that is not explicitly registered. So far so good?

I got to the point where I receive the symbol (IDependency) and need to create a mock of the interface (IDependency). But it is at all possible to map a symbol to an interface with the same name in typescript? I guess not, or else there would not have been a need for symbols in the first place.

And if not, can any clever head suggest a workaround? Like storing information about an interface somewhere at compile time and its mapping to a symbol, so it can be mocked at runtime? Or, perhaps, using Proxies that are so generic that they do not care what type they are mocking?

Eugene Strizhok · Accepted Answer · 2024-10-23 17:47:53Z

I went with the second approach (which only came to my mind as I was posting the question), and my tests now look like this:

testSuiteFor(TestExplorer, function() {
  this.test('should send init request when ILanguageClient is ready', function() {
    this.mock<IExtensionContext>(IExtensionContext, { subscriptions: [] });

    const client = this.mock<ILanguageClient>(ILanguageClient);
    const readyCallback = captor<Function>();

    this.component();

    expect(client.onReady).toHaveBeenCalledWith(readyCallback);
    expect(client.sendRequest).not.toHaveBeenCalled();

    readyCallback.value();

    expect(client.sendRequest).toHaveBeenCalledWith("ut/explorer/init");
  });

  this.test('should register Run profile', function() {
    this.mock<IExtensionContext>(IExtensionContext, { subscriptions: [] });

    const controller = this.mock<ITestController>(ITestController);
    const explorer = this.component();

    expect(controller.createRunProfile).toHaveBeenCalledWith(
      "Run", 
      vscode.TestRunProfileKind.Run, 
      functionBoundTo(explorer.runTests)
    );
  });
});

Today I also stumbled upon typescript-rtti. I think I will try that next.

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