Testing function context ("this") in Mock functions

[matthias-ccri]

🚀 Feature Proposal

Currently Jest doesn't have a strong story for how to test the context of mock function calls.

Motivation

It's well-documented how to test other aspects of calls (arguments and return values), but not the context.

Note: the "context" is the "this" binding which can be set with Function.prototype.call or bind:

const ctx = {};
mockFn.call(ctx, arg1, arg2);

It is currently possible to test the context with mock.instances, but this doesn't seem to be the intended purpose. According to the docs mock.instances seems class-related:

An array that contains all the object instances that have been instantiated from this mock function using new.

In fact, there is an open issue (#7070) discussing how to "fix" instances to only be the return value of new calls. If this happens, there will be no way to test the context of regular calls.

In any case, it would be nice to see context get official support and documentation. Even better if it could get a dedicated expectation clause like toHaveBeenCalledWithContext.

Example

Ideas:

const spy = jest.fn();
const context = {};
spy.call(context);

// a new `toHaveBeenCalledWithContext` expectation
expect(spy).toHaveBeenCalledWithContext(context);
// or a new `mock.contexts` property similar to `mock.instances`
expect(spy.mock.contexts[0]).toBe(context);

Pitch

Context is core to functions. While it may be a lesser-used pattern, it has been around for a long time, in libraries like jquery and lodash. In my case, I want to write a comprehensive test for an EventEmitter, which allows a user to pass a context, which gets applied to the event listener. Personally I prefer not to use context, but the feature exists. I think Jest should be able to support this kind of comprehensive testing. And while it is possible currently, it would be nice to see it more officially supported.

Thanks

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[SimenB]

I don't think we should add matchers for this. but adding it as a context like your seconds suggestion sounds good to me. Wanna send a PR? 🙂

[matthias-ccri]

#12601

[SimenB]

https://github.com/facebook/jest/releases/tag/v28.0.0-alpha.8

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