std/expect: add spyOn function

[kt3k]

https://jestjs.io/docs/jest-object#jestspyonobject-methodname

[gmzacharydovel]

I'll see if I can make a PR on this. Probably next week.

[csvn]

There is already mocks/spying in the @std/testing/mock package? What I wish though is that it would be possible to use these with expect correctly:

import { expect } from "@std/expect";
import { stub } from "@std/testing/mock";

Deno.test(() => {
  using logSpy = stub(console, "log");
  console.log("hello");
  expect(logSpy).toHaveBeenCalledTimes(1);
});

This does not work currently, since getMockCalls() will throw. It's possible to work-around with fn, but it's messy for several reasons:

import { expect } from "@std/expect";
import { stub } from "@std/testing/mock";

Deno.test(() => {
  using logSpy = stub(console, "log", fn(() => {}) as any);
  console.log("hello");
  expect(logSpy.fake).toHaveBeenCalledTimes(1);
});

• The return signature Function is not compatible with stub types, it should likely use a generic
• stub can't be wrapped, because then the disposal won't work, so we need to pass a bogus method to fn
• We must get the function returned by fn, so .fake has to be used on the stub spy

It would be great if @std/testing/mock worked out of the box, or if there at least was simpler way to integrate them. I think it may be a mistake to duplicate similar code in two different @std packages.

[gmzacharydovel]

Hey @csvn I think that is a given. Unfortunately, whenever I think I have some free time I get drowned in some client work. I would implement this such that it fits into the current architecture.

[csvn]

My comment was mostly because the issue title was not clear whether this was about adding something new or re-exporting/wrapping @std/testing/mock. And I was not commenting at all about who was to work on this. Just writing how I hope it could be simplified 😃

[kt3k]

@std/expect generally tries to be compatible with jest APIs (and we currently don't have plan to make @std/expect and @std/testing/mock compatible).

So the idea of this issue is to implement something like:

import { expect } from "@std/expect";
import { spyOn } from "@std/expect/unstable-spy-on";

const spy = spyOn(console, "log");

console.log("hello");

expect(spy).toHaveBeenCalledWith("hello");