Documentation: Not possible to restore initial implementation when mocking?

[mikemorris]

💥 Regression Report

Jest documentation for restoreMocks claims to remove fake implementations and restore initial implementations, but the underlying functionality here appears to have changed to no longer remove fake implementations (and doesn't seem to restore pre-mocked functionality even when called manually after jest.resetAllMocks in an afterEach block).

afterEach(() => {
  jest.resetAllMocks();
  jest.restoreAllMocks();
});

resetMocks does remove fake implementations, but does not restore initial implementations.

The differences between these methods are not clearly explained (#5969, #5143, #4828), and it's unclear if it's even possible to restore pre-mocked functionality.

Neither resetModules nor clearMocks seem to have this functionality either.

To Reproduce

jest.mock("fs");

const fs = require("fs");

it("mocks fs.readFileSync", () => {
  fs.readFileSync.mockReturnValue("foo");
  const file = fs.readFileSync("foo.txt", "utf8");
  expect(file).toEqual("foo");
});

it("restores mock and reads file", () => {
  const file = fs.readFileSync("foo.txt", "utf8");
  expect(file).toEqual("bar");
  // fails with `undefined` when run with --resetMocks
  // fails with `foo` when run with --restoreMocks
});

foo.txt

bar

Expected behavior

The Jest documentation accurately describes the behavior of mock reset/restore functionality and what is or is not possible when mocking.

Link to repl or repo (highly encouraged)

https://github.com/mikemorris/jest-restore-mocks

Run npx envinfo --preset jest

  System:
    OS: macOS High Sierra 10.13.6
    CPU: x64 Intel(R) Core(TM) i5-7267U CPU @ 3.10GHz
  Binaries:
    Node: 8.9.3 - ~/.asdf/shims/node
    npm: 6.4.1 - ~/.asdf/shims/npm
  npmPackages:
    jest: ^23.6.0 => 23.6.0

[angeloocana]

This works:

class Bla {
	foo() {
		return true;
	}
}

const bla = new Bla();

describe("Bla", () => {
	it("should return mock value", () => {
		jest.spyOn(bla, "foo").mockImplementationOnce(() => false);
		const result = bla.foo();
		expect(result).toBe(false);
	});
	
	it("should return real value", () => {
		const result = bla.foo();
		expect(result).toBe(true);
	});
});

or

class Bla {
	foo() {
		return true;
	}
}

const bla = new Bla();

describe("Bla", () => {
	afterEach(() => {
		jest.restoreAllMocks();
	});

	it("should return mock value", () => {
		jest.spyOn(bla, "foo").mockImplementation(() => false);
		const result = bla.foo();
		expect(result).toBe(false);
	});
	
	it("should return real value", () => {
		const result = bla.foo();
		expect(result).toBe(true);
	});
});

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