这是 full minimal repro

给定以下应用:

src/food.js

const Food = {
  carbs: "rice",
  veg: "green beans",
  type: "dinner"
};

export default Food;

src/food.js

import Food from "./food";

function formatMeal() {
  const { carbs, veg, type } = Food;

  if (type === "dinner") {
    return `Good evening. Dinner is ${veg} and ${carbs}. Yum!`;
  } else if (type === "breakfast") {
    return `Good morning. Breakfast is ${veg} and ${carbs}. Yum!`;
  } else {
    return "No soup for you!";
  }
}

export default function getMeal() {
  const meal = formatMeal();

  return meal;
}

我有以下测试:

_tests_/meal_test.js

import getMeal from "../src/meal";

describe("meal tests", () => {
  beforeEach(() => {
    jest.resetModules();
  });

  it("should print dinner", () => {
    expect(getMeal()).toBe(
      "Good evening. Dinner is green beans and rice. Yum!"
    );
  });

  it("should print breakfast (mocked)", () => {
    jest.doMock("../src/food", () => ({
      type: "breakfast",
      veg: "avocado",
      carbs: "toast"
    }));

    // prints out the newly mocked food!
    console.log(require("../src/food"));

    // ...but we didn't mock it in time, so this fails!
    expect(getMeal()).toBe("Good morning. Breakfast is avocado and toast. Yum!");
  });
});

如何根据测试正确模拟Food？？换句话说，我只想为 "should print breakfast (mocked)" 测试用例应用模拟。

我也不想理想地更改应用程序源代码(尽管也许 Food 是一个返回对象的函数是可以接受的 - 仍然无法让它工作)

我已经尝试过的事情:

• 通过 getMeal 围绕 Food 对象进行线程化 + 使用依赖注入(inject)到 formatMeal
  • (这种 IRL 方法的重点是我们不想在整个应用程序周围穿插 Food)
• manual mock + jest.mock() - 答案可能在某处，但由于导入时间异常，很难控制此处的值并在每次测试时重置它
  • 在顶部使用 jest.mock() 会覆盖每个测试用例，我不知道如何更改或重置 Food 每个测试。

最佳答案

简答

设置模拟后，使用require 在每个测试函数中抓取一个新模块。

it("should print breakfast (mocked)", () => {
    jest.doMock(...);
    const getMeal = require("../src/meal").default;

    ...
});

或

将 Food 变成一个函数，并将对 jest.mock 的调用放入模块范围。

import getMeal from "../src/meal";
import food from "../src/food";

jest.mock("../src/food");
food.mockReturnValue({ ... });

...

长答案

Jest 中有一个片段 manual上面写着:

Note: In order to mock properly, Jest needs jest.mock('moduleName') to be in the same scope as the require/import statement.

same manual还指出:

If you're using ES module imports then you'll normally be inclined to put your import statements at the top of the test file. But often you need to instruct Jest to use a mock before modules use it. For this reason, Jest will automatically hoist jest.mock calls to the top of the module (before any imports).

ES6 导入在任何测试函数执行之前在模块范围内解析。因此，要应用模拟，需要在测试函数之外和导入任何模块之前声明它们。 Jest 的 Babel 插件会将 jest.mock 语句“提升”到文件的开头，以便在任何导入发生之前执行它们。 注意 jest.doMock 是 deliberately not hoisted .

可以通过查看 Jest 的缓存目录来研究生成的代码(运行 jest --showConfig 以了解位置)。

示例中的 food 模块很难模拟，因为它是对象字面量而不是函数。最简单的方法是每次需要更改值时强制重新加载模块。

选项 1a:不要使用来自测试的 ES6 模块

ES6 import 语句必须是模块作用域的，但是“好旧的”require 没有这样的限制，可以从测试方法的范围内调用。

describe("meal tests", () => {
  beforeEach(() => {
    jest.resetModules();
  });

  it("should print dinner", () => {
    const getMeal = require("../src/meal").default;

    expect(getMeal()).toBe(
      "Good evening. Dinner is green beans and rice. Yum!"
    );
  });

  it("should print breakfast (mocked)", () => {
    jest.doMock("../src/food", () => ({
      type: "breakfast",
      veg: "avocado",
      carbs: "toast"
    }));

    const getMeal = require("../src/meal").default;

    // ...this works now
    expect(getMeal()).toBe("Good morning. Breakfast is avocado and toast. Yum!");
  });
});

选项 1b:在每次调用时重新加载模块

也可以包装被测函数。

代替

import getMeal from "../src/meal";

使用

const getMeal = () => require("../src/meal").default();

选项 2:注册模拟并默认调用真实函数

如果 food 模块暴露了一个函数而不是文字，它可能会被模拟。模拟实例是可变的，可以在测试之间更改。

src/food.js

const Food = {
  carbs: "rice",
  veg: "green beans",
  type: "dinner"
};

export default function() { return Food; }

src/meal.js

import getFood from "./food";

function formatMeal() {
  const { carbs, veg, type } = getFood();

  if (type === "dinner") {
    return `Good evening. Dinner is ${veg} and ${carbs}. Yum!`;
  } else if (type === "breakfast") {
    return `Good morning. Breakfast is ${veg} and ${carbs}. Yum!`;
  } else {
    return "No soup for you!";
  }
}

export default function getMeal() {
  const meal = formatMeal();

  return meal;
}

__tests__/meal_test.js

import getMeal from "../src/meal";
import food from "../src/food";

jest.mock("../src/food");

const realFood = jest.requireActual("../src/food").default;    
food.mockImplementation(realFood);

describe("meal tests", () => {
  beforeEach(() => {
    jest.resetModules();
  });

  it("should print dinner", () => {
    expect(getMeal()).toBe(
      "Good evening. Dinner is green beans and rice. Yum!"
    );
  });

  it("should print breakfast (mocked)", () => {
    food.mockReturnValueOnce({ 
        type: "breakfast",
        veg: "avocado",
        carbs: "toast"
    });

    // ...this works now
    expect(getMeal()).toBe("Good morning. Breakfast is avocado and toast. Yum!");
  });
});

当然还有其他选择，比如将测试分成两个模块，其中一个文件设置模拟，另一个使用真实模块，或者返回一个可变对象来代替 food<> 模块，因此它可以被每个测试修改，然后在 beforeEach 中手动重置。

关于javascript - 我们如何模拟每个测试与 Jest 的依赖关系？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/53075279/