笑话回顾：安全模拟全局对象的属性和方法-js教程-PHP中文网

笑话回顾：安全模拟全局对象的属性和方法

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发布： 2024-07-19 15:10:32

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Jest Recap: Safely Mock Properties and Methods of Global Objects

长话短说：

您希望避免覆盖/模拟的属性/方法影响其他测试。
对于本地对象（由该测试创建并拥有），您可以（并且应该）使用
- localObject.theAnswer = 42 和
- localObject.calcTheAnswer = jest.fn(() => 42).
对于全局对象，您应该使用
- jest.replaceProperty(globalObject, "theAnswer", 42) 和
- jest.spyOn(globalObject, "calcTheAnswer").mockReturnValue(42).
确保在 afterEach() 挂钩中调用 jest.restoreAllMocks()。

什么？

在一个完美的世界 代码库中，不需要操作全局对象，但是世界 代码库很混乱 - 所以正在测试。

您要不惜一切代价避免一个测试影响另一个测试。无论测试的顺序如何，或者是否跳过某些测试，测试都应该有意义。

模拟属性

模拟值的一种简单方法是将属性设置为测试中所需的任何值。
只要您更改此特定测试拥有（创建）的本地对象中的值就可以了：

describe("override properties of local objects", () => {
    it("works and is harmless", () => {
        const myArray = [1];
        myArray.length = 0;
        expect(myArray).toHaveLength(0);
    });
    it("does not affect the next test", () => {
        const myArray = [1];
        expect(myArray).toHaveLength(1);
    });
});

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如果你对全局对象这样做，它会变得混乱：

describe("don't override properties of global objects", () => {
    it("works before the property is overridden", () => {
        expect(window.innerWidth).toBeGreaterThan(0);
    });
    it("works, but is evil", () => {
        window.innerWidth = 0;
        expect(window.innerWidth).toBe(0);
    });
    it("fails in the test after the property was overridden", () => {
        expect(() => {
            expect(window.innerWidth).toBeGreaterThan(0); // <-- ERROR: expect(received).toBeGreaterThan(expected)
        }).toThrow(Error);
    });
});

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这就是 jest.replaceProperty() 的用途：

describe("use jest.replaceProperty() to override properties of global objects", () => {
    afterEach(() => {
        jest.restoreAllMocks();
    });
    it("works before the property is overridden", () => {
        expect(window.innerWidth).toBeGreaterThan(0);
    });
    it("works and is harmless", () => {
        jest.replaceProperty(window, "innerWidth", 0);
        expect(window.innerWidth).toBe(0);
    });
    it("does not affect the next test", () => {
        expect(window.innerWidth).toBeGreaterThan(0);
    });
});

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模拟方法

方法可以像属性一样被模拟。

describe("override methods of local objects using jest.fn()", () => {
    it("works and is harmless", () => {
        const mySet = new Set([1]);
        mySet.has = jest.fn().mockReturnValue(false);
        expect(mySet.has(1)).toBeFalsy();
    });
    it("does not affect the next test", () => {
        const mySet = new Set([1]);
        expect(mySet.has(1)).toBeTruthy();
    });
});

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如果您在全局对象上使用 myObject.someFunction = jest.fn()，您的测试可能会相互依赖并失去其意义：

describe("don't override methods of global objects using jest.fn()", () => {
    it("works before the method is overridden", () => {
        expect(document.getElementById("foo")).toBeNull();
    });
    it("works, but is evil", () => {
        const el = document.createElement("div");
        document.getElementById = jest.fn().mockReturnValue(el);

        expect(document.getElementById("foo")).toBe(el);
    });
    it("fails in the test after the property was overridden", () => {
        expect(() => {
            expect(document.getElementById("foo")).toBeNull(); // <-- ERROR: expect(received).toBeNull()
        }).toThrow(Error);
    });
});

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我们应该如何模拟全局对象中的方法？这就是 jest.spyOn() 的优点：

describe("use jest.spyOn() to override methods of global objects", () => {
    afterEach(() => {
        jest.restoreAllMocks();
    });
    it("works before the method is overridden", () => {
        expect(document.getElementById("foo")).toBeNull();
    });
    it("works and is harmless", () => {
        const el = document.createElement("div");
        jest.spyOn(document, "getElementById").mockReturnValue(el);

        expect(document.getElementById("foo")).toBe(el);
    });
    it("does not affect the next test", () => {
        expect(document.getElementById("foo")).toBeNull();
    });
});

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