笑话回顾：安全模拟全局对象的属性和方法

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);    });});

describe("don't override properties of global objects", () =&gt; {    it("works before the property is overridden", () =&gt; {        expect(window.innerwidth).tobegreaterthan(0);    });    it("works, but is evil", () =&gt; {        window.innerwidth = 0;        expect(window.innerwidth).tobe(0);    });    it("fails in the test after the property was overridden", () =&gt; {        expect(() =&gt; {            expect(window.innerwidth).tobegreaterthan(0); // <p>这就是 jest.replaceproperty() 的用途：<br></p><pre class="brush:php;toolbar:false">describe("use jest.replaceproperty() to override properties of global objects", () =&gt; {    aftereach(() =&gt; {        jest.restoreallmocks();    });    it("works before the property is overridden", () =&gt; {        expect(window.innerwidth).tobegreaterthan(0);    });    it("works and is harmless", () =&gt; {        jest.replaceproperty(window, "innerwidth", 0);        expect(window.innerwidth).tobe(0);    });    it("does not affect the next test", () =&gt; {        expect(window.innerwidth).tobegreaterthan(0);    });});

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();    });});

describe("don't override methods of global objects using jest.fn()", () =&gt; {    it("works before the method is overridden", () =&gt; {        expect(document.getelementbyid("foo")).tobenull();    });    it("works, but is evil", () =&gt; {        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", () =&gt; {        expect(() =&gt; {            expect(document.getelementbyid("foo")).tobenull(); // <p>我们应该如何模拟全局对象中的方法？这就是 jest.spyon() 的好处：<br></p><pre class="brush:php;toolbar:false">describe("use jest.spyOn() to override methods of global objects", () =&gt; {    afterEach(() =&gt; {        jest.restoreAllMocks();    });    it("works before the method is overridden", () =&gt; {        expect(document.getElementById("foo")).toBeNull();    });    it("works and is harmless", () =&gt; {        const el = document.createElement("div");        jest.spyOn(document, "getElementById").mockReturnValue(el);        expect(document.getElementById("foo")).toBe(el);    });    it("does not affect the next test", () =&gt; {        expect(document.getElementById("foo")).toBeNull();    });});