在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 - 测试也是如此。
您要不惜一切代价避免一个测试影响另一个测试。无论顺序如何，或者是否跳过某些测试，测试都应该有意义。
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);
});
});
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); //
<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", () => {
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);
});
});
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();
});
});
如果你在全局对象上使用 myobject.somefunction = jest.fn() ，你的测试可能会相互依赖并失去它们的意义：
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(); //
<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", () => {
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();
});
});
如果你想确保所有测试都发现系统处于相同（新鲜、干净）状态，则需要在每次测试后恢复模拟的状态。
最简单的解决方案是设置restoremocks配置属性。
最直接的选择是在 aftereach() 中调用 jest.restoreallmocks()
有时您想模拟文件中所有测试的内容。如果您在顶层或在describe()块中使用jest.spyon()和jest.replaceproperty()，则在执行第一个测试后，所有mock将被重置。
在顶层，您可以安全地重写属性和方法，无需 jest.spyon() 和 jest.replaceproperty()。
如果您只想为describe() 块模拟事物，则需要在beforeeach() 挂钩中进行这些调用。