beforeEach()函数用于在每个测试运行之前运行一段代码。它允许我们在每个测试之前设置共享的测试环境或变量。这样可以确保每个测试都在相同的环境下开始。

beforeAll()函数在所有测试之前只运行一次，所以上下文会有影响。

同理afterEach，afterAll，分别是每个测试之后运行，和所有测试之后只运行一次。

代码举例：

安装jest相关依赖

# jest本体
npm install --save-dev jest
# jest的类型声明
npm install --save-dev @types/jest
# typescript中使用
npm install --save-dev ts-jest

代码示例

class Calculator {count: number;constructor(count) {this.count = count;}add(val) {this.count = (this.count + val);return this.count;}subtract(val) {this.count=  (this.count - val);return this.count;}
}export { Calculator };

Test beforeEach

import { Calculator } from "./Calculator";describe("Calculator", () => {let calculator: Calculator;beforeEach(() => {calculator = new Calculator(10);});it("should add numbers correctly", () => {const result = calculator.add(1);expect(result).toEqual(11);});it("should subtract numbers correctly", () => {const result = calculator.subtract(1);expect(result).toEqual(9);});
});

Test beforeAll

import { Calculator } from "./Calculator";describe("Calculator", () => {let calculator: Calculator;beforeAll(() => {calculator = new Calculator(10);});it("should add numbers correctly", () => {const result = calculator.add(1);expect(result).toEqual(11);});it("should subtract numbers correctly", () => {const result = calculator.subtract(1);expect(result).toEqual(9);});
});

可以看出，beforeEach每次厕所都会生成新的calculator，而beforeAll只会生成一次calculator。