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Jest 安装和配置
Jest 安装
由于我们的项目是使用 typescript-library-starter 初始化的，已经内置了 Jest 的安装，但是安装的版本却不是最新的，我们可以对 package.json 中的相关依赖版本做修改，重新安装。
{
"@types/jest": "^24.0.13",
"jest": "^24.8.0",
"jest-config": "^24.8.0",
"ts-jest": "^24.0.2",
"typescript": "^3.4.5"
}
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注意，这里都是目前最新的版本，未来如果有版本升级的话，可以自行更新到最新版本。
更改版本后，在命令行再次执行 npm install 即可安装到相应版本。
Jest 配置
在 package.json 文件中有 jest 字段，对应 Jest 配置：
"jest": {
"transform": {
".(ts|tsx)": "ts-jest"
},
"testEnvironment": "jsdom",
"testRegex": "/test/.*\\.(test|spec)\\.(ts)$",
"moduleFileExtensions": [
"ts",
"tsx",
"js"
],
"coverageThreshold": {
"global": {
"branches": 90,
"functions": 95,
"lines": 95,
"statements": 95
}
},
"collectCoverageFrom": [
"src/*.{js,ts}",
"src/**/*.{js,ts}"
],
"setupFilesAfterEnv": [
"/test/boot.ts"
]
},
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接下来，我们就分别来看这几个配置的含义。
简单地说就是一种转换器配置，比如我们这里的
"transform": {
".(ts|tsx)": "ts-jest"
},
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表示的就是使用 ts-jest 工具把 .ts 和 .tsx 文件内容转换成 JavaScript，因为我们也是使用 TypeScript 编写测试代码，而 Node.js 是不能直接支持 TypeScript 的，所以需要配置转换器。
测试环境。
"testEnvironment": "jsdom"
表示它是一个类浏览器的测试环境，我们可以使用浏览器环境中的一些 API。
要测试文件的正则表达式。
"testRegex": "/test/.*\\.(test|spec)\\.(ts)$"
表示 test 目录下所有以 .test.ts 和 .spec.ts 的文件都需要跑测试。
模块文件扩展名，当你去引入一个模块并没有指定扩展名的时候，它会依次尝试去添加这些扩展名去找你引入的模块文件。
"moduleFileExtensions": [
"ts",
"tsx",
"js"
]
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表示优先找 .ts 的模块、然后是 .tsx，最后是 .js。
测试覆盖率的阈值设定，当我们的测试覆盖率达不到阈值的时候，测试会失败。
"coverageThreshold": {
"global": {
"branches": 90,
"functions": 95,
"lines": 95,
"statements": 95
}
}
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表示全局的代码分支覆盖率要达到 90%，方法覆盖率要达到 95%，代码行数覆盖率达到 95%，声明覆盖率达到 95%。
收集指定文件的测试覆盖率(即使你没为这些文件编写测试)，它的值为 glob patterns 类型。
"collectCoverageFrom": [
"src/*.{js,ts}",
"src/**/*.{js,ts}"
]
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表示收集 src 目录以及它的所有子目录中的 js 和 ts 文件的测试覆盖率。
测试框架安装后立即执行的代码文件列表。
"setupFilesAfterEnv": [
"/test/boot.ts"
]
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表示每次跑具体测试代码之前会先运行
其他关于 Jest 的配置，感兴趣的同学可以去官网做扩展学习。
至此，我们学习了 Jest 的安装和配置，下节课我们就开始编写 ts-axios 库的单元测试。
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