代码转换
- 默认值
- 编写自定义Transformers
  - tip" level="5">tip
例子
检查带有类型的TypeScript
- 将图片转换为其路径

代码转换

Jest在你的项目中以JavaScript的代码形式运行，但是如果你使用一些Node.js不支持的，却可以开箱即用的语法(如JSX，TypeScript中的类型，Vue模板等)那你就需要将代码转换为纯JavaScript，类似于上述语法在构建浏览器时将要做的事情。

Jest通过 transform 配置选项来支持这一点。

转换器是为转换源文件提供同步功能的模块。例如，如果希望能够在Node尚不支持的模块或测试中使用新的语言功能，可以插入许多编译器中的一个，这些编译器将JavaScript的未来版本编译为当前版本。

Jest将会缓存转换后的结果，并且试图让多方因素（比如正在转换的文件源和配置信息被修改等）造成的结果无效

默认值

Jest附带一个现成的转换器-babel Jest。它将自动加载项目的Babel配置，并转换与以下正则表达式匹配的任何文件：/\.[jt]sx$/表示任何.js，.jsx，.ts和.tsx文件。此外，babel-jest还将会注入 ES Module mocking中所提到的Babel插件。

如果你覆盖了transform的配置选项，则babel-jest将不会生效，如果你还想要使用Babel的话，那么就得手动添加它。

编写自定义Transformers

你可以编写专属的Transformer， Transformer的API如下所示：

interface SyncTransformer<OptionType = unknown> {
  /**
   * Indicates if the transformer is capable of instrumenting the code for code coverage.
   *
   * If V8 coverage is _not_ active, and this is `true`, Jest will assume the code is instrumented.
   * If V8 coverage is _not_ active, and this is `false`. Jest will instrument the code returned by this transformer using Babel.
   */
  canInstrument?: boolean;
  createTransformer?: (options?: OptionType) => SyncTransformer<OptionType>;
  getCacheKey?: (
    sourceText: string,
    sourcePath: Config.Path,
    options: TransformOptions<OptionType>,
  ) => string;
  getCacheKeyAsync?: (
    sourceText: string,
    sourcePath: Config.Path,
    options: TransformOptions<OptionType>,
  ) => Promise<string>;
  process: (
    sourceText: string,
    sourcePath: Config.Path,
    options: TransformOptions<OptionType>,
  ) => TransformedSource;
  processAsync?: (
    sourceText: string,
    sourcePath: Config.Path,
    options: TransformOptions<OptionType>,
  ) => Promise<TransformedSource>;
}
interface AsyncTransformer<OptionType = unknown> {
  /**
   * Indicates if the transformer is capable of instrumenting the code for code coverage.
   *
   * If V8 coverage is _not_ active, and this is `true`, Jest will assume the code is instrumented.
   * If V8 coverage is _not_ active, and this is `false`. Jest will instrument the code returned by this transformer using Babel.
   */
  canInstrument?: boolean;
  createTransformer?: (options?: OptionType) => AsyncTransformer<OptionType>;
  getCacheKey?: (
    sourceText: string,
    sourcePath: Config.Path,
    options: TransformOptions<OptionType>,
  ) => string;
  getCacheKeyAsync?: (
    sourceText: string,
    sourcePath: Config.Path,
    options: TransformOptions<OptionType>,
  ) => Promise<string>;
  process?: (
    sourceText: string,
    sourcePath: Config.Path,
    options: TransformOptions<OptionType>,
  ) => TransformedSource;
  processAsync: (
    sourceText: string,
    sourcePath: Config.Path,
    options: TransformOptions<OptionType>,
  ) => Promise<TransformedSource>;
}
type Transformer<OptionType = unknown> =
  | SyncTransformer<OptionType>
  | AsyncTransformer<OptionType>;
interface TransformOptions<OptionType> {
  /**
   * If a transformer does module resolution and reads files, it should populate `cacheFS` so that
   * Jest avoids reading the same files again, improving performance. `cacheFS` stores entries of
   * <file path, file contents>
   */
  cacheFS: Map<string, string>;
  config: Config.ProjectConfig;
  /** A stringified version of the configuration - useful in cache busting */
  configString: string;
  instrument: boolean;
  // names are copied from babel: https://babeljs.io/docs/en/options#caller
  supportsDynamicImport: boolean;
  supportsExportNamespaceFrom: boolean;
  supportsStaticESM: boolean;
  supportsTopLevelAwait: boolean;
  /** the options passed through Jest's config by the user */
  transformerConfig: OptionType;
}
type TransformedSource =
  | {code: string; map?: RawSourceMap | string | null}
  | string;
// Config.ProjectConfig can be seen in code [here](https://github.com/facebook/jest/blob/v26.6.3/packages/jest-types/src/Config.ts#L323)
// RawSourceMap comes from [`source-map`](https://github.com/mozilla/source-map/blob/0.6.1/source-map.d.ts#L6-L12)

可以看出，只有process或processAsync是必须实现的，尽管我们强烈建议也实现getCacheKey，这样我们就不会浪费资源来传输之前从磁盘中读取过的源文件。你还可以使用@jest/create-cache-key-function来帮助你实现它

需要注意的是 ECMAScript module的支持是由supports* 选项指明的。具体来说supportsDynamicImport: true 表示的是这个transformer可以返回ESM和CJS都支持的import()表达式。而 supportsStaticESM: true 则表示的是支持最高级别的import语句，代码将被解释为ESM而不是CJS。阅读 Node’s docs了解ESM和CJS之间的具体差异信息

tip" class="reference-link">tip

Make sure TransformedSource contains a source map, so it is possible to report line information accurately in code coverage and test errors. Inline source maps also work but are slower.

例子

检查带有类型的TypeScript

babel-jest默认情况下会传输TypeScript文件，但Babel并不会对类型校验。如果你需要校验类型你可以使用 ts-jest.

将图片转换为其路径

导入图像是将其包含在浏览器包中的一种方法，但它们不是有效的JavaScript。在Jest中有一种解决方法是将它们的文件名替换成导入值

fileTransformer.js

const path = require('path');
module.exports = {
  process(src, filename, config, options) {
    return `module.exports = ${JSON.stringify(path.basename(filename))};`;
  },
};

jest.config.js

module.exports = {
  transform: {
    '\\.(jpg|jpeg|png|gif|eot|otf|webp|svg|ttf|woff|woff2|mp4|webm|wav|mp3|m4a|aac|oga)$':
      '<rootDir>/fileTransformer.js',
  },
};