如何在角度测试中导入“ M”变量？

Question

为了能够使用物化CSS，我在types.d.ts中定义了一个变量“ M”。但是在测试中我得到了“ M”未定义错误。

我已经在tsconfig.json中的“ typeRoots”下以及在karma.conf.js中的“文件”下导入了“ typings.d.ts”，但这不起作用。

tsconfig.json：

{
  "compileOnSave": false,
  "compilerOptions": {
    "baseUrl": "./",
    "importHelpers": true,
    "outDir": "./dist/out-tsc",
    "sourceMap": true,
    "declaration": false,
    "module": "es2015",
    "types": [
      "materialize-css",
      "jest",
      "typings"
    ],
    "moduleResolution": "node",
    "emitDecoratorMetadata": true,
    "experimentalDecorators": true,
    "target": "es5",
    "typeRoots": [
      "node_modules/@types",
      "src/typings.d.ts"
    ],
    "lib": [
      "es2017",
      "dom"
    ]
  }
}

karma.conf.js：

// Karma configuration file, see link for more information
// https://karma-runner.github.io/1.0/config/configuration-file.html

module.exports = function (config) {
  config.set({
    basePath: '',
    frameworks: ['jasmine', '@angular-devkit/build-angular'],
    plugins: [
      require('karma-jasmine'),
      require('karma-chrome-launcher'),
      require('karma-jasmine-html-reporter'),
      require('karma-coverage-istanbul-reporter'),
      require('@angular-devkit/build-angular/plugins/karma')
    ],
    client: {
      clearContext: false // leave Jasmine Spec Runner output visible in browser
    },
    coverageIstanbulReporter: {
      dir: require('path').join(__dirname, '../coverage'),
      reports: ['html', 'lcovonly'],
      fixWebpackSourcePaths: true
    },
    files: ["src/typings.d.ts"],
    reporters: ['progress', 'kjhtml'],
    port: 9876,
    colors: true,
    logLevel: config.LOG_INFO,
    autoWatch: true,
    browsers: ['Chrome'],
    singleRun: false
  });
};

typings.d.ts：

declare var M;