ng test angular 7 not recognize user defined methods in window object

Trying to run command ng test and result is error: Cannot find name 'isEmpty'

method 'isEmpty' declared as utils in global.ts

const _global = (window)

_global.isEmpty = function(...args): boolean {


    for (let i = 0; i < arguments.length; i++) {
        let object = arguments[i];

        if (object == null ||
            object == undefined ||
            (typeof object === 'object' && Object.keys(object).length == 0) ||
            (typeof object === 'string' && object.trim().length == 0))

            return true;
    }

ng serve added this file in app-module

but ng test cannot recognize

Angular CLI: 7.3.8

Node: 10.15.3

Angular: 7.2.1

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', 'text-summary'],
      fixWebpackSourcePaths: true
    },
    reporters: ['progress', 'kjhtml'],
    port: 9876,
    colors: true,
    logLevel: config.LOG_INFO,
    autoWatch: true,
    browsers: ['Chrome'],
    singleRun: false
  });
};

tsconfig.spec.json

{
  "extends": "../tsconfig.json",
  "compilerOptions": {
    "outDir": "../out-tsc/spec",
    "types": [
      "jasmine",
      "node"
    ]
  },
  "files": [
    "test.ts",
    "polyfills.ts"
  ],
  "include": [
    "**/*.spec.ts",
    "**/*.d.ts"
  ]
}

How to solve this problem ?