title: Fail with JEST and Vue.js "Plugin/Preset files are not allowed to export objects, only functions."
body: <blockquote>
    <p>Hello everyone!</p>
  </blockquote>
  
  <p>i have a problem with jest.
  I want to test a view.js application with jest.
  The application is already well advanced.</p>
  
  <p>I create a very simple test to understand the operation but it always returns me an error.</p>
  
  <pre><code> FAIL  test/unit/specs/sum.test.js
   ● Test suite failed to run
  
    at createDescriptor (node_modules/@babel/core/lib/config/config-descriptors.js:178:11)
    at items.map (node_modules/@babel/core/lib/config/config-descriptors.js:109:50)
        at Array.map (&lt;anonymous&gt;)
    at createDescriptors (node_modules/@babel/core/lib/config/config-descriptors.js:109:29)
    at createPresetDescriptors (node_modules/@babel/core/lib/config/config-descriptors.js:101:10)
    at presets (node_modules/@babel/core/lib/config/config-descriptors.js:47:19)
    at mergeChainOpts (node_modules/@babel/core/lib/config/config-chain.js:320:26)
    at node_modules/@babel/core/lib/config/config-chain.js:283:7
    at buildRootChain (node_modules/@babel/core/lib/config/config-chain.js:120:22)
    at loadPrivatePartialConfig (node_modules/@babel/core/lib/config/partial.js:85:55)
  </code></pre>
  
  <p>Here is the state of my configuration:</p>
  
  <p>package.json</p>
  
  <p>The part <code>scripts</code>:</p>
  
  <pre><code>"scripts": {
    "dev": "webpack-dev-server --inline --progress --config build/webpack.dev.conf.js",
    "start": "npm run dev",
    "unit": "cross-env BABEL_ENV=test karma start test/unit/karma.conf.js --single-run",
    "e2e": "node test/e2e/runner.js",
    "test": "jest",
    "lint": "eslint --ext .js,.vue src test/unit test/e2e/specs",
    "build": "node build/build.js"
  }
  </code></pre>
  
  <p>The part <code>Jest</code>:</p>
  
  <pre><code>"jest": {
    "moduleFileExtensions": [
      "js",
      "json",
      "vue"
    ],
    "transform": {
      ".*\\.(vue)$": "vue-jest",
      "^.+\\.js$": "&lt;rootDir&gt;/node_modules/babel-jest"
    },
    "moduleNameMapper": {
      "^@/(.*)$": "&lt;rootDir&gt;/src/$1"
    }
  }
  </code></pre>
  
  <p>The part <code>devDependencies</code>:</p>
  
  <pre><code>"devDependencies": {
    "@vue/test-utils": "^1.0.0-beta.29",
    "autoprefixer": "^7.1.2",
    "babel-core": "^6.22.1",
    "babel-eslint": "^8.2.1",
    "babel-helper-vue-jsx-merge-props": "^2.0.3",
    "babel-jest": "^24.1.0",
    "babel-loader": "^7.1.1",
    "babel-plugin-istanbul": "^4.1.1",
    "babel-plugin-syntax-jsx": "^6.18.0",
    "babel-plugin-transform-runtime": "^6.22.0",
    "babel-plugin-transform-vue-jsx": "^3.5.0",
    "babel-preset-env": "^1.3.2",
    "babel-preset-stage-2": "^6.22.0",
    "babel-register": "^6.22.0",
    "chai": "^4.1.2",
    "chalk": "^2.0.1",
    "chromedriver": "^2.27.2",
    "copy-webpack-plugin": "^4.0.1",
    "cross-env": "^5.0.1",
    "cross-spawn": "^5.0.1",
    "css-loader": "^0.28.11",
    "eslint": "^4.15.0",
    "eslint-config-standard": "^10.2.1",
    "eslint-friendly-formatter": "^3.0.0",
    "eslint-loader": "^1.7.1",
    "eslint-plugin-import": "^2.7.0",
    "eslint-plugin-node": "^5.2.0",
    "eslint-plugin-promise": "^3.4.0",
    "eslint-plugin-standard": "^3.0.1",
    "eslint-plugin-vue": "^4.0.0",
    "extract-text-webpack-plugin": "^3.0.0",
    "file-loader": "^1.1.4",
    "friendly-errors-webpack-plugin": "^1.6.1",
    "html-webpack-plugin": "^2.30.1",
    "inject-loader": "^3.0.0",
    "jest": "^24.1.0",
    "karma": "^1.4.1",
    "karma-coverage": "^1.1.1",
    "karma-mocha": "^1.3.0",
    "karma-phantomjs-launcher": "^1.0.2",
    "karma-phantomjs-shim": "^1.4.0",
    "karma-sinon-chai": "^1.3.1",
    "karma-sourcemap-loader": "^0.3.7",
  "karma-spec-reporter": "0.0.31",
  "karma-webpack": "^2.0.2",
  "mocha": "^3.2.0",
  "nightwatch": "^0.9.12",
  "node-notifier": "^5.1.2",
  "optimize-css-assets-webpack-plugin": "^3.2.0",
  "ora": "^1.2.0",
  "phantomjs-prebuilt": "^2.1.14",
  "portfinder": "^1.0.13",
  "postcss-import": "^11.0.0",
  "postcss-loader": "^2.0.8",
  "postcss-url": "^7.2.1",
  "rimraf": "^2.6.0",
  "selenium-server": "^3.0.1",
  "semver": "^5.3.0",
  "shelljs": "^0.7.6",
  "sinon": "^4.0.0",
  "sinon-chai": "^2.8.0",
  "style-loader": "^0.20.3",
  "stylus": "^0.54.5",
  "stylus-loader": "^3.0.2",
  "uglifyjs-webpack-plugin": "^1.1.1",
  "url-loader": "^0.5.8",
  "vue-jest": "^3.0.3",
  "vue-loader": "^13.3.0",
  "vue-style-loader": "^3.0.1",
  "vue-template-compiler": "^2.5.2",
  "webpack": "^3.6.0",
  "webpack-bundle-analyzer": "^2.9.0",
  "webpack-dev-server": "^2.9.1",
  "webpack-merge": "^4.1.0"
  </code></pre>
  
  <p>}
  }</p>
  
  <p>The function to be tested</p>
  
  <p>The file name <code>sum.js</code></p>
  
  <pre><code>export default function sum (a, b) {
    return a + b
  }
  </code></pre>
  
  <p>File that must test the function</p>
  
  <p>The file  <code>sum.test.js</code></p>
  
  <pre><code>import sum from '../../../src/components/sum'
  
  describe('sum', () =&gt; {
    it('create sum of 2 numbrers', () =&gt; {
      expect(sum(15, 8)).toBe(23)
    })
  })
  </code></pre>
  
  <p>The file structure and the like this:</p>
  
  <pre><code>src
  -components
  --sum.js
  test
  -unit
  --specs
  ---sum.test.js
  </code></pre>