title: How to resolve "Cannot resolve 'file'" error in webpack?
body: <p>I keep getting this really annoying error in webpack and even when I run <code>webpack --display-error-details</code>, I get no errors. But anytime I run <code>webpack-dev-server</code> I get the following error. I really can't see where I am going wrong in my configuration. </p>
  
  <pre><code>Entry module not found: Error: Cannot resolve 'file' or 'directory' 
  ./frontend/app.jsx
  </code></pre>
  
  <hr>
  
  <pre><code>module.exports = {
    context: __dirname,
    entry: './frontend/app.jsx',
    output: {
      filename: 'bundle.js',
      path: __dirname 
    },
    module: {
      loaders: [
        {
          test: [ /\.js$/, /\.jsx$/],
          exclude: /(node_modules)/,
          loader: 'babel-loader',
          query: {
            presets: ['es2015', 'react']
          }
        },
        {
          test: /\.scss$/,
          use: ['style-loader', 'css-loader', 'sass-loader']
        }
      ]
    },
    devtool: 'source-map',
    resolve: {
      extensions: ['.js', '.jsx', '.scss' ]
    }
  };
  </code></pre>
  
  <hr>
  
  <pre><code>{
    "name": "basicapp",
    "version": "1.0.0",
    "description": "",
    "main": "app.jsx",
    "author": "",
    "license": "ISC",
    "scripts": {
      "start": "webpack-dev-server --env development",
      "dev-server": "nodemon ./server.js localhost 8080",
      "build": "webpack --env production"
    },
    "dependencies": {
      "babel-cli": "^6.24.0",
      "babel-core": "^6.24.0",
      "babel-loader": "^6.4.1",
      "babel-preset-es2015": "^6.24.0",
      "babel-preset-react": "^6.23.0",
      "css-loader": "^0.27.3",
      "express": "^4.15.2",
      "express-graphql": "^0.6.3",
      "lodash": "^4.17.4",
      "node-sass": "^4.5.1",
      "react": "^15.4.2",
      "react-dom": "^15.4.2",
      "react-redux": "^5.0.3",
      "react-router": "^4.0.0",
      "redux": "^3.6.0",
      "sass-loader": "^6.0.3",
      "webpack": "^2.3.2"
    },
    "devDependencies": {
      "html-webpack-plugin": "^2.28.0",
      "node-sass": "^4.5.1",
      "nodemon": "^1.11.0",
      "sass-loader": "^6.0.3"
    }
  }
  </code></pre>