Module not found: Error: Can't resolve 'sass-loader'

Question

New to webpack. Trying to get sass-loader to play nice with my react project, have followed tutorials. Config seems correct, but result is always "Can't resolve 'sass-loader'".

I suspect this is some glaringly obvious error, but no amount of searching for it or googling has led me to it yet. Any help appreciated.

Error

ERROR in ./ui/index.js
Module not found: Error: Can't resolve 'sass-loader' in '/root/src'
 @ ./ui/index.js 19:0-33
 @ multi webpack-hot-middleware/client?reload=true babel-polyfill ./ui/index.js

What I have done so far (on said pre-existing react project):

npm i --save-dev node-sass sass-loader

package.json, under devDependencies

"node-sass": "^4.7.2",
"sass-loader": "^6.0.6",
"webpack": "~2.2.1",
"webpack-dev-middleware": "~1.12.0",
"webpack-hot-middleware": "~2.20.0",
"webpack-node-externals": "~1.6.0"

index.js

import React from 'react';
import ReactDOM from 'react-dom';
import { HashRouter } from 'react-router-dom';
import App from './src/App';

require('./src/styles/main.scss');

ReactDOM.render(<HashRouter><App /></HashRouter>, 
document.getElementById('root'));

webpack.config.react.js:

const path = require('path');
const webpack = require('webpack');

const ExtractTextPlugin = require('extract-text-webpack-plugin');
const HtmlWebpackPlugin = require('html-webpack-plugin');

const sourcePath = path.join(__dirname, './ui');
const extractStyle = new ExtractTextPlugin('styles.css');

//======break to module rules=======

module: {
  rules: [
    {
      test: /\.jsx?$/,
      include: sourcePath,
      exclude: /node_modules/,
      loader: 'babel-loader',
      query: {
        presets: ['react'],
      },
    },
    {
      test: /\.scss$/,
      include: sourcePath,
      exclude: /node_modules/,
      use: extractStyle.extract('sass-loader?sourceMap'),
    },
    {
      test: /\.css$/,
      exclude: sourcePath,
      loader: extractStyle.extract('css-loader'),
    },
  ],
},

//======= break again for plugins =====

plugins: [
  extractStyle,
  new HtmlWebpackPlugin({
    template: 'ui/index.html',
    title: 'name',
  }),
  new webpack.HotModuleReplacementPlugin(),
  new webpack.NamedModulesPlugin(),
  new webpack.optimize.OccurrenceOrderPlugin(),
  new webpack.NoErrorsPlugin(),
  new webpack.DefinePlugin({
    'process.env.NODE_ENV': JSON.stringify('development'),
  }),
]

Possible confounder: This is all running in a development docker container. It's been rebuilt and npm install has run again, though.

Answer 1

Go to the project and run below two commands

npm install sass-loader -D

npm install node-sass -D