错误，找不到node-sass,Module build failed: Error: Cannot find module 'node-sass'

 error  in ./src/views/Login.vue

Module build failed: Error: Cannot find module 'node-sass'
Require stack:
- D:\Vue\workspace\hellovue\node_modules\sass-loader\dist\index.js
- D:\Vue\workspace\hellovue\node_modules\sass-loader\dist\cjs.js
- D:\Vue\workspace\hellovue\node_modules\loader-runner\lib\loadLoader.js
- D:\Vue\workspace\hellovue\node_modules\loader-runner\lib\LoaderRunner.js
- D:\Vue\workspace\hellovue\node_modules\webpack\lib\NormalModule.js
- D:\Vue\workspace\hellovue\node_modules\webpack\lib\NormalModuleFactory.js
- D:\Vue\workspace\hellovue\node_modules\webpack\lib\Compiler.js
- D:\Vue\workspace\hellovue\node_modules\webpack\lib\webpack.js
- D:\Vue\workspace\hellovue\node_modules\webpack-dev-server\lib\Server.js
- D:\Vue\workspace\hellovue\node_modules\webpack-dev-server\bin\webpack-dev-server.js
    at Function.Module._resolveFilename (internal/modules/cjs/loader.js:793:17)
    at Function.Module._load (internal/modules/cjs/loader.js:686:27)
    at Module.require (internal/modules/cjs/loader.js:848:19)
    at require (internal/modules/cjs/helpers.js:74:18)
    at getDefaultSassImpl (D:\Vue\workspace\hellovue\node_modules\sass-loader\dist\index.js:198:10)
    at Object.loader (D:\Vue\workspace\hellovue\node_modules\sass-loader\dist\index.js:80:29)

 @ ./node_modules/vue-style-loader!./node_modules/css-loader?{"sourceMap":true}!./node_modules/vue-loader/lib/style-comp
iler?{"vue":true,"id":"data-v-26084dc2","scoped":true,"hasInlineConfig":false}!./node_modules/sass-loader/dist/cjs.js?{"
sourceMap":true}!./node_modules/vue-loader/lib/selector.js?type=styles&index=0!./src/views/Login.vue 4:14-370 13:3-17:5
14:22-378
 @ ./src/views/Login.vue
 @ ./src/router/index.js
 @ ./src/main.js
 @ multi (webpack)-dev-server/client?http://localhost:8080 webpack/hot/dev-server ./src/main.js

解决办法：
后来搜索出cnpm install node-sass --save
这个命令可能有用，但是要事先安装cnpm。所以npm i cnpm时又是一通报错。

所以用npm install -g cnpm --registry=https://registry.npm.taobao.org ，从淘宝镜像那下载，然后cnpm下载成功。

最后输入cnpm install node-sass --save。
npm run dev运行成功。