報錯信息：
Webpacker can't find application.js in .../app/public/packs/manifest.json. Possible causes:
1. You want to set webpacker.yml value of compile to true for your environment
unless you are using the `webpack -w` or the webpack-dev-server.
2. webpack has not yet re-run to reflect updates.
3. You have misconfigured Webpacker's config/webpacker.yml file.
4. Your webpack configuration is not creating a manifest.
Your manifest contains:
{
}
報錯原因：node.js版本太低
在用rails new project_name命令生成項目目錄時會看到一個node.js版本警告信息（項目生成時注意看命令行的打印信息，后面新建一個項目時才看到）
1. 更新node.js
1.1 輸入命令sudo npm config set registry https://registry.npm.taobao.org，把npm的包源設置為淘寶的鏡像
1.2 輸入命令sudo npm install n -g，來安裝n這個工具，n這個工具是用於更新node版本的工具
1.3 輸入命令sudo n stable，安裝最新穩定版的nodejs
2. 安裝Webpacker
輸入命令bundle exec rake webpacker:install